NEET Biology Notes – Evolution

NEET Biology Notes – Evolution Introduction

• The theory of evolution maintains that the different kinds of organisms that we see today have evolved from common ancestors over millions of years.
• This theory is one of the most important concepts in biology.
• The distinguished scientist Theodosius Dobzhansky has said, “Nothing in biology makes sense except in the light of evolution.”
• For more than a century, the theory of evolution has exerted a very strong influence not only on our thinking about biology, but also on developments in other disciplines such as sociology, politics, economics, and religion.
• Life originated on the earth between 3000 and 4000 million years ago in the form of unicellular organisms.
• How did these simple cells lead to (or evolve into) organisms as large as a whale or a Sequoia (redwood) tree and structures as complex and delicate as the eye and the brain?
• The theory of evolution by natural selection was put forward by Charles Darwin and Alfred Russel Wallace towards the middle of the 19th century.
• It has provided us with a scientific framework for understanding the evolutionary changes that have occurred and continue to take place in the biological world.

From Origin Of Earth To Origin Of Life

• Evolution is a slow, continuous, and irreversible process of change.
• Origin of earth: The big bang theory proposes that the universe had an explosive beginning. The universe originated about 20 billion years ago by a big bang (thermonuclear explosion) of a dense entity. About 4.6 billion years ago, our solar system was probably created when the gaseous cloud called solar nebula started to collapse under the force of its own gravity, until it became a flattened spinning disc of atoms and particles. Its central region heated up and became a star.
• The earth is about 4.6 billion years old, and the oldest rocks that have persisted in recognizable form are about 3.8 billion years old. For many years, scientists believed that such ancient rocks did not contain any fossils, but they knew that fossils were simply too small to be seen clearly without an electron microscope.
• The oldest microfossils discovered so far are that of cyanobacteria that appeared 3.3-3.5 billion years ago.
• Massive limestone deposits called stromatolites became frequent in the fossil record about 2.8 billion years ago. Produced by cyanobacteria, stromatolites were abundant in virtually all freshwater and marine communities until about 1.6 billion years ago.
• The fossil records indicate that unicellular protists the first eukaryotes-appeared about 1.5 billion years ago.
• The basic unit of evolution is population.
• According to recent literature, the first non-cellular forms of life could have originated 3 billion years ago. These were giant molecules (RNA, protein, polysac- charides, etc.). These capsules reproduced their molecules perhaps. The first cellular form of life did not possibly originate till about 2000 million years ago.

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Theories On The Origin Of Life

• Theory of special creation: It states that life was created by supernatural power in the form that has not undergone any change. It was given by Father Suarez. God created life in six days from materia prima and man was created by Him on the sixth day. According to this theory, the earth is about 4000 years old.
• Theory of catastrophism: It was given by Cuvier. According to him, after a gap of certain period (called age), the world undergoes a catastrophe (sudden calamity) that kills almost all living organisms and then God creates a new generation or new life from inorganic matter.
• Theory of biogenesis (i.e., life from life, omnis vivum ex vivo): This theory was proved by Redi, Spallanzani, and Pasteur independently. They disproved (refuted) the theory of spontaneous generation (abiogenesis). Francesco Redi (1668) proved that flies could not arise from putrefying meat without their eggs. Spallanzani (1767) demonstrated that the putrefaction of meat is due to microbes in the air and it can be prevented by boiling and sealing the meat in air-tight containers. Pasteur gave a definite proof of life arising from pre- existing life using microbes and sterilization methods. He performed “swan neck flask” experiment.
• Cosmozoic theory (theory of panspermia): This theory was given by Richter (1865) and Helmholtz (1884) and supported by Arrhenius (1908). They suggested that life reached the earth from some heavenly body through meteorites. According to this theory, panspermia (primitive form of life, as suggested by Arrhenius) consisted of spores or seeds (sperms) and microbes that existed throughout the universe and produced different forms of life on this earth.
• Abiogenic or Chemical Origin of Life
  Majority of scientists are of the opinion that life originated from inanimate matter. Since the theory of abiogenic origin or chemi- cal evolution of life is the only one that provides an explanation that can be tested, most scientists have tentatively accepted it.

Oparin-Haldane Hypothesis

• Alexander I. Oparin (1894-1980), a Russian bio- chemist, and J.B.S. Haldane (1892-1964), a British scientist, put forward the concept that the first living organism evolved from non-living material. They also suggested the sequence of events that might have oc- curred. In 1923, Oparin postulated that life originated on the earth at some point of time in the remote past and under the conditions no longer observed. In his book, The Origin of Life (1938), Oparin submitted “abiogenesis first, but biogenesis ever since.” Oparin’s theory is known as primary abiogenesis.
• According to Oparin and Haldane (1929), the sponta- neous generation of early molecules might have taken place if the earth once had more reducing atmosphere compared to the present oxidizing atmosphere. Oparin and Haldane agreed that the primeyal earth contained little, if at all, oxygen. Perhaps in the primitive atmos- phere, oxygen in the free gaseous state was virtually absent. Therefore, no degradation of any organic com- pound arising in the primeval earth could have taken place.
• As there was no ozone layer in the atmosphere, any absorption of UV radiations, which is lethal to our pre- sent lives, was not possible in the primeval earth.
• The early gas cloud was rich in hydrogen, being pre- sent in the combined form in methane (CH), ammonia (NH3), and water vapor (H2O).
• Moreover, the atmospheric water vapor along with early gas cloud condensed into drops of water and fell as rain that rolled down the rock surfaces and accumu- lated to form liquid pools and oceans. In the process, the erosion of rocks and the washing of minerals (e.g., chlorides and phosphates) into the oceans were inevi- table. Thus, Haldane’s hot dilute soup was produced and the stage was set for the combination of various chemical elements.
• Atmospheric chemicals and those in water produced small precursor molecules such as amino acids, sugars, and nitrogenous bases. These precursor molecules then combined resulting in the appearance of proteins, polysaccharides, and nucleic acids.
• The energy sources for such reactions of organic synthesis were the UV radiations (solar radiation), cosmic rays, electrical discharges (lightning), intense dry heat (volcanic eruption), and radioactive decay of various elements on the earth’s surface. Once formed, the organic molecules accumulated in water because their degradation was extremely slow in the absence of any life or enzyme catalysts. Such transformation is not possible in the present oxidizing atmosphere because oxygen or microorganisms will decompose or destroy the living particle that may arise by mere chance.

Experimental Evidence for Abiogenic Molecular Evolution of Life

• Harold C. Urey (1893-1981), an astronomer, accorded the first adequate recognition of Oparin-Haldane’s view on the origin of life in 1952.
• Urey asked his student Stanley L. Miller, a biochemist, to replicate the primordial atmosphere as propounded by Oparin and Haldane.
• Miller (1953) made the first successful simulated experiment to assess the validity of the claim for the origin of organic molecules in the primeval earth’s conditions.

• The abiotic synthesis of biomolecules is studied under the following headings:
  • Chemogeny: It is the synthesis of organic molecules by chemical reactions.
  • Biogeny: It is the formation of self-replicating biomolecules in broth (primordial hot soup or warm little pond).
  • Cognogeny: It is the evolution of various forms of life or the diversification of existing groups.

Enclosing the Prebiotic Systems

• The experiments of Miller and other scientists demonstrate that prebiotic molecules could have been formed under the conditions which most likely existed on the early earth.
• Still, the formation of prebiotic soup of small molecules does not necessarily lead to the origin of life.
• For the origin of life, at least three conditions needed to have been fulfilled:
  • There must have been a supply of self-replicators, i.e., self-producing molecules.
  • Copying of these replicators must have been subject to error via mutation.
  • The system of replicators must have required a perpetual supply of free energy and partial isolation from the general environment.
• The high temperature prevailing in the early earth would have easily fulfilled the second condition, that is, the requirement of mutation. The thermal motion would have continually altered the prebiotic molecules.

Evidences Of Evolution

Evidences from Anatomy

• Homologous organs
  • These organs have similar basic structure and developmental origin.
  • The organisms that possess such organs are said to have originated from common ancestor.
  • Consider, for example, the seal’s flipper, the bat’s wing, the horse’s foot, the cat’s paw, and the human’s hand.
  • In plants, a thorn of Bougainvillea differs from a tendril of Cucurbita in its function; both are located in a similar (auxiliary) position and have similar origin.
  • Thorns and tendrils are considered homologous.

• • Homologous organs show divergent evolution. It means that similar structures developed along different directions due to adaptions to different needs or adaptive radiation (which is the development of dissimilar functional structures in closely related group of organisms).

• Analogous organs
  • These are organs that are not anatomically similar though they perform similar functions.
  • For example, the wings of birds and butterfly look alike; they perform the similar function of flying but are not anatomically or structurally similar.
  • Even the wings of birds and bats are also analogous structures which have different origins.
  • Other examples are the flippers of penguin and dolphin (the former is a bird and the latter is a mammal), and the eyes of octopus and mammals (both differ in retinal position; still the function is the same).
  • In plants, sweet potato (root modification) and potato (stem modification) is another example of analogy. Both are meant for the storage of food but are modifications of different parts of plant.
  • Now, what is the reason of the development of analogous structures?
  • The possible explanation may be that it is the similar habitat that has resulted in the selection of similar adaptive features in different (distantly related) groups of organisms put toward the same function.
  • This phenomenon is termed as adaptive convergence or convergent evolution. It is the opposite of adaptive radiation as seen in homologous structures.
• Vestigial organs
  • These are believed to be the remnants of organs that were complete and functional in the ancestors.
  • The study of vestigial organs offers an evolutionary explanation of such rudimentary vestiges by stating that adaptations to the new environment of the organism have made these structures redundant.
  • Such structures are called vestigial organs.

