Evolution

Class 12 | Pure Science — Zoology | Chapter 6

Class 12 Unit 6 Pure_Science

UNIT 2 CHAPTER 6 — EVOLUTION

"Each has his own tree of ancestors, but at the top of all sits probably arboreal"

Chapter Outline

6.1 Origin of life — Evolution of life forms
6.2 Geological time scale
6.3 Biological evolution
6.4 Evidences for biological evolution
6.5 Theories of biological evolution
6.6 Mechanism of evolution
6.7 Hardy Weinberg principle
6.8 Origin and evolution of man
6.9 Isolating mechanisms
6.10 Speciation
6.11 Extinction of animals

Learning Objectives

• Understands the evolution of life on earth.
• Gains knowledge on theories of evolution.
• Interprets evidences (anatomical, embryological and geological) of evolution.
• Learns the principles of biological evolution.
• Understands the importance of gene frequencies in a population.
• Studies the geological time scale.
• Understands the importance of isolating mechanisms in speciation.

The term evolution is used to describe heritable changes in one or more characteristics of a population of species from one generation to the other. The present state of mankind on earth is the outcome of three kinds of evolution — chemical, organic and social or cultural evolution. Radiometric dating of meteorites yields an estimated age for the solar system and for earth as around 4.5 to 4.6 billion years. The new born earth remained inhospitable for at least few hundred million years. At first it was too hot. This is because the collisions of the planetesimals that coalesced to form earth released much heat to melt the entire planet. Eventually outer surface of the earth cooled and solidified to form a crust. Water vapour released from the planet's interior cooled and condensed to form oceans. Hence origin of life can be reconstructed using indirect evidences. Consequently, biologists have turned to gather disparate bits of information and filling them together like pieces of jig saw puzzle. Many theories have been proposed to explain the origin of life. Few have been discussed in this chapter.

6.1 Origin of Life — Evolution of Life Forms

Theory of special creation states that life was created by a supernatural power, respectfully referred to as "God". According to Hinduism, Lord Brahma created the Earth. Christianity, Islam and most religions believe that God created the universe, the plants and the animals.

According to the theory of spontaneous generation or Abiogenesis, living organisms originated from non-living materials and occurred through stepwise chemical and molecular evolution over millions of years. Thomas Huxley coined the term abiogenesis.

Big bang theory explains the origin of universe as a singular huge explosion in physical terms. The primitive earth had no proper atmosphere, but consisted of ammonia, methane, hydrogen and water vapour. The temperature of the earth was extremely high. UV rays from the sun split up water molecules into hydrogen and oxygen. Gradually the temperature cooled and the water vapour condensed to form rain. Rain water filled all the depressions to form water bodies. Ammonia and methane in the atmosphere combined with oxygen to form carbon-dioxide and other gases.

Do You Know?

Coacervates (large colloidal particles that precipitate out in aqueous medium) are the first pre-cells which gradually transformed into living cells.

According to the theory of biogenesis, life arose from pre-existing life. The term biogenesis also refers to the biochemical process of production of living organisms. This term was coined by Henry Bastian.

According to the theory of chemical evolution, primitive organisms in the primordial environment of the earth evolved spontaneously from inorganic substances and physical forces such as lightning, UV radiations, volcanic activities, etcetera. Oparin (1924) suggested that the organic compounds could have undergone a series of reactions leading to more complex molecules. He proposed that the molecules formed colloidal aggregates or 'coacervates' in an aqueous environment. The coacervates were able to absorb and assimilate organic compounds from the environment.

Haldane (1929) proposed that the primordial sea served as a vast chemical laboratory powered by solar energy. The atmosphere was oxygen free and the combination of CO₂, NH₃ and UV radiations gave rise to organic compounds. The sea became a 'hot' dilute soup containing large populations of organic monomers and polymers. They envisaged that groups of monomers and polymers acquired lipid membranes and further developed into the first living cell. Haldane coined the term prebiotic soup and this became the powerful symbol of the Oparin-Haldane view on the origin of life (1924 to 1929).

Oparin and Haldane independently suggested that if the primitive atmosphere was reducing and if there was appropriate supply of energy such as lightning or UV light then a wide range of organic compounds can be synthesized.

6.2 Geological Time Scale

The duration of the earth's history has been divided into eras that include the Paleozoic, Mesozoic, and Cenozoic. Recent eras are further divided into periods, which are split into epochs. The geological time scale with the duration of the eras and periods with the dominant forms of life is shown below.

