CBSE Class 12 Biology Revision Notes Chapter 7

Class 12 Biology Chapter 7 Notes

Class 12 Biology Chapter 7 explains Evolution. Evolution in Biology is the historical study of living forms on Earth. Along with humans, dramatic changes in flora and fauna are also a part of evolution.

In preparing Biology Chapter 7 Class 12 Notes, the CBSE stated norms and regulations are followed, which cover all topics and provide students with a thorough explanation.Class 12 Biology Students can easily study, revise, and lessen their exam load with the help of the Class 12 Biology Chapter 7 Notes. These notes were written by the academic staff at Extramarks using information from the NCERT textbook and other relevant sources.

Students may register on Extramarks to access the Class 12 Biology Notes Chapter 7 and CBSE Class 12 Syllabus. Along with the Syllabus, students may also refer to CBSE Revision Notes, CBSE Sample Papers, CBSE Important Questions, CBSE Extra Questions, and CBSE past years’ question papers.

Key Topics Covered In Class 12 Biology Chapter 7 Notes

Following are the topics included in Chapter 7 Biology Class 12 Notes:

• Origin of Life
• Evolution of Life Forms – A Theory
• What is the Evidence for Evolution?
• What is Adaptive Radiation?
• Biological Evolution
• Mechanism of Evolution
• Hardy – Weinberg Principle
• A Brief Account of Evolution
• Origin and Evolution of Man

Introduction

The study of the history of life forms on earth is called evolutionary biology. Life is the inherent capacity of the organism to utilise  materials like light, water, gases, and food.

The essential requirements for the origin of life were the formation of the earth and the initial atmosphere.

The four conditions for the origin of life are

• The primitive atmosphere with little or no oxygen.
• Water and various organic and inorganic molecules are the right chemicals.
• Source of energy
• Infinite time

THEORIES OF THE ORIGIN OF LIFE:

Theory of special creation: There are three important postulates in the theory of special creation. They are

• All forms of life were created at the same time or at short intervals with no connection.These include microorganisms, fungi, algae, plants, and animals.
• They did not undergo any changes since their creation. They are created in the same form in which they exist in the present.
• Their bodies and organs have been specially designed to fully meet the needs of the environment in which they exist.

Theory of spontaneous generation:

It stated that life repeatedly originated from non-living materials in a spontaneous manner.

Theory of catastrophism:

It states that a catastrophe has preceded several creations due to geological disturbance.

Cosmozoic or interplanetary theory:

According to this theory, life reached Earth from other heavenly bodies in the form of resistant spores of  simple organisms in meteorites or spaceships. They found fertile soil and grew here, and then various life forms evolved.

Theory of eternity of life:

According to this theory, life exists at present and will continue to do so forever, changing only in form.

The origin of life can be explained as

• Free atoms came together as the earth cooled down in the atmosphere and formed inorganic molecules.
• Simple organic monomers were produced as a result of interactions between inorganic molecules.
• Further interaction gave rise to large, complex organic polymers, for example, proteins, lipids, polysaccharides, nucleic acids, and nucleotides.
• Microspheres were formed as a result of the aggregation of large organic polymers. This could absorb and release materials, divide, and grow.
• Activities were controlled by nucleic acids.
• Evolution of a lipid-protein membrane.
• Growth and division became regular and precise.
• Formation of progenitors, which is the first living cell.
• The first living cell was anaerobes and heterotrophs.
• Some primitive heterotrophs evolved into the first autotrophs.

THEORIES OF EVOLUTION:

There are four theories of evolution namely:

• Lamarck’s theory of inheritance of acquired characters or Lamarckism
• Darwin’s theory of natural selection or Darwinism
• De Vries mutation theory
• The modern concept of evolution.

Lamarck’s theory of inheritance of acquired characters or Lamarckism:

Extramarks Class 12 Biology Chapter 7 Notes: The theory had three factors: new needs; character acquisition, which includes innate tendency; use and disuse of organs and the effect of the environment; and character inheritance.

Lamarck’s theory was explained with examples in his writing:

• A giraffe is a mammal with a long neck and long forelimbs. These evolved from deer-like ancestors who had small necks and small forelimbs. These animals  lived in a place where leaves were the only source of food for them, and by making continuous efforts to reach the leaves, they stretched their neck and forelimbs. This was the reason behind the elongated neck and forelimbs. This increase  was transmitted from one generation to the next. This process continued, and after a number of generations, the present-day giraffe with a long neck and long forelimbs was produced.
• Aquatic birds like ducks and swans have arisen from terrestrial ancestors. The latter used to go into the water for food and protection, so they spread their toes widely and stretched their skin at the base so that they could rest on the water. So this leads to the development of the web between the toes.
• The lizard-like reptiles with fully developed skin were the ancestors of the snakes. These ancestors, out of fear of the larger and more powerful animals, started living in narrow crevices or holes. They continuously stretched their body to accommodate themselves in the narrow space, and their limbs were not used, so they became longer and cylindrical. The permanent disuse of the limbs led to their disappearance.
• Cave animals lost their eyes, as they were not used while they lived in the dark environment.
• The deer’s speed was developed through constant efforts to run to protect itself from enemies.