• • The rudiment of the reptilian jaw apparatus and the rudiments of the hind limbs of python and Greenland whales are some examples of vestigial organs.
  • In humans, many vestigial structures indicate a relationship with other mammals, including the primates.
  • For instance, the muscles of the external ear and scalp are rudimentary and often non-functional.
  • But these are common to many mammals where they are functional.
  • The reduced tailbone and nictitating membrane of the eye, the appendix of cecum, rudimentary body hair, and wisdom teeth are examples of vestigial organs.
  • The appendix of man is thought to be a remnant of the large cecum-the storage organ for cellulose digestion in herbivorous mammals.
  • Similarly, the non-functional vestiges of the pelvic girdle in python and porpoise show, for instance, that they originally evolved from four-footed ancestors.
• Atavism
  • The sudden reappearance of an ancestral character is called atavism.
  • For example, tail in new born human baby.
  • The winged petiole of Citrus represents that the unifoliate condition is derived from the trifoliate leaf.

Biogeographical Evidences

• The study of the patterns of distribution of animals and plants in different parts of the earth is called biogeography.
• Alfred Russel Wallace (1823-1913) divided the whole world into six major biogeographical regions or realms.

• • Palaearctic: Europe and Asia, north of the tropics, north-western corner of Africa, including the Atlas Mountains.
  • Nearctic: North America exclusive of the tropics, Alaska, Canada, United States, and Mexico.
  • Neotropical: Central America including low lands of Mexico, islands of the Caribbean, and all of South America.
  • Ethiopian: Africa (with exception of the Atlas Mountains) and Madagascar and adjacent islands.
  • Oriental: Tropical part of Asia (including India); south of the Himalaya Mountains; and eastward through Sumatra, Java, Borneo, and Philippines.
  • Australian: Australia; Tasmania; New Guinea; and all islands of the Indonesian archipelago that lie to the east of Borneo; beginning with Celebes.
• Biogeographic map of the world is that in which the six major biogeographic realms are present.
• Geologists believe that millions of years ago, all continents we demarcate today were in the form of a single land mass.
• On account of geological changes, especially the movements of crustal plates below the surface of the earth, huge land masses broke off and drifted apart from one another.
• As these land masses (continents) moved away, the seas separated them and acted as barriers to the free movement of organisms among the continents.
• Because of variable environmental conditions prevailing on the different continents, over centuries, plants and animals evolved independently in each biogeographical region.
• Consider, for example, two instances that show similarities in the pattern of distribution of plants and animals between two land masses which were once the part of a larger land mass.
  • The flora and fauna on each of the Galapagos Islands a chain of 22 islands in the Pacific Ocean on the west coast of South America-resemble those of the South American mainland with which the Galapagos Islands were once connected.
  • Magnolias, tulips, and sassafras are found naturally growing in the eastern USA and in China. Hence, these show disjunct distribution. This means that these floras have different groups that are related but widely separated geographically.
• The distribution patterns of the present-day animals and plants as well as fossils are best explained on the basis of the theory of evolution.
• The birds on Galapagos Islands show differences in bills and feeding habits.
• The bills of several of these species resemble those of different, distinct families of birds on the mainland.
• All these birds are thought to have evolved from a single common ancestor.

What is Adaptive Radiation?

• During his journey, Darwin went to Galapagos Islands. There he observed an amazing diversity among creatures.
• Of particular interest were small black birds, later called Darwin’s finches, which amazed him.
• He realized that there were many varieties of finches on the same island.
• All the varieties, he conjectured, evolved on the island itself.
• From the original seed-eating features, many other forms with different sizes of beaks arose, enabling them to become insectivorous and vegetarian finches.
• These birds on the Galapagos Islands show difference in bills and feeding habits, but still resemble the birds present on the original mainland.
• Hence, we have seen that different species have evolved from single common ancestor.

• This process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation, of which Larwin’s finches represent one of the best example.
• The clusters of species that have been formed on the Galapagos Islands (Tasmanian wolf) are clear examples of species formation arising by microevolutionary divergence from an ancestral form occupying different habitats of microevolution leading to macroevolution.
• Another example is Australian marsupials.

• A number of marsupials, each different from the other, evolved from an ancestral stock but within the Australian continent.
• When more than one adaptive radiation appears to have occurred in isolated geographical areas (representing different habitats), one can call this convergent evolution.
• Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be similar to a corresponding marsupial (e.g., placental wolf and Tasmanian wolf marsupial)

Embryological Evidences

• The sequence of embryonic development in different vertebrates shows striking similarities.
• Gill clefts and notochord appear in the embryonic development of all vertebrates from fishes to mammals.
• The notochord is replaced by the vertebral column in all adult vertebrates.
• Similarly, gills are replaced by lungs in adult amphibians, reptiles, and mammals.
• Such similarities in embryonic development once again reinforce the idea of evolution from common ancestors.
• Occasionally, embryonic features such as tail and gill slits persist in adults.

• According to Ernst Haeckel, ontogeny (development of embryo) is the recapitulation of phylogeny (the ancestral sequence).
• This view was summarized by his biogenetic law: Ontogeny recapitulates phylogeny.
• Developmental evidence for evolution is also available from plants.
• It is generally believed that mosses and ferns are more evolved than algae.
• Protonema of mosses resembles certain green algae.
• This provides a clue to their evolutionary relationship.
• Both bryophytes and pteridophytes have ciliated sperms and require water for fertilization.
• Gymnosperms do not need water for fertilization.
• But Cycads and Gingko, the primitive gymnosperms, have ciliated sperms like pteridophytes.
• This suggests that gymnosperms have descended from pteridophyte-like ancestors.
• The occurrence of ancestral traits in embryo is called palaeogenesis.

Palaeontological Evidences

• Fossils are the remains and/or impressions of organisms that lived in the past few centuries. Palacontologists have painstakingly built up extensive collections of fossils from all over the world.
• Fossil record has helped in building the broad historical sequence of biological evolution.
• Phylogeny, the evolutionary history of the organism, can sometimes be reconstructed with the help of fossils.
• Horse, elephant, and man are good examples of relatively complete reconstructions of phylogeny.
• Besides form and structure, the habits and behavior of extinct species can be inferred from well-preserved fossils.
• It is also possible to reconstruct the entire habitat of an organism from fossils.
• Fossils also indicate the connecting links between two groups of organisms.
• Archaeopteryx shows the features of both reptiles and birds.

• The reptilian characters of Archaeopteryx are as follows:
  • The body axis is more or less lizard-like.
  • A long tail is present.
  • The bones are not pneumatic.
  • The jaws are provided with similar teeth.
  • A weak sternum is present.
  • Free caudal vertebrae are present as found in lizards.
  • The hand bears typical reptilian plan and each finger terminates in a claw.
• The avian characters of Archaeopteryx are as follows:
  • Feathers are present on the body.
  • The two jaws are modified into a beak.
  • The forelimbs are modified into wings.
  • The hindlimbs are built on the typical avian plan.
  • There is an intimate fusion of the skull bones as seen in the birds.
• By careful analysis of the distribution of fossils in different strata of rocks, the time in history when different species were formed or became extinct can be inferred.

Timeline of Evolution

• When scientists first began to study and date fossils, they had to find some way to organize the different time periods from which the fossils came.
• They divided the earth’s past into large blocks of time called eras.
• Eras were further subdivided into smaller blocks of time called periods, and some periods, in turn, were subdivided into epochs.
• The major geological eras, with their approximate dates in millions of years.

Fossil Parks

• Our country has rich deposits of fossil plants spanning a gap of 3500 million years.
• 20 million years old fossil forests have been discovered and studied by the Birbal Sahni Institute of Palaeobot any, Lucknow. These forests need to be systematically studied and conserved for scientific understanding and enlightenment. Some excellent localities that can be raised to the status of national fossil parks are as follows:
  • 50 million years old fossil forests preserved in the sediments between the streaming lava flow that poured out into the Deccan country at Mandla district, Madhya Pradesh.
  • 100 million years old fossil forest located in Raj-mahal Hills, Bihar.
  • 260 million years old coal-forming forests in Orissa.

Microfossils and Fossil Fuel Exploration

1. Paleobiological study helps in understanding and locating coal and hydrocarbon sources.
2. Palynofossils-tiny microscopic spores, pollen, and other vegetal remains of the past-assist us in inter- preting ancient environmental conditions favorable for organic matter accumulation and its conversion to fossil fuels by transformation and subsequent thermal alteration.
3. By the quantitative analysis of microfossils, it is possible to determine the approximate location and con- figuration of nearshore marine deposits, which are in turn responsible for the formation and accumulation of hydrocarbons.
4. The main sources of hydrocarbons are phytoplankton, marine, and terrestrial algae as well as lipid-rich plant remains.
5. Thus, the study of fossil plants offers an effective tool in stratigraphic geology and can be exploited in tapping organic fuel resources.

Evolution Of Modern Horse

Eohippus (Hyracotherium)

• The evolution of modern horse began in Eocene epoch.
• The first fossil named Eohippus (dawn horse) was in North America.
• This horse was about the size of a fox or terrier dog (a type of small dog for unearthing foxes), only 40 cm high at the shoulders.
• It had short head and neck.
• The forelimbs were with four complete fingers (2, 3, 4, and 5) and one splint of the first finger and the hindlimbs with three functional toes (2, 3, and 4) and one splint of the fifth toe. (Splints are non-functional reduced fingers and toes of horse.)
• Teeth were with incomplete cement.
• Molar teeth had no serrations.
• Low-crowned molar teeth were adapted to browse soft lush vegetation.

Mesohippus

• Mesohippus, the intermediate horse, evolved from Hyracotherium about three crore years ago during Oligocene epoch.
• It was of the size of modern sheep, about 60 cm high at the shoulders.
• Forefeet had three fingers and one splint of the fifth finger and hind feet possessed three toes, but the middle one was longer than the others and supported most of the body weight.
• Molar teeth had some serrations.