Era	Years (in million)	Period	Epoch	Fauna	Flora
Cenozoic	1	Quaternary	Recent (Holocene)	Age of Mammals	Angiosperms Monocotyledons
Cenozoic	6	Quaternary	Pleistocene	Age of Human being	Age of Angiosperms - Dicotyledons
Cenozoic	10	Tertiary	Pliocene	Human evolution	Age of Angiosperms - Dicotyledons
Cenozoic	15	Tertiary	Miocene	Mammals and Birds	Age of Angiosperms - Dicotyledons
Cenozoic	20	Tertiary	Oligocene	Mammals and Birds	Age of Angiosperms - Dicotyledons
Cenozoic	100	Tertiary	Eocene	Mammals and Birds	Age of Angiosperms - Dicotyledon
Cenozoic	100	Tertiary	Paleocene	Mammals and Birds	Age of Angiosperms - Dicotyledon
Mesozoic	125	Cretaceous (Golden age of Reptiles)	Rise of Dinosaurs		Sphenopsides, Ginkgos, Gnetales, (Dicotyledons)
Mesozoic	150	Jurassic (Golden age of Reptiles)	Rise of Dinosaurs		Herbaceous lycopods, Ferns, Conifers, Cycads
Mesozoic	180	Triassic (Golden age of Reptiles)	Rise of Dinosaurs
Paleozoic	205	Permian		Mammal like reptiles	Arborescent lycopods
Paleozoic	230	Carboniferous	Pennsylvanian	Earliest reptiles	Seed ferns and Bryophytes
Paleozoic	255	Carboniferous	Mississippian	Earliest Amphibians and abundant Echinoderms
Paleozoic	315	Devonian		Age of fishes	Progymnosperms
Paleozoic	350	Silurian		Earliest fishes and land invertebrates	Zosterophyllum
Paleozoic	430	Ordovician		Dominance of Invertebrates	Appearance of first land plants
Paleozoic	510	Cambrian		Fossil invertebrates	Origin of algae
Precambrian	3000	Upper		Multicellular organisms
Precambrian	3000	Middle		Appearance of eukaryotes
Precambrian	3000	Lower		Planktons prokaryotes

The Paleozoic era is characterized by abundance of fossils of marine invertebrates. Towards the later half, other vertebrates (marine and terrestrial) except birds and mammals appeared. The six periods of Paleozoic era in order from oldest to the youngest are Cambrian (Age of invertebrates), Ordovician (fresh water fishes, Ostracoderms, various types of Molluscs), Silurian (origin of fishes), Devonian (Age of fishes, many types of fishes such as lung fishes, lobe finned fishes and ray finned fishes), Mississippian (earliest amphibians, Echinoderms), Pennsylvanian (earliest reptiles), Permian (mammal like reptiles).

Mesozoic era (dominance of reptiles) called the Golden age of reptiles, is divided into three periods namely, Triassic (origin of egg laying mammals), Jurassic (Dinosaurs were dominant on the earth, fossil bird Archaeopteryx) and Cretaceous (extinction of toothed birds and dinosaurs, emergence of modern birds).

Cenozoic era (Age of mammals) is subdivided into two periods namely Tertiary and Quaternary. Tertiary period is characterized by abundant mammalian fauna. This period is subdivided into five epochs namely, Paleocene (placental mammals), Eocene (Monotremes except duck billed Platypus and Echidna, hoofed mammals and carnivores), Oligocene (higher placental mammals appeared), Miocene (origin of first man like apes) and Pliocene (origin of man from man like apes). Quaternary period witnessed decline of mammals and beginning of human social life.

The age of fossils can be determined using two methods namely, relative dating and absolute dating. Relative dating is used to determine a fossil by comparing it to similar rocks and fossils of known age. Absolute dating is used to determine the precise age of a fossil by using radiometric dating to measure the decay of isotopes.

6.3 Biological Evolution

Formation of protobionts: Abiotically produced molecules can spontaneously self assemble into droplets that enclose a watery solution and maintain a chemical environment different from their surroundings. Scientists call these spheres as 'protobionts'. Liposomes are lipids in a solution that can self assemble into a lipid bilayer. Some of the proteins inside the liposomes acquired the properties of enzymes resulting in fast multiplication of molecules.