Neo-Lamarckism:

Neo-Lamarckism is the revival of Lamarckism in a modified form. It states that:

• Germ cells are not always immune to the effects of the environment.
• Germ cells may be affected directly by the environment without any effect on the somatic cells, like Tower’s potato beetles.
• Germ cells, like Harrison’s moths, can pass on acquired or somatic variation to their offspring.
• The organism is  “whole” and is made up of many organs. One organ may affect the functioning of another organ.
• The interaction between genes and the environment results in somatic traits.
• The gene expression is not affected by the environment.
• The acquired characters visualised by Lamarck do not exist.
• Lamarckism successfully explains the existence of vestigial organs in animals.

Darwin’s theory of natural selection:

Extramarks Class 12 Biology Chapter 7 Notes state that on July 1, 1858, the theory of natural selection was announced by Charles Robert Darwin and b Russel Wallace at the Linnean Society of London meeting. Other names for the theory include Darwin-Wallace theory and Darwinism.Darwinism was published in the book “The Origin of Species using Natural Selection” in 1859.

There are factors on which the theory of natural selection is based. They are:

• Rapid multiplication
• Food and space which are the limited environmental resources
• Struggle for existence
• Variation
• Survival of the fittest
• Inheritance of the useful variations
• Formation of the new species.

Rapid multiplication and limited food and space lead to the struggle for existence. The space in the universe remains constant. There is a carrying capacity of the environment, and it does not allow the population to grow beyond the limit, so equilibrium is attained. So it is said that the population fluctuates around this equilibrium.

The struggle for existence and variation leads to the survival of the fittest. Natural selection and the inheritance of useful variations over many generations lead to the formation of new species.

Every individual needs space to live and food to eat. These are the primary necessities of life, and the competition for these needs is referred to as the struggle of existence.

There are three types of struggle:

• Intraspecific struggle
• Interspecific struggle
• Environmental struggle.

The intraspecific struggle is the keenest form, and it occurs between individuals of the same species who share the same needs. Examples include war and Cannibalism.

The interspecific struggle occurs between the individuals of the different species.

Environmental struggle refers to the difficulty encountered by the animal with environmental changes like heat, cold, drought, flood, storm and light.

The evidence of Darwinism is shown by entomophily. Industrial melanism is an appropriate example of natural selection.

Some of the key points of Darwin’s theory are:

• Adaptation to the environment is the key to survival.
• A selective advantage is any variation that gives one individual an advantage over another.
• Individuals with unfavourable variations are rejected by nature and are said to have a selective disadvantage.
• The principal cause of natural selection is the environment. Some examples of natural selection are the resistance of mosquitos to DDT and sickle cell anaemia.
• The Lederberg replica plating technique supported Darwinism. This technique demonstrated that the adaptation results from a pre-existing mutant form of bacteria. The introduction of penicillin represents a change in the environment, and it gives a selective advantage to those bacteria that are already penicillin-resistant over those that are penicillin-sensitive.
• Darwin’s theory could not recognise the source of variations.

The drawbacks of Darwin’s theory are:

• Minute fluctuating variations were considered as principal factors.
• There was no differentiation between somatic and germline mutations.

The examples which go against Darwinism are:

• Inheritance of small variations
• Perpetuation of vestigial organs
• Over-specialisation of organ

Neo-Darwinism:

To remove the objections, as covered in the Class 12 Biology Chapter 7 Notes the original theory of natural selection was refined and called Neo-Darwinism.

Important features of Neo – Darwinism is:

• This is the modification in light of genetic research.
• Various causes of variation are incorporated.
• Genetic variations, or mutations, are considered inheritable and the raw material for evolution.
• The population is the unit of evolution according to this concept.
• Differential reproduction leads to changes in gene frequency. This is natural selection.
• Reproductive isolation is an essential factor in speciation.
• Both mutation and natural selection are responsible for evolution.
• The members of each species are different. This is due to differences in genetic patterns and variations of environmental influences.

Some of the supplemental theories of Darwinism are

• Artificial selection
• Sexual selection
• Theory of pangenesis

By practising selective breeding, artificial selection is made for better crop varieties and domestic animals.

Secondary sexual characters do not play any significant role in the struggle for existence, and according to Darwin, their origin is beyond the purview of natural selection. His theory of sexual selection is based on assumptions. They are:

Males are more predominant than females in number, and there is competition for females among males. In this struggle, those with superior qualities are favoured, while those with inferior qualities are eliminated.

Darwin proposed the pangenesis theory to explain how characters’ transmission occurs from one generation to the next. According to this theory, every somatic cell produces gemmules, and the site for collecting these gemmules is the actual germ cells coming from the different somatic cells. This theory was completely discarded.