Merychippus

• Merychippus, the ruminating horse, arose from Mesohippus in Miocene epoch about two crore years ago.
• It was of the size of a small pony, about 100 cm high at the shoulders.
• It had a longer neck.
• Its forelimbs and hindlimbs had three fingers and three toes, the middle finger and toe being longer than the others and supported the entire body weight.
• There was no splint.
• Teeth were longer with cement.
• Molar teeth had well-developed serrations.

Pliohippus

• Pliohippus, the Pliocene horse, evolved from Merychippus in Pliocene epoch about one crore years ago. It was the size of modern pony, about 120 cm high at the shoulders.
• Its every forelimb and hindlimb had one complete finger and one complete toe and two splints hidden beneath the skin.
• Pliohippus is, therefore, referred to be the first one toed horse.
• The molar teeth were long with well-developed cement and serrations.
• Teeth were adapted for eating grass.

Equus

• Equus is the modern horse that arose from Pliohippus in Pleistocene epoch about nine to ten lakh years ago in North America and later spread throughout the world except Australia.
• It is about 150 cm high at the shoulders.
• It has a long head and a long neck.
• Each forelimb and hindlimb of the modern horse has one finger and one toe and two splints.
• The crowns of molar teeth are elongated with enameled ridges and are highly suitable for grinding.
• During the evolution of horse, there was
  • General increase (with occasional decrease) in size,
  • Progressive loss of toes,
  • Lengthening of toes that were retained,
  • Lengthening of limbs in general,
  • Enlargement of brain, especially cerebral hemi- spheres,
  • Increase in height, and
  • Increase in the complexity of molar teeth and an enlargement of the last three premolars until they came to resemble molars.

Evolution of Vertebrates and Major Groups of Plants

• The patterns of evolution of vertebrates and major groups of plants are conspicuously different.
• The major groups of vascular plants have left relatively small number of fossils which even show gaps (fossil-less dark periods).
• There are relatively a few major lineages, and all lineages are very distinct from one another.
• Instead of showing gradual and continuous change through time, the major lineages appeared suddenly in the fossil record.
• After that, they persisted with little fundamental change for hundreds of millions of years.
• The existence of many major subdivisions of vascular plants living today can be recognized about 345 mil- lion years ago on the basis of their distinctive reproductive structure.
• All primitive land plants reproduce via tiny spores contained in the sporangia. The major taxonomic groups are distinguished by the position of sporangia on the plant.
• The sporangia are terminal, located at the tip of the plant in the most primitive Psilopsida.
• These are placed at the base of the leaves in Lycopsida (represented in the modern flora by Lycopodium and Selaginella).

• The sporangia are arranged in whorls at the top of the plant in Sphenopsida (horsetails).
• Fossil evidences document that these basic patterns have been maintained for more than 350 million years. A few, if any, intermediates are known between these patterns.
• The origin of seeds in the land plants was achieved about 345 million years ago in lineages recognized as ancestral to all more advanced vascular plants.
• The last major evolutionary advancement among vascular plants was the emergence of flowering plants (the angiosperms) about 140 million years ago.
• But the fossils left no clue as to their ancestors.
• The fossil records also indicate that nearly all living orders of angiosperms and most of the characters of their modern-day representatives evolved from them.
• The continuous change of a character within an evolying lineage is termed as evolutionary trend.
• A lineage is an evolutionary sequence arranged in linear order from an ancestral group to a descendant group.
• The number of trends in any lineage is, therefore, the same as the number of characters evolving.
• A trend may be progressive (simple to complex, from unicellular to multicellular) or retrogressive (complex to simple from bacteria to virus).

Brief Accout Of Evolution

• About 2000 million years ago (mya), the first cellular forms of life appeared on earth.
• The mechanism of how non-cellular aggregates of giant macromolecules could evolve into cells with membranous envelop is not known.
• Some of these cells had the ability to release O2.
• The reaction could have been similar to the light re- action in photosynthesis where water is split with the help of solar energy captured and channelized by appropriate light harvesting pigments.
• Slowly, single-celled organisms became multi-cellular life forms.
• By about 500 mya, invertebrates were formed and active.
• Jawless fishes probably evolved around 350 mya.
• Sea weeds and a few plants were existent probably around 320 mya.
• We are told that the first organisms that invaded land were plants.
• They were widespread on land when animals invaded land.
• Fishes with stout and strong fins could move on land and go back to water.
• There are no specimens of these left with us.
• However, these were ancestors of modern day frogs and salamanders.
• The amphibians evolved into reptiles.
• These lay thick-shelled eggs that do not dry up in the sun unlike those of amphibians.
• Again, we only see their modern day descendentsturtles, tortoises, and crocodiles.
• This was about 350 mya.
• In 1938, a fish caught in South Africa happened to be a Coelacanth which was earlier thought to be extinct.

• These animals called lobefins evolved into the first amphibians that lived on both land and water.
• In the next 200 million years or so, reptiles of different shapes and sizes dominated the earth.
• Giant ferns (pteridophytes) were present but they all fell to form coal deposits slowly.
• Some of these land reptiles went back into water to evolve into fish-like reptiles probably 200 mya (e.g., Ichthyosaur).
• The land reptiles were, of course, the dinosaurs.
• The biggest of them-Tyrannosaurus rex-was about 20 ft in height and had huge fearsome dagger-like teeth.
• About 65 mya, dinosaurs suddenly disappeared from the earth.
• We do not know the true reason.
• Some say climatic changes killed them. Some say most of them evolved into birds.

• The truth may lie in between.
• Small-sized reptiles of that era still exist today.
• The first mammals were like shrews.
• Their fossils are small sized.
• Mammals were viviparous and protected their unborn young inside the mother’s body.
• Mammals were more intelligent in sensing and avoiding danger at least.
• When reptiles came down, mammals took over this earth.
• In South America, there were mammals resembling horse, hippopotamus, bear, rabbit, etc.
• Due to continental drift, when South America joined North America, these animals were overridden by North American fauna.
• Due to the same continental drift, the pouched mammals of Australia survived because of lack of competition from any other mammal.
• Lest we forget, some mammals live wholly in water. Whales, dolphins, seals, and sea cows are some examples.
• Evolutions of horse, elephant, dog, etc., are special stories of evolution.
• The most successful story is the evolution of man with language skills and self-consciousness.

Theories Of Evolution

Lamarck’s Theory of Evolution

• Lamarck’s theory is often called as the theory of inheritance of acquired characters or the theory of use and disuse of organs.
• The first attempt to explain the origin of species and their adaptation to the environment was done by Jean Baptiste Lamarck (1744-1829).
• He was the greatest French naturalist.
• Lamarck’s theory was published in 1809 (year of Darwin’s birth) in his book “Philosophie Zoologique.”
• According to this theory, organisms undergo changes to adapt themselves to the environment.
• The changes acquired by organisms during their lifetime are passed on to the next generation.
• He took the example of long neck of Giraffe. They continuously stretched their neck to reach the vegetation on trees.
• This acquired change was passed on to the next generation.
• He also gave the principle of use and disuse.
• The use of an organ will lead to strengthening of the organ, and disuse will lead to weakening of the organ.
• Lamarck arranged his theory in the form of four postulates.
  • Internal forces tend to increase the size of the body.
  • The formation of new organs is the result of the need or want continuously felt by organisms Doctrine of appetency/desires.
  • The development and power of action of an organ is directly proportional to its use.
  • All changes acquired by the organism during its life are transmitted to the offsprings by the process of inheritance.

Darwin’s Theory of Evolution

• Charles Robert Darwin put forward the concept of natural selection as the mechanism of evolution.
• The theory was put forward along with Alfred Russell Wallace.
• Darwin had written the book “Origin of Species.”
• Darwin was greatly influenced by “An Essay on Population” written by Thomas Rev Malthus. He was also influenced by Charles Lyell’s essays on “Principles of Geology.”
• Darwin was a British naturalist.
• In 1831, at the age of 22, he was appointed on a world survey ship of British government, HMS Beagle.
• For five years on this ship, Darwin explored the fauna and flora of continents and islands.
• Branching descent and natural selection are the two key concepts of the Darwinian theory of evolution.
• According to Wallace’s chart, the main points of Dar- win’s theory of natural selection were as follows:
  • High rate of reproduction
  • Total number almost constant
  • Struggle for existence
  • Variations
  • Survival of fittest
  • Natural selection
• All successful organisms have a high biotic potential or reproductive rate.
• The organisms produce a large number of offsprings that can possibly survive.
• For example, a mouse produces a dozen mice at a time. A rabbit produces six young ones in a litter (there are four litters in a year). A rabbit starts reproducing at the age of 6 months.
• Not all but only some individuals that survive reach adulthood, and those which reach adulthood, reproduce at different rates. This is called differential reproduction.
• The success in survival and reproduction depends on the characteristic traits of an organism. For example, only those rabbits will survive which are the fastest. There is “struggle for existence” and, in this, there will be “survival of the fittest.” The phrase “survival of the fittest” was first used by Herbert Spencer. The same context was asserted by Darwin as “natural selection.”
• So, evolution is the change in the genetic composition of the population which is brought about by natural selection (which acts upon the variability in population).

Causes of Variations

• Mutation is the ultimate source of variations.
• At the next level is recombination.
• Intermingling of two widely separated populations also causes variation.