The coacervates with nucleoprotein and nutrients had a limiting surface membrane that had the characters of a virus or free-living genes. Subsequently, number of genes united to form 'proto viruses' somewhat similar to present day viruses. Two major cell types that appeared during this time were significant. One form of the earliest cell contained clumps of nucleoproteins embedded in the cell substance. Such cells were similar to the Monera. They are considered as ancestral to the modern bacteria and blue green algae. The other form of earliest cells contained nucleoprotein clumps that condensed into a central mass surrounded by a thin membrane. This membrane separated nucleoproteins from the cell substances. Such cells were referred to as Protista.

When the natural sources of food in the ocean declined in course of time, the ancestors of Monera and Protista had to evolve different methods for food procurement. These may be summarized as parasitism, saprophytism, predator or animalism and chemosynthesis or photosynthesis. When the number of photosynthetic organisms increased there was an increase in the free O₂ in the sea and atmosphere.

Example: CH₄ + 2O₂ → CO₂ + 2H₂O

4NH₃ + 3O₂ → 2N₂ + 6H₂O

The atmospheric oxygen combined with methane and ammonia to form CO₂ and free nitrogen. The presence of the free O₂ brought about the evolution of aerobic respiration which could yield large amounts of energy by oxidation of food stuffs. Thus Prokaryotes and Eukaryotes evolved.

Experimental approach to the origin of life: Urey and Miller (1953), paved way for understanding the possible synthesis of organic compounds that led to the appearance of living organisms. In their experiment, a mixture of gases was allowed to circulate over electric discharge from a tungsten electrode. A small flask was kept boiling and the steam emanating from it was made to mix with the mixture of gases (ammonia, methane and hydrogen) in the large chamber that was connected to the boiling water. The steam condensed to form water which ran down the 'U' tube. Experiment was conducted continuously for a week and the liquid was analysed. Glycine, alanine, beta alanine and aspartic acid were identified. Thus, Miller's experiments had an insight as to the possibility of abiogenetic synthesis of large amount of variety of organic compounds in nature from a mixture of sample gases in which the only source of carbon was methane. Later in similar experiments, formation of all types of amino acids, and nitrogen bases were noticed.

6.4 Evidences for Biological Evolution

6.4.1 Paleontological Evidences

Paleontology is the study of prehistoric life through fossils. Fossils are described as the true witnesses of evolution or documents of various geological strata of evolution. Fossilization is the process by which plant and animal remains are preserved in sedimentary rocks. They fall under three main categories.

1) Actual remains: The original hard parts such as bones, teeth or shells are preserved as such in the earth's atmosphere. This is the most common method of fossilization. When marine animals die, their hard parts such as bones, shells, etc., are covered with sediments and are protected from further deterioration. They get preserved as such as they are preserved in vast ocean; the salinity in them prevents decay. The sediments become hardened to form definite layers or strata. For example, Woolly Mammoth that lived 22 thousand years ago were preserved in the frozen coast of Siberia as such. Several human beings and animals living in the ancient city of Pompeii were preserved intact by volcanic ash which gushed out from Mount Vesuvius.

2) Petrifaction: When animals die the original portion of their body may be replaced molecule for molecule by minerals and the original substance being lost through disintegration. This method of fossilization is called petrifaction. The principle minerals involved in this type of fossilization are iron pyrites, silica, calcium carbonate and bicarbonates of calcium and magnesium.

3) Natural moulds and casts: Even after disintegration, the body of an animal might leave indelible impression on the soft mud which later becomes hardened into stones. Such impressions are called moulds. The cavities of the moulds may get filled up by hard minerals and get fossilized, which are called casts. Hardened faecal matter termed as coprolites occur as tiny pellets. Analysis of the coprolites enables us to understand the nature of diet the pre-historic animals thrived on.

Activity

Visit any museum nearer to your school with your teacher and identify the bones of different animals including mammals. The famous Egmore Museum is in Chennai.

6.4.2 Evidences from Comparative Anatomy

Similarities in structure between groups of organisms are accepted as indicators of relationship. For example, a comparative study of the forelimbs of different vertebrates exhibits a fundamental plan of similarity in structure. These relationships can be studied under homologous organs, analogous organs, vestigial organs, connecting links and atavistic organs.

Homologous structures: In vertebrates, comparative anatomical studies reveal a basic plan in various structures such as fore limbs and hind limbs. Fore limbs of vertebrates exhibit anatomical similarity with each other and is made of similar bones such as humerus, radius, ulna, carpals, metacarpals and phalanges. Structures which are similar in origin but perform different functions are called homologous structures that brings about divergent evolution. Similarly, the thorn of Bougainvillea and the tendrils of Cucurbita and Pisum sativum represent homology. The thorn in the former is used as a defence mechanism from grazing animals and the tendrils of the latter is used as a support for climbing.