Objections to Darwinism:

• There is no mention of an explanation regarding the adaptability of some of the organs in their initial stages, which are used only when they are completely developed. For example, protective colouration, mimicry, and the electric organs of the fish.
• There is no explanation for vestigial organs, over-specialisation such as antlers in Irish deer and mammoth tusks, degeneration, and regeneration of specific organs.
• There is no difference mentioned between heritable and non-heritable variations.
• Mutations were referred to as “sports of nature” during the evolution process.
• The theory of natural selection does not explain the sterility of hybrids and the occurrence of neutral flowers in plants.
• Several transitional forms in the evolution of species by natural selection are not explained. These include some small and useful variations.

De Vries mutation theory:

In 1901, the mutation theory was put forth by Hugo de Vries. According to him, mutations are the raw materials for evolution. Natural selection works on mutations. It preserves the mutants with useful mutations and eliminates those with harmful mutations. The mutants with useless mutations coexist with the parent species.

The statement of the theory could be explained as

In a single generation, new species emerge from pre-existing species due to the sudden appearance of markedly discontinuous, inheritable genetic variations known as mutations.There is a transition from one species to another.There are four types of plants:

• Being progressive entails acquiring new characteristics.
• Retrogressive means certain traits are lost or reduced.
• Degressive plants are those that have a low survival rate.
• Plants that resembled their parents but produced variants at times are said to be inconstant.

Some of the important features of mutation theory are:

• Mutations are the raw material for evolution.
• Sudden appearance.
• Immediate effects
• Mutants differ from their parents, but there are no intermediate stages between the two.
• In several individuals of a species, the same mutations can appear.
• Mutations are recurrent and appear in all directions.
• They are inheritable and have a genetic basis.
• It is a discontinuous and jerky process.
• Beneficial mutations are selected by nature.
• Lethal mutations are rejected by nature.

The evidence for the theory is the Ancon sheep, a short-legged variety of sheep that an ordinary sheep produced in a single generation in 1891. This mutation benefited the farmers, as these sheep could not jump over the low stone fences. Another example could be Oenothera Lamarckian, which has 14 chromosomes, and the mutants produced from it have 16,20,22,24,28, and 30 chromosomes. This shows that the mutants are independent species.

The present or modern concept of evolution:

The modern concept of evolution synthesises Darwin’s and De Vries’s theories. G.L.Stebbin explained the synthetic theory in the book “Process of Organic Evolution.” The modern concept of various factors include genetic variation in the population, natural selection, speciation, and reproductive isolation.

Genetic variations of the population:

• Migration: Breeding immigrants with the host population adds new alleles to the gene pool of the host population. This addition of alleles is called gene migration.
• Nonrandom mating: The frequency of bright colours in the next generation is increased when the female selects a more brightly coloured male.
• Genetic drift refers to the chance elimination of the genes of certain traits when a population group migrates or dies of natural calamity. It alters the gene frequency, eliminates alleles, and fixes other alleles. This helps reduce the genetic variability of the population.

Founder effect: It occurs when founders, a small group of people, leave their homes to find a new settlement. The founder effect is the formation of a different genotype in a new settlement.

Bottleneck effect: Earthquakes, floods, etc., are natural calamities that reduce the size of the population and randomly kill individuals. The genetic pool of the small surviving population is not representative of the genetic pool of the original population. This is a condition where genetic variability is reduced, called the bottleneck effect.

• Mutation: Mutation alters the base sequence in a gene and is the major source of genetic variation. The mutation includes gene mutation, chromosomal aberration, gene recombination and hybridisation.

In hybridisation, the genes of two populations are mixed.

The phenotypes produced have specific advantages for survival.

Artificial Selection:

The process of inbreeding the desired organisms by man is called artificial selection.

The importance of artificial selection is:

• Animal breeders can produce improved varieties of animals like dogs, cows, goats, sheep, etc.
• The varieties of useful plants can be improved, like wheat, rice, sugarcane, cotton, pulses, vegetables, fruits, etc.
• Many crop plant species, like cabbage, broccoli, kale, cauliflower, etc., have been produced from a common wild mustard species by selective breeding.

Comparison of artificial selection with natural selection:

Artificial selection closely resembles natural selection, except that

• The role of nature is played by a man.
• The traits selected are the area of interest of the humans.
• In a shorter period, many important results can be achieved.

Mutation as the cause of adaptation:

• Joshua Lederberg and Esther Lederberg demonstrated the genetic basis of adaptation in bacteria with the help of an experiment called the Lederberg replica plating experiment.
• The experiment clearly showed that the basis of the adaptation was the pre-existing gene mutation, which led to the selection of the mutant bacteria by nature.

Natural selection:

Natural selection is a creative force that spreads genetic novelty. This is the modern view of evolution. There are three types of natural selection. They are stabilising selection, directional selection, and disruptive selection.

In stabilising selection, the average phenotype is the most adaptable and is preferred over extreme phenotypes in either direction.

In directional selection, the extreme phenotype is favoured, and the distribution curve shifts in that direction. The examples include industrial melanism, the evolution of the modern horse Equus, and pesticide resistance in mosquitoes.