Weakness of Darwinism

• He was not able to explain the cause of discontinuous variations observed by him in nature and the mode of transmission of variants to the next generation.
• In 1868, Darwin put forward the theory of pangenesis. According to this theory, every organ of the body pro- duces minute hereditary particles, called pangenes or gemmules, and these are carried through the blood into the gametes.
• Weismann’s theory of germplasm (1892) rejected Darwin’s theory of pangenesis.
• He established that the germ (sex) cells are set apart from other body (somatic) cells early in the embryonic development. So, only the changes in the germplasm affect the characteristics of future generations.
• Alfred Wallace (1823-1923), a naturalist from Dutch East Indies, was working on Malay Archipelago (present Indonesia).
• He had written the book “On the Tendencies of Varieties to Depart Indefinitely from the Original Type.”

Mutation Theory

• In 1901, Hugo de Vries proposed the mutation theory on the basis of his observation on a wild variety of evening primrose, Oenothera lamarckiana.
• According to this theory, new species originate as a result of large, discontinuous variations that appear suddenly.
• The main features of mutation theory are as follows:
  • Mutations arise from time to time amongst individuals of a naturally breeding population.
  • Mutations are heritable and establish new forms or species.
  • Mutations are large and sudden and are totally different from the fluctuating variations of Darwin, which are small and directional.
  • Mutations may occur in any direction.

Hardy-Weinberg Principle

• The five basic processes that affect the Hardy-Wein- berg equilibrium and cause variations at genetic level are as follows:
  • Mutation.
  • Gene migration
  • Genetic drift
  • Recombination
  • Natural selection
• The Hardy-Weinberg principle states that the proportions of different alleles will stay the same in a large population if mating occurs at random and the above mentioned forces are absent.
• In algebraic terms, the Hardy-Weinberg principle is written as an equation.
• Its form is what is known as a binomial expansion.
• For a gene with two alternative alleles, A and a, the frequency of allele A can be expressed as p and that of alternative allele a as q. Because these are the only two alleles, p + q must always be equal to one. The equation looks like this:
• For example, if q is the frequency of allele a, then the Hardy-Weinberg equation states that q2 is the percentage of individuals homozygous for allele a, say 16%. q2=0.16,q=0.4

Mutation

Replica Plate Experiment of Lederberg and Lederberg

• Mutations are random (indiscriminate) with the respect to the adapative needs of organisms.
• Most mutations are harmful or with no effect (neutral) on the bearer.
• Mutation rates are very slow.
• The Lederberg replica plating experiment, a beautiful example of the genetic basis of a particular adaptation, was demonstrated in bacteria by an ingenious method devised by Joshua Lederberg and Esther Lederberg.

• E. coli bacteria are usually grown in the laboratory by plating dilute suspensions of bacterial cells on semi-solid agar plates.
• After a period of growth, discrete colonies appear on the agar plates.
• Each of these colonies originates from a single bacterium through a large number of cell divisions.
• They then created several replicas of this master plate by a simple procedure.
• A sterile velvet disc, mounted on a wooden block, was gently pressed on the master plate.
• Some bacteria from each colony adhered to the velvet.
• By pressing this velvet on to new agar plates, they obtained exact replicas of the master plate, because the few bacteria transferred by the velvet formed colonies on the new agar plates.
• However, when they attempted to make replicas using plates containing an antibiotic such as penicillin, most colonies found on the master plate did not grow on the replica plates.
• The few colonies that did grow were obviously resistant to penicillin.
• How did the bacteria acquire the ability to grow in a new environment (here, agar medium, containing penicillin)? In other words, what was the origin of this adaptation?
• A Lamarckian interpretation of this adaptation would have been that penicillin somehow induced a change in one or more bacteria, enabling them to grow in the presence of penicillin.
• A Darwinian view is that there were, in the original suspension of bacteria from which the master plates were prepared, a few bacteria carrying mutant genes which conferred on them the ability to survive the action of penicillin and form colonies.
• These mutations, which had arisen by chance and not induced by penicillin, were present only in small numbers in the original suspension.
• Lederberg’s experiment provided evidence that mutations are actually preadaptive.
• These kinds of mutations are regarded as advantageous mutations.
• These appear without exposure to the environment.
• Actually, preadaptive mutations express themselves only after exposure to the new environment to which the organisms are to adapt themselves.
• The new environment does not induce the formation; it only selects the preadaptive mutations that occurred earlier.

Migration

• Migration, defined in genetic terms as the movement of individuals from one population into another, can be a powerful force in upsetting the genetic stability of natural populations.
• If the characteristics of the newly arrived animal differ from those already there, the genetic composition of the receiving population may be altered, if the newly arrived individual or individuals can adapt to survive in the new area and mate successfully.
• Gene pool: The total collection of all genes and their alleles in a population is called gene pool. Thus, gene pool will have all genotypes, i.e., genes of the organisms.
• Gene flow: If genes are exchanged between two different populations of a species, it is gene flow.

Genetic Drift/Sewall Wright Effect/Non-Directional Factor

• Natural selection is not the only force responsible to bring about changes in gene frequencies. There is the role of chance or genetic drift also.
• Genetic drift causes a change in gene frequency by chance in a small population.
• In a small population, the individual alleles of a gene are represented by a few individuals in the population.
• These alleles will be lost if these individuals fail to reproduce.
• Allele frequencies appear to change randomly, as if the frequencies were drifting genes. Thus, a random loss of alleles in small population is genetic drift.
• A series of small populations that are isolated from one another may come to differ strongly as a result of genetic drift.
• Genetic drift has following two ramifications:
  • Bottle-neck effect:
    • It is the decrease in genetic variability in a population, e.g., cheetah population in Africa decreased due to hunting.
    • Their decreased numbers have limited cheetahs’ genetic variability, with serious consequences.
    • The present cheetah population is susceptible to a number of fatal diseases.
    • If any of these diseases attacks the cheetah population, the path of extinction of cheetah cannot be reversed.
  • Founder’s effect:
    • When one or a few individuals are dispersed and become the founders of a new, isolated population at some distance from their place of origin, the alleles that they carry are of special significance.
    • Even if these alleles are rare in the source. population, they will be a significant fraction of the new population’s genetic endowment.
    • This effect by which rare alleles and combinations of alleles may be enhanced in new populations is called founder’s effect.
    • Founder’s effect is particularly important in the evolution of organisms on islands such as Galapagos Islands which Darwin visited. . Most of the kinds of organisms that occur in such areas are probably derived from one or a few initial founders.
    • Fixation of new mutations: Genetic drift fixes new alleles-genes that arise by mutation-from time to time and eliminates the original gene, thereby changing the genetic makeup of small population.

Recombination

• Gene recombination is also an important source of variation.
• It occurs during crossing-over at the time of meiosis [free assortment (selection) of genes at the time of gamete formation], random union of gametes at the time of fertilization, and even chromosomal aberrations.
• They cause reshuffling of gene recombinations which provide new combinations of existing genes and alleles.
• This is the entity of gene recombination.
• Gene recombination can occur not only between genes but also within genes resulting in the formation of a new allele.
• Since it adds new alleles and combination of alleles to the gene pool, it is an important process during evolution which causes variations.

Natural Selection

• Natural selection causes allele frequencies of a population to change. Depending upon which traits are favored, natural selection can produce different results.
• Forms of selection: There are three kinds of natural selections:
  • Stabilizing selection (normalizing selection): When selection acts to eliminate both extremes from an array of phenotypes, the frequency of the intermediate type which is already the most com- mon is increased.
  • Directional selection (progressive selection): When selection acts to eliminate one extreme from an array of phenotypes, the genes determining this extreme become less frequent in the population. Industrial melanism in peppered moth, Biston betularia, provides good example of directional selection from nature.
  • Disruptive selection (diversifying selection): In some situations, selection acts to eliminate, rather than favor, the intermediate type. Individuals at both extremes are favored.

Examples of Natural Selection

Industrial Melanism

• Industrial melanism was first studied by R.A. Fisher and E.B. Ford and, in recent time, by H.B.D. Kettlewell.
• One of the most striking example which demonstrates the action of natural selection is the industrial melanism in England.

• The peppered moth, Biston betulania, with a dull gray color or white color was abundant in England before the Industrial Revolution.
• A black-colored form of the same moth (melanic, a dominant mutant differing in a single gene), carbon- aria, was very rare.
• Within a couple of hundred years, however, the proportion of carbonaria increased to almost 90%.
• These moths rest on tree trunks.
• Before the Industrial Revolution, the tree trunks used to be covered with gray-colored lichen.
• The dull gray moth easily blended with this back-ground, while the black moth stood out conspicuously and was, therefore, more susceptible to predation by birds.
• With the advent of the Industrial Revolution, large-scale burning of coal became common.
• The enormous amount of smoke produced resulted in the deposition of particulate matter on tree trunks, turning them black.
• As a result, the gray moths now became more conspicuous than the black variety and, hence, more susceptible to predation.
• The frequency of black-colored moths in the population, therefore, increased.
• Gradual replacement of coal by oil and electricity as well as the improved methods of controlling soot production reduced the soot deposition on the trees.
• Conditions then became more suitable for the survival of gray moths. Consequently, their frequency once again increased.
• Thus, reduction in pollution is now correlated with reverse evolution.
• Industrial melanism, as this phenomenon is called, is thus a particularly interesting example which clearly brings out the action of natural selection.
• This has been observed in about 70 different species of moths, and in several other European countries as well.
• This understanding is supported by the fact that in areas where industrialization did not occur, e.g., in rural areas, the count of melanic moths was low.
• This showed that in a mixed population, those that can better-adapt, survive and increase in population size. Remember that no variant is completely wiped out. Similarly, excess use of herbicides, pesticides, etc., has only resulted in the selection of resistant varieties in a much lesser time scale.
• This is also true for microbes against which we employ antibiotics or drugs against eukaryotic organisms/cell.
• Hence, resistant organisms/cells are appearing in a time scale of months or years and not centuries.
• These are examples of evolution by anthropogenic action.
• This also tells us that evolution is not a direct process in the sense of determinism.
• It is a stochastic process based on chance events in nature and chance mutations in the organisms.