Analogous structures: Organisms having different structural patterns but similar function are termed as analogous structures. For example, the wings of birds and insects are different structurally but perform the same function of flight that brings about convergent evolution. Other examples of analogous organs include the eyes of the Octopus and of mammals and the flippers of Penguins and Dolphins. Root modification in sweet potato and stem modification in potato are considered as analogous organs. Both of these plants have a common function of storage of food.

Vestigial organs: Structures that are of no use to the possessor, and are not necessary for their existence are called vestigial organs. Vestigial organs may be considered as remnants of structures which were well developed and functional in the ancestors, but disappeared in course of evolution due to their non-utilization. Human appendix is the remnant of caecum which is functional in the digestive tract of herbivorous animals like rabbit. Cellulose digestion takes place in the caecum of these animals. Due to change in the diet containing less cellulose, caecum in human became functionless and is reduced to a vermiform appendix, which is vestigial. Other examples of vestigial organs in human beings include coccyx, wisdom teeth, ear muscles, body hair, mammae in male, nictitating membrane of the eye, etcetera.

Connecting link: The organisms which possess the characters of two different groups (transitional stage) are called connecting links. Example: Peripatus (connecting link between Annelida and Arthropoda), Archeopteryx (connecting link between Reptiles and Aves).

Atavistic organs: Sudden appearance of vestigial organs in highly evolved organisms is called atavistic organs. Example: presence of tail in a human baby is an atavistic organ.

6.4.3 Embryological Evidences

Embryology deals with the study of the development of individual from the egg to the adult stage. A detailed study of the embryonic development of different forms makes us to think that there is a close resemblance during development. The development of heart in all vertebrates follows the same pattern of development as a pair of tubular structures that later develop into two chambered heart in fishes, three chambered in amphibians and in most reptiles and four chambered in crocodiles, birds and mammals; indicating a common ancestry for all the vertebrates.

Hence scientists in the 19th century concluded that higher animals during their embryonic development pass through stages of lower animals (ancestors). Ernst Von Haeckel propounded the "biogenetic law or theory of recapitulation" which states that the life history of an individual (ontogeny) briefly repeats or recapitulates the evolutionary history of the race (phylogeny). In other words, "Ontogeny recapitulates Phylogeny". The embryonic stages of a higher animal resemble the adult stage of its ancestors. Appearance of pharyngeal gill slits, yolk sac and the appearance of tail in human embryos are some of the examples.

The biogenetic law is not universal and it is now thought that animals do not recapitulate the adult stage of any ancestors. The human embryo recapitulates the embryonic history and not the adult history of the organisms. The comparative study of the embryo of different animals shows structural similarities among themselves. The embryos of fish, salamander, tortoise, chick and human start life as a single cell, the zygote, and undergo cleavage to produce the blastula, change to gastrula and are triploblastic. This indicates that all the above said animals have evolved from a common ancestor.

Molecular evidences: Molecular evolution is the process of change in the sequence composition of molecules such as DNA, RNA and proteins across generations. It uses principles of evolutionary biology and population genetics to explain patterns in the changes of molecules. One of the most useful advancement in the development of molecular biology is proteins and other molecules that control life processes are conserved among species. A slight change that occurs over time in these conserved molecules (DNA, RNA and protein) are often called molecular clocks. Molecules that have been used to study evolution are cytochrome c (respiratory pathway) and r RNA (protein synthesis).

6.5 Theories of Biological Evolution

6.5.1 Lamarck's Theory

Jean Baptiste de Lamarck, was the first to postulate the theory of evolution in his famous book Philosophie Zoologique in the year 1809. The two principles of Lamarckian theory are:

1. The theory of use and disuse: Organs that are used often will increase in size and those that are not used will degenerate. Neck in giraffe is an example of use and absence of limbs in snakes is an example for disuse theory.

2. The theory of inheritance of acquired characters: Characters that are developed during the life time of an organism are called acquired characters and these are then inherited.

The main objection to Lamarckism: Lamarck's "Theory of Acquired characters" was disproved by August Weismann who conducted experiments on mice for twenty generations by cutting their tails and breeding them. All mice born were with tail. Weismann proved his germplasm theory that change in the somatoplasm will not be transferred to the next generation but changes in the germplasm will be inherited.