When disruptive selection results in many new species, it is called adaptive radiation. This kind of selection is rare.

Reproductive isolation:

It preserves the integrity of a species by checking hybridisation. The main significance of reproductive isolation is that it is essential for forming new species. There are two types of reproductive isolation. They are prezygotic or premating and postmating or postzygotic.

The types of premating isolation mechanisms are:

1. Geographical isolation: sea, mountains, deserts, and rivers are the physical barriers.
2. Genetic isolation refers to interspecific sterility.
3. Ecological isolation disallows interbreeding.
4. Temporal isolation means two species fail to interbreed because of seasonal variation.
5. Behaviour isolation means differences in courtship and mating behaviour.
6. Physiological isolation means functional incompatibility in mating.
7. Mechanical isolation means differences in the size and structure of genitalia.
8. Many animals shed their gametes in water for external fertilisation, which is referred to as gametic.

Postmating isolation mechanism: these are operable after mating and have the following types:

1. Incompatibility
2. Hybrid inviability
3. Hybrid sterility
4. Hybrid breakdown

Difference between natural and artificial selection:

In artificial selection, the process is artificial, it is conducted by men on a limited scale in specific labs, and the traits selected for improvement are beneficial to humans. In a short period of time, the results are achieved.

Natural selection is a natural phenomenon, and it is conducted on a vast scale all over the world. The traits that are selected for evolution are beneficial to the species. Results are achieved over a longer period of time.

The examples of natural selection can be elaborated as follows:

• Industrial melanism has been noted in about 70 species of moths in England and in about 100 species in the Pittsburgh region of the USA. Many other European countries also exhibit industrial melanism.
• Dairymen give antibiotics to the cattle in their feed. This makes the cattle fatten faster. But cattle have become the breeding ground for antibiotic-resistant bacteria, so many countries have banned the use of antibiotics in cattle feed. This should be followed by others as well.

Hardy-Weinberg equilibrium law:

G.H. Hardy, an English mathematician and W.Weinberg, a German physician, independently developed a method to relate  evolutionary changes and population genetics in 1908. This is known as the Hardy-Weinberg law.

According to this law, the relative frequency of alleles in the population of sexually reproducing organisms remains constant from generation to generation as long as the:

• The population is large, and the changes in allele frequencies due to chance or accident are insignificant.
• Random mating
• Mutation does not occur; if it appears, it must reach a state of equilibrium.
• The members of the population survive, and their reproductive rates are equal.
• This law explains why a population as a whole, with all its variations and genotype frequencies, remains unchanged for many generations.

Speciation:

It is defined as the process of formation of one or more new species from an existing one by evolutionary means. This is of three types: multiplicative speciation, phyletic speciation, and speciation by fusion.

The method of speciation:

The members of a species seldom live together as a single large population. Demes are the small ,interbreeding populations in which a species exists.

Types of speciation:

There are three types of speciation. They are multiplicative, instantaneous, and speciation occurs through fusion.

• Multiplicative speciation increases the number of species and is the most common mode of speciation. It is further of two types, gradual speciation and instant speciation.

Gradual speciation occurs within populations and spans generations.This can be allopatric or sympatric speciation. Allopatric speciation takes place with geographical speciation, and there is a subdivision of the original population with the formation of the barriers such as the mountain, land bridges, oceans, etc. There is no involvement of geographical location in sympatric speciation.The new species are formed within a single population, and it may result from Polyploidy.

• In instant speciation, it occurs through individuals and in a single generation by hybridisation involving allopolyploidy.
• In Phyletic speciation, there is a replacement of one species by another without increasing the number of the species. Eohippus evolved into Mesohippus, and itself became extinct.
• In speciation by fusion, there is the merging of two different species into one new species by the breakdown of previously existing reproductive isolation, which leads to a reduction in the total number of species.

HYBRIDISATION AND POLYPLOIDY – INSTANT SPECIATION:

• Hybridisation combines the best traits of both parent species into the new species.
• The latter has better chances of survival when compared to the parents.
• If the new species combines the worst characteristics of both parents, then that is its biggest disadvantage, and the species is unlikely to survive.
• Unlike chromosomes cannot pair in meiosis, and the gametes do not receive the full chromosome complement, one of each kind.
• If in an interspecific hybrid, the number of chromosomes becomes double by a form of chromosome mutation called allopolyploidy, then meiosis can occur, and the normal gametes are formed.
• Raphanobrassica was created by crossing two 18-chromosome plants, radish and cabbage
• Species of higher plants have chromosome numbers that are multiples of the same basic number.
• In wheat, there are species with 14, 28, and 42 chromosomes.
• The species of rose have 14, 28, 42, and 56 chromosomes.
• The most important role played in the evolution of plants is polyploidy.
• Polyploidy is not common in animals.

Species concept:

The term “species” refers to “form or kind.”There is a difference between a cow and a buffalo. The father of human taxonomy is Linnaeus, who described individual species in terms of their physical form.