Change in Genotypic Frequencies

• If the alleles for gray and black colors are denoted by G and B, respectively, the genotypes of moths would be GG, GB, and BB.
• Since B is dominant, GB and BB will be black.
• Due to greater predation by birds on the black (melanic) phenotype, the proportion of B in the population was maintained at a much lower value than that of G.

Resistance of Mosquitoes to Pesticides

• Mosquitoes have always been a major health hazard, especially as they are responsible for the spread of diseases such as malaria and filaria.
• When DDT was first introduced to control mosquitoes, it was tremendously successful; most mosquitoes were sensitive to DDT and were, therefore, killed.
• However, DDT has now become ineffective against mosquitoes.
• This is explained as follows:
  • In the original population of mosquitoes, some individuals were resistant to DDT.
  • In the absence of DDT, such resistant individuals were few because they had no advantage over the DDT-sensitive mosquitoes.
  • However, when DDT was used on a large-scale, only resistant genotypes were able to survive and reproduce.
  • As a result, over a period of time, almost entire population came to consist of the resistant type, which made DDT quite ineffective.
• Evolution is, thus, a change in gene frequencies in a population in response to changes in the environment-in this case, the introduction of DDT.
• The principle of natural selection, thus, helps us to understand why such chemical insecticides would remain useful only for a limited time.

Artificial Selection

• Some genetic variability is always present in a population.
• Some alleles make organisms better adapted to the environment and, thus, make them more successful in survival and reproduction.
• As a result, the frequency of such alleles in a population gradually increases.
• This is called selection; these alleles are, thus, “selected” over the other alleles.
• This process operating in natural populations is, therefore, called natural selection.
• The process of natural selection, acting on variability inherent in the population, over millions of years, has given rise to the great diversity we see in the biological world.
• Ancestry of different breeds can be traced to wild rock pigeon (artificial selection).
• Man has been using a similar process for improving the qualities of domesticated plants and animals for centuries.

• Plant-breeding and animal-breeding are very similar to the action of natural selection, the difference being that the role of nature is played by man.
• The criteria for selection are based on human interests

• To obtain cows with high milk yield, the dairy scientists monitor milk production of a large number of cows.
• Only the calves produced by cows that are high-yielders are chosen to breed and form the next generation.
• When this process is repeated (i.e., artificial selection is applied) for many generations, a population of cows with high milk yield is obtained.
• Here, the task of selection is done by man.

Speciation And Isolation

• Speciation is the formation of one or more new species from an existing species.
• The crucial episode in the origin of species occurs when the gene pool of a population is severed from other populations of the parent species and gene flow no longer occurs.
• Speciation can take place in two modes based on the geographical relationship of the new species to its ancestral species.
• When a population, formerly continuous in range, splits into two or more geographically isolated populations and forms new species, the mode of speciation is called allopatric speciation.
  This can happen by subdivision of the original population when a geographical barrier such as a creeping glacier, a land bridge (e.g., Isthmus of Panama), an ocean, or a mountain cuts across a species range.
  Alternatively, a small number of individuals may colonize a new habitat which is geographically separated from the original range.
  Darwin’s finches that formed separate species in the Galapagos Islands and the Australian marsupials that radiated to form new species are its examples.
• In the second speciation mode, a subpopulation becomes reproductively isolated in the midst of its parent population; this is sympatric speciation. So, sympatric speciation is the formation of species within a single population without geographical isolation.
  The usually quoted example of sympatric speciation comes from polyploidy-the multiplication of the normal chromosome number.
  This can happen when chromosomes fail to segregate at meiosis or replicate without undergoing mitosis.

Species Concept

• Species is the basic unit of classification.
• The term was coined by John Ray (1693).
• Most taxonomists define species as morphologically distinct and reproductively isolated natural population or group of populations where individuals resemble one another more closely than the members of other species; have a similar anatomy, karyotype, and bio- chemicals; interbreed freely; and form a genetically closed system. There are three basic concepts about species.
  • Morphospecies concept
    • It is the earliest concept of species.
    • Davis and Heywood (1963) have defined it as the assemblage of individuals with morpho- logical features in common, and separable from other such assemblages by correlated morphological discontinuities in a number of features.
    • However, the number of morphological characters chosen for separating species varies from taxonomist to taxonomist.
    • “Lumpers” combine all populations with broadly similar traits into a single species while “splitters” separate various populations with even minor morphological differences into distinct species.
  • Biological species concept
    • Though first proposed by Buffon (1753), biological species concept was formulated by Mayr (1942).
    • According to it, a biospecies (biological species, biological species concept) is a sexually interbreeding or potentially interbreeding group of individuals which is reproductively isolated from other species and is, therefore, separated from others by the absence of genetic exchange.
    • Normally, species are distinct from one an- other by both morphological traits and reproductive isolation.
    • However, sibling species are those distinct species which are almost identical morphologically but are distinct from each other due to the absence of interbreeding, e.g., Drosophila pseudoobscura and D. persimilis.
    • Biological species concept is, therefore, mainly based on the absence of cross-fertilization between the members of two species.
    • Cross-fertilization tests carried out by taxonomists on the individuals of morphologically and geographically separated populations have resulted in the revision of species and the grouping of many of them into single species, e.g., several species of North American sparrows as subspecies and the races of a single song sparrow, Passarella melodia.
    • The only problem in using reproductive isolation is the absence of sexual reproduction in several organisms-prokaryotes, some protists, some fungi, and some plants (e.g., commercial banana) and animals.
    • Further, cross-fertilization experiments cannot be performed on such a large number of species that occurs in varied geographical areas.
    • Reproductive isolation cannot be used as a criterion in case of fossils.
    • Living organisms and fossils can be grouped only on the basis of their morphology and biochemistry.
    • Mayr (1987) has named morphologically grouped asexual species as paraspecies while Ghiselin (1987) has named them pseudospecies.
  • Evolutionary species concept
    • All evolutionary taxonomists have been in search of a proper definition of species which is the basic unit of classification.
    • One such definition has been given by Simpson.
    • According to Simpson (1961), “an evolutionary species is a lineage (an ancestor-descendent sequence of population) evolving separately from others and with its own unitary evolutionary role and tendencies.”
    • The concept stresses on evolutionary isolation with sexual isolation being its one aspect.
    • It is more dependent on differences, which can be morphological, genetic, behavioral, and ecological, to know evolutionary distance.
    • However, evolution does not occur simultaneously in all traits.
    • Neither its rate nor direction (in which it is occurring) are the same.
    • Reproductive isolation may be defined as the existence of intrinsic barrier to the in- terbreeding in natural populations. Each of these intrinsic barriers is called a reproductive isolating mechanism. According to Mayr (1942), reproductive isolating mechanisms are the biological properties of individuals which prevent the interbreeding of naturally sympatric populations.
    • Reproductive isolation in the form of hybrid sterility is known since long. In the labora- tory or in zoos, hybrids can be produced between species that do not interbreed in na- ture. Horses and donkeys are two different species; a hybrid, mule, is produced from the mating of a male donkey and a mare (female horse).
    • Similarly, mating between stallion (male horse) and female donkey results in a hybrid called hinny.
    • Both mule and hinny are sterile.
    • There are examples of species that can produce fertile hybrids in captivity. You might have heard about the famous “tigon”-a hybrid of African lioness (Panthera leo) and Asian tigers (Panthera tigris)-which is fertile. No barrier to hybridization between these species has evolved during their long isolation from each other. Natural selection has not favored a reduction in hybridization for the simple reason that no hybridization has been possible. Other examples of species that breed in captivity and produce fertile hybrids are the mallard (a duck) and the pintail duck, the polar bear and the Alaskan brown bear, and the platy and swordtail fish. But these species do not interbreed at all in natural condition.

Modern Synthetic Theory Of Evolution

• Evolution on the grand scale of geological time is called macroevolution.
• Evolution at the genetic level is called microevolution.
• Studies of how individual traits evolve within natural populations provide powerful evidence that natural selection can be a powerful agent of microevolutionary change within species. The progressive change in allele frequencies within the population is microevolution.
• The unit of evolution is population.
• The unit of natural selection is individual.
• The modern synthetic theory of evolution is the result of the work of a number of scientists, namely T. Dobzhansky, R.A. Fisher, J.B.S. Haldane, Sewall Wright, and Stebbins.
• The synthetic theory includes the following factors:
  • Gene mutations
  • Changes in chromosome structure and number
  • Genetic recombination
  • Natural selection
  • Reproductive isolation
  • Migration
  • Hybridization

Neutral Theory Of Evolution

• According to Kimura, most of the mutations are neutral and are not eliminated from the population.
• This is against natural selection.
• Kimura proposed that speciation is not due to the selection of advantageous genotypes but due to the elimination of deleterious alleles and random selection of neutral alleles.
• It emphasized that most mutations are of neutral value and genetic drift is responsible for divergence.
• It means that all mutations are alike in adaptive value. It is only chance or random drift which delineates a novel collection of mutants into a group divergent from the parental population.