The various concepts of species are stated as below in our Class 12 Biology Chapter 7 Notes

• Morphological species concept: This was given by Davis and Heywood.Each individual of the species differs in morphological characteristics. Morphospecies, or morphological species, are those that are formed based on morphology.
• Biological species concept: In 1942, Ernest Mayer advanced the concept. It includes species of a population where the members interbreed freely among themselves and produce viable and fertile young ones. They cannot reproduce with the members of the other species. Members of the same species are reproductively compatible but, at the same time, reproductively isolated from the other species. Biospecies or biological species are recognised based on their reproductive potential. Sibling species are those that are morphologically identical. Drosophilia pseudoobscura and Drosophilia persimilis are similar species but cannot interbreed as they are reproductively isolated. Polytypic species are united species; biologists sometimes combine morphologically and geographically separate species into a single species based on successful interbreeding. The various species of North American sparrows are geographical subspecies of song sparrow, Passarella melodia.
• Cohesive species concept: In 1989, Alan R. Templeton broadened the concept of biological species by including asexual organisms and downplaying interbreeding in organisms that reproduce `sexually. The cohesion concept mentions that some species produce fertile hybrid offspring by interbreeding. The hybrids may sometimes mate successfully with one of the parent species. Panthera leo and Panthera tigris produce fertile young ones. Mare and mareess produce a sterile mule. The female mule produces young ones with the male horse.
• Ecological species concept: The main focus of this concept is the position and function of species in the environment. For example, two populations of animals that look identical would qualify as two ecological species if each inhabits a particular type of environment, such as a freshwater pond that has a unique set of chemical, physical, and biological conditions.
• Evolutionary species concept: Species are made up of populations of similar individuals who differ from other species and breed among themselves and rarely with the other species. They are reasonably stable. They tend to change throughout many generations due to flexibility and plasticity. The concept refers to the character of phenotype, breeding behaviour and the evolutionary unit of a group or organism’s status.

The lumpers and the splitters are the two groups of evolutionary biologists. The lumpers tend to group populations or smaller taxa into a single species or a larger taxon. The splitters tend to recognise the minute differences and, on that basis, divide the same population or a large taxon into separate species or small taxa.

The other name for evolutionary species is the chronospecies. The concept was suggested in 1961 by George Gaylord Simpson. The concept is related to the incorporation of the change in the organisms.

EVOLUTION:

Evolution is known as the formation of newer types of organisms from pre-existing ones through modification. Charles Darwin was the first to use the term “organic evolution”, which means descent with modification.

This states that the present-day complex forms of plants and animals have arisen from earlier simpler forms through gradual changes.

The relationship among organisms can be explained as follows:

• Living material: Protoplasm exists as units called cells.
• Spontaneous movements as the force for them arises within the bodies.
• Uptake of matter like food, oxygen, water, and salt
• Uptake of energy.
• Metabolism: All organisms use matter to build up and maintain their bodies, and this energy is used to operate the processes of life.
• Growth
• Respiration
• Release of waste products. This process is called excretion
• All organisms respond to external and internal stimuli. This is called irritability.
• Coordination of parts
• All organisms reproduce. It involves DNA replication, cell division, the transmission of hereditary information, and the expression of genetic information.
• Individuality
• Struggle for existence as there is competition for food, space, mate and safety.
• Homeostasis
• Healing and regeneration
• Heredity, evolution and adaptation.
• Organised life cycle
• Differentiation
• Proper environment
• Form and size.

Organic evolution:

The meaning of the term evolution is to unroll or unfold. Inorganic evolution means changes in elements with time. Biological or organic evolution is a change in living organisms over time.

Organic evolution is one of the key concepts in biology. In 1973, Theodosius Dobzhansky remarked that nothing in biology makes sense except in the light of evolution.

Morphological and anatomical pieces of evidence of organic evolution:

• Body organisation:

All plants and animals are made up of cells that aggregate to form tissues. The tissue gathers to form organs, and the organs cooperate to form the organ systems. In sponges, the cells function independently, and there is no tissue organisation, so this gives a cellular level of organisation. In coelenterates, the cells  aggregate to form tissues but do not form organs, so there is tissue-level organisation. From Platyhelminthes onward, there is an organ system level of organisation.

• Gradual modification: The heart shows gradual modification, the actual mechanism of organic evolution. The heart is two-chambered in fish, three-chambered in amphibians, incompletely four-chambered in reptiles, and completely four-chambered in crocodiles, birds, and mammals.
• Homologous organs have similarities in embryonic origin and construction but differ in their external forms and functions. They are found in forms showing adaptive radiation from a common ancestor, so these provide evidence of divergent evolution. Examples of homologous organs are vertebra forelimbs, insect legs, insect mouth parts, vertebrae epidermal derivatives, tails and vertebral column.

Thorns of Bougainvillea and tendrils of Passiflora are also examples of homologous organs. They have different looks and help the plant climb differently, but they both arise in the axillary position and are  modified branches.

• Analogous organs

These are those organs that have the same function and are superficially alike. The main difference lies in the fundamental structure and the embryonic origin.

The analogy is based on the convergent evolution of the parallel evolutions in the separate lineages.Examples of analogous organs are insect and bird wings, fins and flippers, cephalopod, and vertebrate eyes, leaves and cladodes and tuberous roots and potatoes.

• Connecting links

Connecting links means those living organisms that possess characteristics of two different groups of organisms.

Connecting links are the living, intermediate forms. The various examples of connecting links are mentioned in

• Vestigial organs

They are degenerate and useless structures that were once large and functional in other animals.Some of the features of these organs are their small size, rudimentary nature, and non functionality. Some examples are the vermiform appendix, coccyx, plica semilunaris, pinna muscles, and wisdom teeth in men. Other examples are the rudiments of hindlimbs in pythons and boas, pelvic girdles in whales, splint bones of horses, wings of Kiwis, eyes, and cave animals.

• Atavism

Atavism is also referred to as “reversion.”It is the unexpected reappearance of ancestral characteristics in an organism that would not normally occur.

Examples of atavism are short tails in babies, the power of moving pinna in certain cases, large canines, cervical fistula, tail, hairs on the body and the face, homodont dentition in cetaceans, etc. These examples show that there is a tendency among animals to develop ancestral characteristics.

The principle of atavism is known as Dollo’s law and was proposed by L. Dollo. According to this law, living organisms do exhibit evolutionary irreversibility. There are three types of atavism. They are family atavism, race atavism, and teratological atavism.

Family atavism includes the transmission of individual characteristics or characteristics within the members of a family in latent condition for several generations and their sudden reappearance in the offspring after several generations. This is controlled at the level of genes, and Mendelian inheritance laws explain it.

Atavism is a term used in racial studies to describe the reversal of one or more characters of one race appearing in individuals of another race.For example, the profuse growth of hair on the body and face of an Irish dogman, etc.

Atavism in teratology means the appearance of those abnormal characters in a race that were normal in other, supposedly ancestral races. The presence of a cervical fistula in a man corresponds to the gill slits, an example of atavism in teratology.

Embryological evidence of organic evolution:

1. All animals start life as a zygote. A blastula is formed on the cleavage of the zygote, which changes into a gastrula. Three primary germ layers are formed: ectoderm, mesoderm, and endoderm. The entire animal develops from these three layers. These germ layers produce the same set of organs in all animals. The similarity in early development and the same fate of all three germ layers are significant. This suggests that animals have evolved from common ancestors.
2. The early embryos of all vertebrates resemble each other in shape and structure.
3. Recapitulation theory: In 1828, Von Baer gave Von Baer’s law which was called biogenetic law by Ernst Haeckel in 1866.

The concept can be summarised as “ontogeny repeats phylogeny.”Ontogeny is the life history of an individual. Phylogeny is the evolutionary history of an animal race.The organism repeats its ancestral history during its development.

Proteonema, which appears early in the development of moss, and a fern gametophyte that resembles filamentous green algae in structure, physiology, and growth pattern are two examples.

The seedlings of the Acacia tree first develop simple leaves but later produce compound leaves. This shows that these evolved from plants with simple leaves.

1. Temporary nonfunctional embryonic structures:

An interesting case of this evidence can be explained as the bird embryo develops tooth buds that disappear before hatching. No food is taken through the mouth by the embryo. They live on the fluid yolk, and their teeth are of no use to them.

The birds have evolved through the modification of the toothed reptiles. They lost teeth in the course of modification, and ancestral characteristics like the presence of teeth are only present for a short time.

Palaeontological evidence of organic evolution:

Palaeontology means the study of past life based on the fossil record. The organic remains like bones, hairs, nails, claws, and shells of the organisms found in the rocks are called fossils.

Italian Leonardo da Vinci is the father of palaeontology.

Cuvier is the founder of modern palaeontology. Fossils are the remains of extinct organisms buried and preserved by natural processes.

Charles Darwin was the first person to show that fossils provide direct evidence for organic evolution.

1. Formation of rocks:

Igneous rocks are formed by the solidification of the originally hot earth.

The action of rain, wind, heat, and cold breaks igneous rocks into smaller particles that form soil. Rain transports soil particles into streams and rivers, and then into lakes and oceans, where they settle.With due time, the layers deposited change into rocks, which are called sedimentary rocks.The deep layers of a sedimentary rock melt due to heat and pressure and then solidify again to form a new rock. This is called metamorphic rock. The sedimentary rock called limestone melts and solidifies to form a metamorphic rock called marble.

1. Formation of fossils:

In forming sedimentary rocks, the dead animals of the sea or large lakes and of the land are carried to the sea or large lakes by rivers, sink, and get buried in the rocks. The oxygen supply is cut off, and the condition prevails to prevent decay. These preserved animals are called fossils.

The problems in fossil formation are:

1. Dead animals may be destroyed and eaten up by the animals.
2. Many animals die on land and never reach the ocean for fossilisation.
3. The marine forms have a greater chance for preservation.
4. Only hard parts of animals are preserved in the process of fossil formation.
5. The fossils on the seafloor are not accessible for study.
6. The study of fossils is difficult and technical.