Place of Humans in the Animal Kingdom

• Today human evolution is being studied due to the following reasons:
  • There is homology in the chromosomes of man and great apes. The banding patterns of human chromosomes number 3 and 6 are compared with those of particular autosomes in the chimpanzee. It shows a common origin for man and chimpanzee.
  • Today, besides autosomal chromosomes, V-chromosomes and mitochondrial DNA are being studied, as they are uniparental in origin and do not take part in recombination.
  • Evidence from blood proteins: It has been proved by the blood protein tests that man is the most closely related to great apes (chimpanzee and gorilla).
  • Evidence from blood groups: Blood groups A and B are found in apes and not in monkeys.
  • Evidence from hemoglobin: There is 99% homology in the hemoglobin of man and gorilla.
• Human beings are vertebrates and belong to class Mammalia.
• Mammals evolved from primitive reptiles in early Tri- assic period, about 210 million years ago.
• But for nearly 150 million years, mammals existed as relatively inconspicuous group of small rat-like creatures, completely dominated by the large number of gigantic reptiles of the Mesozoic age.
• It is only after the great extinction of dinosaurs and other large reptiles that mammals diversified and began to occupy earth’s many different habitats.
• Within class Mammalia, human beings belong to order Primate a group that originated about 65 mya and includes not only monkeys and apes but also lorises, lemurs, and tarsiers.
• Anthropoid apes, or the ancestors of monkeys, apes, and humans, evolved about 36 mya; and hominids, or the ancestors of apes and humans, evolved about 24 mya.
• Today, apes are represented by two families, namely, Pongidae (which includes chimpanzees, gorillas, and orangutans) and Hylobatidae (which includes gibbons).
• Chimpanzee and gorilla are restricted to Africa whereas orangutans and gibbons are found only in Asia.
• Humans belong to the family Hominidae in which Homo sapiens is the only living species.

Early Human Ancestors

• Tracing the evolution of human beings, both by fossil hunting and molecular methods, is one of the most exciting and active areas of research in biology.
• The fossil evidence cleatly indicated that genera such as Ramapithecus and Sivapithecus were the forerunners of hominids.
• A genus called Australopithecus appeared in Africa about 5 mya and ultimately gave rise to Homo about 2 mya.
• But even Australopithecus had a brain measuring only about 350-450 cm3.

• The most important change that must have occurred during the three million years or so between the appearance of Australopithecus and that of our genus must, therefore, have been a phenomenal increase in brain size, all the way up to 1400-1450 cm3-a characteristic of our species.
• A combination of molecular data and a modern interpretation of the fossil record suggests that gibbons probably diverged from the main line of hominid evolution about 10 mya, that the orangutan did so about 8 mya, and the ancestors of gorilla and chimpanzee, about 4 mya.
• Gorilla and chimpanzee separated from each other only 2.3 mya.

Place and Sequence of Human Evolution

• There is evidence that almost all of hominid evolution occurred in Africa and Asia and that the evolution of human species took place in Africa.
• Several species belonging to genus Homo can be rec- ognized from the fossil record. For example, Homo habilis lived in Africa about 2 mya and had a larger brain than Australopithecus; it used tools and was bipedal.
• Another species, Homo erectus, appeared about 1.7 mya, used fire, and is believed to have migrated to Asia and Europe.
• The fossils of the so called “Java man” and “Peking man” belong to Homo erectus. It was replaced by Homo sapiens.
• A primitive form of Homo sapiens, called Neanderthal man (Homo sapiens neanderthalensis), was common in Europe and Asia.
• Neanderthal men resembled us, though they were rela- tively short and stocky and more powerfully built.
• Neanderthals made tools and used animal hides as clothing.
• They built hut-like structures for dwelling and buried their dead.
• There is evidence that an abrupt transition occurred all over Europe whereby the Neanderthal man was wiped out. It gave way to the more efficient cousin the Cro- Magnon-about 34,000 years ago.
• The Cro-Magnon people left behind very elaborate cave paintings showing the attainment of a form of culture not unlike our own.
• After the last glacial period (about 10,000 years ago), modern Homo sapiens sapiens began to spread all over the globe. They cultivated plants, domesticated animals, and reached enormous population sizes.
• Homo sapiens appeared in Africa about 5,00,000 years ago and probably replaced Homo erectus there.
• But in Asia, Homo erectus appears to have survived for another 2,50,000 years when it was finally replaced by Homo sapiens migrating from Africa.

Human Evolution

Place or Origin of Man

• It has been established that Dryopithecus is one of the oldest fossil which in turn evolved into apes and man.
• The origin and evolution of man can be studied in the following three major headings: prior to ape man, ape-man including prehistoric man, and true man including the living modern man.

Prior to Ape Man

• Dryopithecus discovery
  • The fossil of Oryopithecus africanus was discovered from the Miocene rocks of Africa and Eu- горе.
  • It lived about 15 mya.
  • Dryopithecus and Ramapithecus were hairy and walked like gorillas and chimpanzees.
  • Ramapithecus was more man-like while Dryo-pithecus was more ape-like.
  • Dryopithecus is the direct ancestor of modern-day apes.
    Characteristics
    • It was ape-like, but had arms and legs of the same length.
    • Heels in its feet indicate its semi-erect pos- ture.
    • It had large brain, a large muzzle, and large canines.
    • It was without brow ridges.
    • It was arboreal, knuckle-walker, and ate soft fruits and leaves.
    • Oryopithecus africanus is regarded a com- mon ancestor of man and apes (gibbons, or- angutan, chimpanzee, and gorilla).
    • It is also called proconsul.
• Proconsul discovery
  • Proconsul africanus or o. africanus was discovered by Louis S.B. Leakey in 1948 from the rocks around Lake Victoria of Kenya, Africa.
  • It lived in early Miocene epoch.
    Characteristics
    • It was morphologically intermediate between apes and man in many features.
    • It had rounded man-like forehead and long, pointed, ape-like canines.
    • It moved on land on all the four limbs and, hence, is not considered amongst the direct ancestors of man.
    • Proconsul gave rise to the ancestors of chimpanzee and gorilla in the Pliocene, about 4 mya.
    • Chimpanzee and gorilla diverged from each other only about 2.3 mya, in Pleistocene epoch.
• Sivapithecus discovery
  • This fossil was discovered from the middle and late Pliocene rocks of Shivalik Hills of India. Hence, it is named Sivapithecus.
    Characteristics
    • It was like Dryopithecus.
    • Its forelimbs, skull, and brain were like those of monkeys, while the face, jaws, and teeth resembled those of apes.
• Ramapithecus discovery
• It has been established that in late Miocene epoch, Oryopithecus gave rise to Ramapithecus (Rama- the hero of Indian legend, pithecus-ape), which was on the direct line of human evolution.
• Ramapithecus survived from late Miocene to Pliocene.
• Thus, he appeared about 14-15 mya.
• The fossil of Ramapithecus was discovered by Edward Lewis (1932) from the Pliocene rocks of Shivalik Hills of India.
• Kenyapithecus wickeri was discovered by L.S.B. Leakey (1962) from the Pliocene rocks of Kenya (Africa).
• It was similar to Ramapithecus. But Ramapithecus was older than Kenyapithecus.

Ape-Man Including Prehistoric Man

• Australopithecus (first ape-man) discovery
  • The early human stock gave rise to Australopithecus.

• • It is the connecting link between apes and man.
  • Raymond Dart (1924), a South African anthropologist, discovered the fossil of Australopithecus africanus (African ape-man) from Pliocene rocks near Tuang in Africa.
  • A. africanus appeared about 5 mya.
  • Actually, the skull discovered by Dart was of a 5-6 year old baby. So, it is also called the Tuang baby.
  • Some fossils of A. africanus were also discovered from Pleistocene epoch.
  • Two mya, australopithecines probably lived in East African grasslands.
  • Evidence shows that they hunted with stone weapons but essentially ate fruits.
    Characteristics
    • Australopithecus africanus was about 1.5 m high and had human as well as ape characters.
    • It had bipedal style of locomotion, ate om- nivorous diet, and had erect posture.
    • It had human-like teeth, but had more of an ape brain than a human brain.
    • Its brain capacity was about 500 cc-similar to that of an ape.
    • He lived in caves.
    • Brow ridges projected over the eyes.
    • It did not have chin.
    • There was lumbar curve in the vertebral col- umn.
    • The pelvis was broad. Australopithecus afri- canus existed until about 1.5 mya and gave rise to Homo habilis about 2 mya.
    • Australopithecus africanus also gave rise to man-like apes called Australopithecus robus- tus and Australopithecus boisei along a sep- arate line that ended blindly. (They did not give rise to any other creatures.)
    • In 1981, Donald Johanson found a 3.2 mil- lion years old skeleton of a female human ancestor.
    • He nicknamed it Lucy.
    • Lucy’s scientific name is Australopithecus afarensis.
    • Six species of Australopithecus are known.
    • These are A. africanus (African ape-man and southern ape or Taung baby), A. afarensis (Lucy), A. ramidus, A. aethiopicus, A. robus- tus, and A. boisei. So we can say that Austra- lopithecus had two main types.
      • Gracile type: Australopithecus afarensis (Johanson) was represented by fossil Lucy with small brain, small molar teeth, pelvic girdles, and short fingers like humans.
      • Robust type: A. robustus (also originally called Paranthropus) had heavier body structure, massive check tooth, and cranial capacity of 600 cm2.
        (Other examples are Zinjanthropus/A. boisei of R. Leakey, Africa; and Megan- thropus from Java.)
• Homo habilis (able or skillful man, the toolmaker, or “handy man”) discovery
  • Louis S.B. Leakey and his wife Mary Leakey (1960) discovered the fossils of Homo habilis from the Pleistocene rocks of Olduvai Gorge in East Africa.
  • He lived in Africa about 2 mya.
  • The first human-like being-the hominid-was H. habilis. He probably did not eat meat.
    Characteristics
    • He was about 1.2-1.5 m tall.
    • He had bipedal locomotion and moved erect.
    • He had about 650-800 cc cranial capacity.
    • Teeth were like that of modern man. e. Homo habilis (habilis-mentally able or skillful) was the first toolmaker and used tools of chipped stones extensively.
    • He is also called handy man because heaps of tools found with these fossils included sharpened stones which indicate that Homo habilis was capable of “making tools.”
    • He also led community life in caves and greatly cared for the young ones.
• Homo erectus (erect man)
  • Homo erectus appeared about 1.5 million years ago, in middle Pleistocene.