The age of rocks and fossils can be determined using radioactive uranium and carbon-14 dating techniques. The half-life of C14 is 5600 years.

Evidence of the evolution of major groups of plants:

1. Vascular plants have left a smaller number of fossils.
2. There are only a few major lineages that are clearly distinct from one another.
3. Psilopsida (primitive group) have sporangia at the terminal position.
4. The modern vascular group Lcopsida, e.g. Lycopodium and Selaginella, and sporangia, are at the base of the leaves.
5. In horsetails, the sporangia lie in whorls at the top of the plants.
6. Angiosperms appeared 140 million years ago.

Geological time scale:

It is a chronological order of evolutionary history based on the study of fossils.

Biogeographical evidence:

This is based on the following:

1. Discontinuous distribution results in mutated and slightly different but closely related species of the same genus, such as alligators, mangolicas, Tulips sassafras, and so on.
2. The continent has peculiar flora and fauna, for example, there are pouched animals in Australia. This is a restricted distribution.
3. The animals and plants of oceanic islands resemble those of the mainland yet include distinct species. Darwin finches from Galapagos Island, for example.
4. Convergent or parallel evolution explains how natural selection can cause animals from different groups, such as marsupials, to adapt to similar habitats in order to survive.

Physiological and biochemical evidence:

1. Digestion, respiration, muscle contraction, and nerve impulse transmission are  similar  metabolic processes in all animals.
2. Similar enzymes exist in all animals, like trypsin, which is present in everything from sponges to humans.Hormones are present in all vertebrates and resemble each other chemically.
3. If the thyroid glands are removed from the tadpoles, they cannot turn into  frogs, so it can be concluded that the thyroid gland plays an important role in the metamorphosis of a tadpole into a frog. Such a larva starts metamorphosis if fed on the thyroid gland of aother animal.
4. The composition of blood and lymph is similar in all vertebrates.
5. Occurrence of biochemical homology and molecular homology.

Human evolution:

Introduction:

Humans developed from primates or ape-like ancestors. The book written by Charles Darwin called “Descent of Man and Selection About Sex” explains his idea related to man’s ancestry. The man originated in Central Asia, more precisely.

Branches in human evolution:

• The study of human evolution and culture is known as Anthropology, and it deals with the fossils of  prehistoric and living man.
• The sequential arrangement of stages in evolution is called Genealogy.
• The study of the human fossil record is called paleoanthropology.

Humans are close in phylogeny to the apes. The taxonomic position of the modern human is given in the table below.

Prehuman Evolution:

• Origin of mammals: About 195 million years ago, mammals evolved in the Jurassic period. The evolution occurred from the cynodont reptiles, which branched off in cotylosaurs (the stem reptiles). These early mammals were small, shrew-like, and insectivorous. They lead a nocturnal life in the trees to save themselves from huge reptiles. They survived and dominated due to their sensitivity to the changing environment. Their brains were more developed, allowing them to adapt to different situations.

• Origin of primates: The fossil record showing man’s evolution is incomplete. About 65 million years ago, the primates originated from shrew-like insectivores at the beginning of the tertiary period. These were long-tailed squirrel-like creatures. They gave rise to prosimians, meaning before monkeys and apes.
• Adaptation for arboreal life: The five major morphological changes that took place to adapt to life on trees Bipedal locomotion, grasping hands and feet, stereoscopic vision, a snout reduction, and brain enlargement are all examples.

• Simian evolution: Simians are monkeys, apes, and humans. The new world monkeys, old world monkeys, and tailless primates arose independently from the tarsioid stock. This took place in the Oligocene epoch, 36 million years ago.

Monkeys branched off early and developed the capacity for running, leaping on, and swinging from branch to branch. Their brain was enlarged, and it provided them with the ability to learn and perform precise movements.

The apes lived in the trees most of the time and were larger than the monkeys. They had difficulty balancing the branches, so they started swinging, a movement called brachiation. The arms and fingers were longer and stronger. The shoulder joints were very mobile, and the enlarged brain acquired a convulsion pattern similar to that of human brain.

Man’s ancestors left the trees, and the requirements for ground life were developed. Man became the most intelligent creature as the brain’s cerebral cortex grew. The brain was highly developed, leading to tool making, oral and written languages for communication, thinking about problems, and remembering incidents.

Asia and Africa were the sites of human evolution.

• Ramapithecus was ape-like and lived on tree tops but travelled on the ground for eating. Their jaw bones and teeth were like humans. They ate hard nuts and seeds like modern men. They could walk erect on their feet and went extinct about 7 million years ago.
• The first ape-man was Australopithecus.
• Australopithecus afarensis appeared in South Africa about 4 or 5 million years ago in the Pliocene. They had creatures that were both humans and apes. He was 1.05metres tall with a fully erect posture, an herbivore diet, and bipedallocomotion. There is no evidence to show whether they made tools or not. Canines and incisor teeth were small, like the modern man. About 2 million years ago, in the Pleistocene, Australopithecus afarensis gave rise to  Homo habilis.
• Australopithecus africanus appeared 2.5 million years ago in Africa. This ape-man had small canines, walked fully erect, was a carnivore, and was involved in the hunting of small game. They had larger jaws than modern men.