• • He probably ate meat.
  • He is called middle Pleistocene man. H. erectus evolved from H. habilis.
  • He was about 1.5-1.8 m tall.
  • H. erectus males were probably larger than fe- males.
  • He had erect posture.
  • His skull was flatter than that of modern man.
  • He had protruding jaws, projecting brow ridges, small canines, and large molar teeth.
  • The cranial capacity was 900 cc.
  • Cranium was domed to accommodate the large brain.
  • He was omnivorous.
  • He made more elaborate tools of stones and bones, hunted big game animals, and perhaps knew the use of fire.
  • H. erectus includes three fossils: Java ape-man, Peking man, and Heidelberg man.
    • Java ape-man Discovery
      • In 1891, Eugene Dubois discovered a fossil from Pleistocene rocks in Central Java (Island of Indonesia).
      • He named it as Pithecanthropus erectus.
      • Pithecanthropus means “ape-man.”
      • Mayer, in 1950, named it as Homo erectus erectus.
        Characteristics
        • Body was 1.65-1.75 m tall and weight was about 70 kg.
        • Legs were long and erect, but body was slightly bent when moving.
        • Chin was inconspicuous and nose was somewhat broader.
        • Forehead was low and receding, but brow ridges were high, like those of apes.
        • Skull cap was thick and heavy and flat- tened in front.
        • Cranial capacity was 800-1000 cc (aver- age 950 cc).
        • Lower jaw was large and heavy.
        • Teeth were large, but quite like those of modem man, except larger canines of the lower jaw.
        • Lips were thick and protruding.
        • He was omnivorous and cannibal.
        • Perhaps, he was the first prehistoric man to make use of fire for hunting, defense, and cooking.
    • Peking man discovery
      • W.C. Pei (1924) discovered the fossils of Peking man from the limestone caves of Choukoutein near Peking (Beijing-capital of China was formerly known as Peking) and named them Sinanthropus.
      • Davidson Black (1927) named it Sinan- thropus pekinensis.
      • Mayer (1950) renamed it as Homo erec- tus pekinensis (a subspecies).
      • The Pleistocene rocks from which the fossils of the Peking man were excavated are about 6 lakh years old.
        Characteristics
        • Placing Java ape-man and Peking man as subspecies of H. erectus has a sound basis, because of close similarities between the two.
        • The body structure was quite similar in both.
        • Being about 1.55-1.60 m tall, the Pe- king man was slightly shorter and a little lighter and weaker.
        • The only noticeable difference of the Pe- king man from the Java ape-man was its large cranial capacity, which ranged from 850 cc to 1100 cc.
        • Like Java ape-man, the Peking man was omnivorous and cannibal.
        • There is a clear evidence of the use of fire by him.
        • It has been confirmed that both Java and Peking men used to live in caves in small groups or tribes.
        • The tools of Peking man were relatively more sophisticated.
  • Heidelberg man discovery
    • In 1908, one of the most perfect fossil jaw belonging to middle Pleistocene was found by workmen working near Heidelberg, Germany.
    • It was shown to Otto Schoetensack, who gets the credit for its discovery. It was named Homo erectus heidelbergensis.
      Characteristics
      • He had lower jaw with all the teeth.
      • Teeth were human-like.
      • The massive jaw was apelike.
      • He used tools and fire.
      • Cranial capacity is believed to be about 1300 cc, which is intermediate between that of erect man (H. erectus) and Neanderthal man (H. sapiens neanderthalensis).
      • Thus, it is regarded as intermediate between pithecanthropines and Neanderthals.

True Man Including the Living Modern Man

• Neanderthal man (Homo sapiens neanderthalensis) discovery
  • The fossils of Neanderthal man were first obtained from Neanderthal Valley in Germany in the late Pleistocene epoch by C. Fuhlrott (1856).
  • Later, many other fossils were excavated in various countries by different palaeontologists.

• • Characteristics
    • He had slightly prognathous face.
    • Neanderthal man walked upright, as we do, and had low brows, receding jaw, and high domed head.
    • If there was anything truly different about him, it was that he was much stockier than we are.
    • Cranial capacity was 1300-1600 cc.
    • Neanderthal man existed half-a-million years ago, but was most numerous from about 100,000 years ago.
    • He became extinct 30,000 years ago.
    • Neanderthal man was the legendary cave dweller, having heavy brow ridge and humped back.
    • He was adapted to a cold environment and encountered a succession of glaciers that passed over most of the northern temperate regions of the world.
    • He was not only a skilled hunter but also a true predator-a specialization that did not happen among hominids before or after him.
    • Neanderthal man was cannibal and fashioned the skin into clothing to protect himself against the harsh climate.
    • Natural caves became camp-sites that were illuminated and heated by fire.
    • It is believed that he buried his dead with flowers and tools. He may have had a religion.
    • It is usually considered that Homo sapiens neanderthalensis did not evolve into Homo sapiens.
• Cro-Magnon man (Homo sapiens fossilis) discovery
  • He has been known as Cro-Magnon man because his fossils were first discovered in 1868 from the Cro-Magnon rocks of France by MacGregor.
  • Cro-Magnon man emerged about 34,000 years ago in Holocene epoch.
  • Thus, he is regarded as the most recent ancestor of today’s man.
    Characteristics
    • The Cro-Magnon man had, like us, about 1.8 m tall, well-built body.
    • His face was perfectly orthognathous with a narrow, elevated nose; broad and arched forehead; moderate brow ridges; strong jaws with man-like dentition; and a well-developed chin.
    • His cranial capacity was, however, somewhat more than ours, being about 1650 cc.
    • It is, therefore, believed that the Cro-Magnon man was somewhat more intelligent and cultured than the man of today.
    • He could walk and run faster and lived with families in caves.
    • He made excellent tools and even orna- ments-not only of stones and bones, but also of elephant tusks.
    • His tools included spears, bows, and arrows, as he was omnivorous.
    • The use of skin clothes by this man is also confirmed.
    • A number of cave paintings done by the Cro- Magnon man have been discovered.
    • He was the direct ancestor of the living modern man.
    • Prehistoric cave art developed about 18,000 years ago.
• Living modern man (Homo sapiens sapiens) discovery
  • Further evolution of man after the Cro-Magnon man involves the evolution of culture rather than that of anatomy.
  • Homo sapiens sapiens appeared about 25,000 years ago in Holocene epoch and started spreading all over the world about 10,000 years ago.
  • Agriculture came around 10,000 years ago and human settlements started.

Modern Humans

Homo sapiens

• The evolutionary journey to modern humans ends with the appearance, about five hundred thousand years ago, of Homo sapiens (wise man), i.e., our own species.
• We are newcomers to the human family-H. sapiens has not been around nearly as long as H. erectus was.
• Still humans have changed quite a bit since those early days.

Homology in Chromosomes of Man and Great Apes

• The somatic cells of humans contain 46 chromosomes (44 autosomes and 2 sex-chromosomes).
• Human chromosomes are usually obtained by cultur- ing certain types of white blood cells from the peripheral blood.
• They can then be treated with specific stains to produce characteristic bands along the length of each chromosome.
• The pattern of banding so obtained is unique for each pair of chromosomes.
• Banding techniques enable the identification of individual chromosomes and their parts.
• The diploid number of chromosomes in gorilla, chimpanzee, and orangutan is 48.
• Comparisons have been made between banded chromosomes of man and those of the great apes.
• The total amounts of DNA in human diploid cells and great apes are not dissimilar.
• But what is most interesting from an evolutionary viewpoint is that the banding pattern of individual human chromosomes is very similar and, in some in- stances, identical to the banding pattern of apparently homologous chromosomes in the great apes.
• Diagrammatic representations of the banding pattern of human chromosome numbers 3 and 6 are compared with those of particular autosomes in chimpanzee.
• This remarkable similarity in the fine structural organization of chromosomes is understandable only in terms of a common origin of man and chimpanzee.