They gave rise to Australopithecus robustus and Australopithecus boisei.

• Homo habilis is the tool maker. He lived in Africa  about 2 million years ago. He had small canines with a light jaw, lived in caves, cared for young ones, was carnivorous, hunted large animals, and was more man-like. He first used tools made of chipped stones extensively.Homo habilis, which means “skillful man,” was given to him..
• Homo erectus was called the “Erect Man.” He evolved 1.7 million years ago in the Pleistocene and was tall with an erect posture. The tools used were more elaborate and made of stones and bones. He was skilled with fire and enjoyed large hunting games.The facial features was a flat skull, protruding jaws, brow ridges projecting, and small canines and large molar teeth.
• The first fossil of Homo neanderthalensis called Neanderthal man, was found in the Neander valley of Germany by C. Fuhlrott in 1856. They were named Homo sapiens neanderthalensis as they presented the primitive form of Homo sapiens. They were different from modern men as they had flat craniums, a sloping forehead, large, thin orbits, heavy brow ridges, protruding jaws, strong mandibles, and no chin. The Neanderthals built hut-like shelters, and they lived in caves. Their hunting skills were developed, and they used animal hides for clothing. They had many other qualities, like fire use, burying of the dead, customs, and social structure, and they were omnivores. They did not practise agriculture or animal domestication.

Modern Man:Homo sapiens fossils were the early or fossil modern man.MacGregor discovered their first fossil in 1868 in Cro-Magnon rocks in France. This is why they are also called Cro-Magnon man or Homo sapiens fossils. They appeared rapidly, indicating that they must have evolved elsewhere and migrated. They emerged around 34,000 years ago and were 1.8 metres tall with fewer hairs on their bodies. They had a large skull, strong jaws, closed teeth, wisdom teeth, a high forehead, a broad flat face, a narrow nose with elevation, deep set eyes, a prominent chin, and inconspicuous brow ridges. They lived with families, walked and ran erect, made sophisticated tools, were expert hunters, used fire, wore animal skin clothes, and made ivory ornaments.They were omnivores and did not practise agricultural or animal domestication. The living man’s direct ancestor is Cro-Magnon man.The living modern man is the Homo sapiens sapiens and appeared 25000 years ago. About 10,000 years ago, they began to spread all over the world. The morphological changes were minor and related to thinning of skull bones with a slight reduction in the cranial capacity and the development of 4 curves in the vertebral column. The brain power was superior, but the body power was inferior to the ancestors. The qualities lacking include an effective biting mechanism, a protective covering, poor smell and hearing. The eyesight is feeble. He became the dominant animal on the earth and started cultivating plants and domesticating animals. They are responsible for a unique cultural development in the animal kingdom.

Morphological changes involved in human evolution:

• Erect posture. The erect posture is associated with a shift of the foramen magnum from the posterior to the lower side.
• Facial changes like flattening.
• Bipedal gait on legs alone
• Changes in the skeletal and muscular system
• Changes in the brain include increased brain size, infolding of the cortex, memory, and intelligence.
• Broadened forehead.
• Brow ridge reduction.
• Stereoscopic vision increases the number of rods and cones for more acute vision.
• Reduced size of pinnae
• Nose elevation and narrowing. Reduced sense of smell.
• Chin formation
• Neck mobility
• Hands as grasping organs due to the opposability of the thumb
• Learning skills
• New tool making
• Body hair reduction
• Development of curves in the vertebral column
• A pelvic girdle with broad ilia provides viscera support by acquiring a bowel-like form.
• Increased height
• Increased range of vision
• Continuous gametogenesis in the gonads.
• Speech
• Communication
• Social behaviour and hominisation

Some common features of ancestry between great apes and man include:

• Similarities in chromosome number, DNA content, and branding patterns of chromosomes.
• Human DNA differs from chimpanzee DNA in only 1.8 percent of its base pairs.
• The blood protein test has added evidence that man is closest to the great apes.
• Homology in haemoglobin between apes and man is 99%.
• There are four types of blood groups in man: A, B, AB, and O. In apes, blood groups A and B are found but not in monkeys. So this shows that there is a close evolutionary relationship between humans and great apes.

Difference between the primitive atmosphere and the modern atmosphere:

The primitive atmosphere was reducing in nature, and it lacked free oxygen. The most common element in the primitive atmosphere was hydrogen. There was no ozone layer, so the UV radiation freely reached the atmosphere, and it was very hot. It favours chemical evolution.

The modern atmosphere is oxidising by nature and has about 20.95%  oxygen and less than 0.4% hydrogen in traces. The atmosphere has a thick ozone layer, which minimises the UV radiation that reaches the earth. The temperature is moderate, and it disallows chemical evolution.

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