Some Important Points

• Mars has CO2 and water vapors and is supposed to have life. CO2 is present in traces. It has no green-house effect. Hence, it is very cold and does not support life. Mercury and moon do not have any sign of life due to the absence of water vapors. This extra terrestrial origin of life was visualized by Hoyle and Wickramasinghe.
• Darwin used the term “warm little pond” for early hot sea, rich in biomolecules. This primitive sea was alkaline.
• K. Bahadur exposed ammonia, formaldehyde, and ferrous chloride to strong sunlight and obtained a mixture of amino acids called Jivam.
• Variation in behavior: Cicada insect has a life span of 17 years. It emerges from soil, remains alive for 5 weeks, and then dies after mating and laying eggs. Dolphins can imitate and laugh. Bat can detect small insects of size (0.0a mm). Male Baya (weaver bird) of India builds its elaborate nest and decorates it with colorful petals to attract female Baya.
• Multiformity among organisms: Internal differentiation increases with the progress in evolution. Human beings are one of the most recently evolved animals. They show the following features:
  • The total length of blood vessels in our body is 96,000 km.
  • The fastest nerve impulse travels at the rate of 532 km/h.
  • The internal area of our lungs is 93-100 m2 which is 40 times the external surface area of our body.
  • Human brain has 10,000 million nerve cells.
  • We have more body hair than apes but shorter and softer.
  • O, disappears from the atmosphere at 16 km height.
  • We remain blind for 30 min/day by blinking our eyes.
  • Bones are as strong as concrete and as hard as granite but far lighter than both.
    We retain only 18% of what we learnt yesterday.
• Synapsid reptiles were mammal-like reptiles that gave rise to mammals. They had a single temporal fossa on the lateral side of skull and heterodont teeth. They originated in Permian period. They are extinct.
• In 1858, Dr. P.L. Sclater divided, for the first time, the earth into six regions (realms) according to the distribution of birds. Later on, Alfred Russel Wallace (1876) classified the earth into six regions (realms) for all ani- mals and plants.
• In all animals, early development is similar, i.e., passing through morula → blastula→ gastrula stages, showing their common origin.
• Early embryos of all vertebrates show basic similarity in having somites, tail, gill clefts, notochord, etc. These traits can be explained as the characters of evolution.
• Any vertebrate organ also passes through different stages during development. For example, mammalian heart is initially two-chambered, then it becomes three-chambered, and then becomes four-chambered. The development of all triploblastic animals starts from zygote and undergoes similar changes to form gastrula having three primary germ layers (ectoderm, mesoderm, and endoderm) which have same fate in organogenesis. Early embryos of different vertebrates resemble in possessing similar structures such as gill slits, notochord, and tail. Not only this, but in the course of development, at different stages, an embryo looks like the embryo of different phyla forms which the given organism has evolved.
• It can be explained on the basis of recapitulation theory (von Baer)/biogenetic law (Haeckel) which states that ontogeny (developmental history of an individual) repeats phylogeny (developmental history of race).
• Types of fossils
  • Macrofossils: These are larger than 1 cm in size.
  • Unusual fossils: These form by sudden preservation of entire organism, e.g., Solnhofen limestone quarry of Southern Germany containing fossils of Archaeopteryx.
  • Gastroliths: These are found in abundance in the body cavities of certain reptiles.
  • Molds and casts: The material surrounding the fossil hardens and preserves the outer details. The actual bodies disintegrate and are removed by slippage of the ground leaving hardened cavities called molds. When molds are filled with natu- ral deposits, they are called casts, e.g., fossils of Pompeii city buried in the volcanic ash of Mount Vesuvius in 79 AD.
• Preservation in ice: In the woolly mammoths from Siberia, the flesh is so well preserved that it can be fed to dogs. It was discovered from Lena Delta in 1790 and Siberia in 1901.
• Fossils in petroleum springs and asphalts: These were found in Rancho La Brea now in Los Angeles.
• Fossils in resins and ambers: Fossil flies in amber from the Baltic forests of Europe during Oligocene period.
• The process of fossilization to preserve finer details is known as histometabasis.
• Mummies: The bodies of dead animals or plants become dehydrated in deserts and are preserved as mummies.
• T. Dobzhansky wrote the book “Genetics and Origin of species.”
• Darlington wrote the book “The Evolution of Genetic Systems.”
• Darwin wrote “Descent of Man and Selection in Relation of Sex” in which he put forward his theory of evolution of man from ape-like ancestors.
• Law of superposition: The lower stratum of geological formation was the first to be deposited and is the oldest.
• Willston’s rule: During the evolution of lineage, serially homolog parts tend to reduce in number but get more and more differentiated, e.g., prawn’s leg.
• Allometry: The study of differential growth rate was called allometry.
• Missing links: Fossils that act as transition between two present-day groups of organisms are called missing links. For example, Archaeopteryx-a fossil of crow-sized toothed bird-acts as a link between reptiles and birds.
• Empedocales (493-435 BC) is regarded as the father of the concept of evolution.
• Seymouria (extinct reptile) is a connecting link between Amphibia and Reptilia.
• Lycaenops (extinct reptile) is a connecting link between reptiles and mammals.
• Wallace’s line: In 1863, A.R. Wallace drew an imaginary dividing line on the map between the Oriental and Australian realms (regions). This line is known as Wallace’s line.
• Sibling species: Species that morphologically look similar but are reproductively isolated are called sibling species.
• Living fossils: A living fossil is a living animal of ancient origin with many primitive characters. A living fossil has been living as such from the time of origin without many changes.
• Eugenics: It is the branch of science that deals with the improvement of human race genetically. It can also be divided into two types: Negative eugenics and positive eugenics. Under negative eugenics, people with inferior and undesirable (dysgenic) traits are prevented from reproducing.

• Homo sapiens or modern man is a member of order Primate, sub-order Anthropoidea. Primates are supposed to have evolved from primitive, tiny, insect-eating quadruped, similar to modern tree shrews, which lived between 75-60 mya during Eocene period. These belong to order Insectivora. Two evolutionary lines diverged leading to present-day prosimians (treeshrews, lemurs, lorises, and tarsier) and the Anthropoidea (including old-world and new-world monkey, ape, and man).
• The first (ape+man) ancestor originated in Oligocene period 30-35 mya under the name Propliopithecus. (Its fossils were found in the Fayum deposits of Egypt.) It is represented by fossil jaws and teeth. Aegyptopithecus is contemporary of Propliopithecus (Kahira).
• Dryopithecus: The Oligocene ancestor gave rise to the Miocene group of (apes + man) called Dryopithecus (formerly known as Proconsul). It lived in Africa and Asia. It had semi-erect posture with hindlimbs and forelimbs of the same size. Hands and skull were monkey like, forehead was human-like, and jaws and dentition were ape-like. Sivapithecus, discovered by ChopraSimon team, is another fossil ape from the Shivalik Hills in India (derived from Dryopithecian stock).
• An aberrant branch from Oreopithecus evolved in late Miocene early Pliocene, Oreopithecus, which later on became extinct. Ramapithecus (Kenyapithecus) originated 1415 mya; it had a few teeth and fragments of jaw. It is believed to have evolved from Dryopithecus in Shivalik Hills in India during late Miocene and early Pliocene.
• Grimaldi: From the caves in village Grimaldi on the Mediterranean coast, cranial capacity 1655 cc is believed to have given rise to Negroid stock.
• Chancelade: In rock shelter near Chancelade in Dordogne France, cranial capacity 1450 cc gave rise to modern Eskimos.
• Modern man: H. sapiens sapiens evolved about 25,000 years ago but spread to various parts of the world about 10,000-11,000 years ago. There is thinning of skull bones, slight reduction in cranial capac- ity (1400-1450 cm3), four flexors in vertebral column, and slight rising of skull cap. Modern man underwent cultural evolution:
  • Paleolithic (age of tools of stones and bones and cave paintings),
  • Mesolithic (age of animal husbandry, development of language, reading, and writing),
  • Neolithic (development of agriculture, manufacture of pottery, and clothes),
  • Bronze Age, and
  • Iron Age.
• Forgery of Piltdown man (Eoanthropus dawsoni): Charles Dawson in 1921 reconstructed the skull from the cranium of modern man and the lower jaw of ape. The fossil skull is known as Piltdown, after the English hamlet where it was found.
• Cranial capacities

• Transitional forms connecting Home erectus with Homo sapiens have been uncovered from Europe. These are Steinheim skull (Germany), Swanscombe skull (second interglacial period), Fontechvade skulls (France, third interglacial period), and Ehringsdorf skull. All of these are called early H. sapiens. The course of evolution of man started in Africa.

 

Assertion – Reasoning Questions

In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R).

1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).
2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
3. If Assertion is true but Reason is false, then mark (3).
4. If both Assertion and Reason are false, then mark (4).

Question 1. Assertion: Interspecific hybrids are usually sterile.

Reason: Interspecific hybrids receive chromosomes from two different species.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 2. Assertion: Magnolias, tulips, and Sassafras are found in Eastern United States and Eastern China only.

Reason: These are examples of restricted distribution.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 3. Assertion: Cretaceous period is called age of dinosaurs.

Reason: Fishes originated in Devonian period.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 4. Assertion: Theory of special creation attributes the origin of life to a vitalistic event.

Reason: According to this theory, the God is creator of life.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 5. Assertion: Both mule and hinny are sterile.

Reason: These are the examples of hybrid sterility.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 6. Assertion: The earliest organisms were anaerobes, having arisen in a sea of organic molecules, and were chemoheterotrophs.

Reason: Before the supply of organic molecules exhausted, some of the heterotrophs might have evolved into autotrophs.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 7. Assertion: There are chances of breakdown of isolating mechanism in allopatric speciation.

Reason: Allopatric speciation is rapid process of speciation.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 8. Assertion: Balanced polymorphism is directly related with directional selection.

Reason: Directional selection favors the maximum dominacy of characters.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 9. Assertion: Artificial selection is highly beneficial for humans.

Reason: Artificial selection is carried out by man.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 10. Assertion: Batesian mimicry is a form of mimicry in which an edible species resembles an inedible one.

Reason: Batesian mimicry is a form of protective mimicry.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 11. Assertion: There is no life on moon.

Reason: Water is absent on moon.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 12. Assertion: The first living organisms on earth were autotrophs.

Reason: They were capable of performing chemosynthesis.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 13. Assertion: Base analogs induced transition.

Reason: Base analogs perform forbidden pairing.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 14. Assertion: Sympatric species are geographically isolated.

Reason: Sympatric species are reproductively isolated.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 15. Assertion: Somatic mutations are sometimes inheritable.

Reason: Some organisms show vegetative propagation.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 16. Assertion: Plants having odd number of sets of chromosomes are fertile.

Reason: Plants having even number of sets of chromosomes are sterile.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 17. Assertion: Colchicine induces polyploidy.

Reason: Colchicine causes disjunction of chromosomes.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 18. Assertion: Change in structure of chromosome is called chromosomal aberration.

Reason: Substitution is an example of chromosomal aberration.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 19. Assertion: The first life originated in water.

Reason: Conditions were favorable for the origin of life in water.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 20. Assertion: A single mutation may produce a new species.

Reason: Mutation may cause major variation in genetic material and these are inheritable.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 21. Assertion: Evolution is not occurring at present.

Reason: Evolution takes a very long time to occur.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 22. Assertion: Analogous organs show common ancestry.

Reason: Analogous organs show evolution.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 23. Assertion: Lung fish is a connective link between fishes and amphibia.

Reason: Lung fishes show characters of both fishes and amphibia.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 24. Assertion: Bird’s embryo shows tooth buds for some time.

Reason: Ontogeny repeats phylogeny.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 25. Assertion: Liger in a hybrid animal.

Reason: Liger is fertile.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).