Neo-Lamarckism: The followers of Lamarck (Neo Lamarckists) like Cope, Osborn, Packard and Spencer tried to explain Lamarck's theory on a more scientific basis. They considered that adaptations are universal. Organisms acquire new structures due to their adaptations to the changed environmental conditions. They argued that external conditions stimulate the somatic cells to produce certain 'secretions' which reach the sex cells through the blood and bring about variations in the offspring.

6.5.2 Darwin's Theory of Natural Selection

Charles Darwin explained the theory of evolution in his book The Origin of Species by Natural Selection. During his journey around the Earth, he made extensive observations of plants and animals. He noted a huge variety and remarkable similarities among organisms and their adaptive features to cope up to their environment. He proved that fittest organisms can survive and leave more progenies than the unfit ones through natural selection. Darwin's theory was based on several facts, observations and influences. They are:

1. Over production (or) prodigality of production: All living organisms increase their population in larger number. For example, Salmon fish produces about 28 million eggs during breeding season and if all of them hatch, the seas would be filled with salmon in few generations. Elephant, the slowest breeder that can produce six young ones in its life time can produce 6 million descendants at the end of 750 years in the absence of any check.

2. Struggle for existence: Organisms struggle for food, space and mate. As these become a limiting factor, competition exists among the members of the population. Darwin denoted struggle for existence in three ways:

• Intra specific struggle between the same species for food, space and mate.
• Inter specific struggle with different species for food and space.
• Struggle with the environment to cope with the climatic variations, flood, earthquakes, drought, etcetera.

3. Universal occurrence of variations: No two individuals are alike. There are variations even in identical twins. Even the children born of the same parents differ in colour, height, behavior, etcetera. The useful variations found in an organism help them to overcome struggle and such variations are passed on to the next generation.

4. Origin of species by Natural Selection: According to Darwin, nature is the most powerful selective force. He compared origin of species by natural selection to a small isolated group. Darwin believed that the struggle for existence resulted in the survival of the fittest. Such organisms become better adapted to the changed environment.

Objections to Darwinism: Some objections raised against Darwinism were:

• Darwin failed to explain the mechanism of variation.
• Darwinism explains the survival of the fittest but not the arrival of the fittest.
• He focused on small fluctuating variations that are mostly non heritable.
• He did not distinguish between somatic and germinal variations.
• He could not explain the occurrence of vestigial organs, over specialization of some organs like large tusks in extinct mammoths, oversized antlers in the extinct Irish deer, etcetera.

Neo Darwinism: Neo Darwinism is the interpretation of Darwinian evolution through Natural Selection as it has been modified since it was proposed. New facts and discoveries about evolution have led to modifications of Darwinism and is supported by Wallace, Heinrich, Haeckel, Weismann and Mendel. This theory emphasizes the change in the frequency of genes in population arises due to mutation, variation, isolation and Natural selection.

6.5.3 Mutation Theory

Hugo de Vries put forth the Mutation theory. Mutations are sudden random changes that occur in an organism that is not heritable. De Vries carried out his experiments in the Evening Primrose plant (Oenothera lamarckiana) and observed variations in them due to mutation. According to de Vries, sudden and large variations were responsible for the origin of new species whereas Lamarck and Darwin believed in gradual accumulation of all variations as the causative factors in the origin of new species. Hugo de Vries believed that Mutations are random and directionless, but Darwinian variations are small and directional.

Do You Know?

Hugo de Vries believed that speciation are due to Mutation and called saltation (single step large mutation).

Salient Features of Mutation Theory

• Mutations or discontinuous variation are transmitted to other generations.
• In naturally breeding populations, mutations occur from time to time.
• There are no intermediate forms, as they are fully fledged.
• They are strictly subjected to natural selection.

6.5.4 Modern Synthetic Theory

Sewell Wright, Fisher, Mayer, Huxley, Dobzhansky, Simpson and Haeckel explained Natural Selection in the light of Post-Darwinian discoveries. According to this theory gene mutations, chromosomal mutations, genetic recombinations, natural selection and reproductive isolation are the five basic factors involved in the process of organic evolution.

1. Gene mutation refers to the changes in the structure of the gene. It is also called gene or point mutation. It alters the phenotype of an organism and produces variations in their offspring.

2. Chromosomal mutation refers to the changes in the structure of chromosomes due to deletion, addition, duplication, inversion or translocation. This too alters the phenotype of an organism and produces variations in their offspring.

3. Genetic recombination is due to crossing over of genes during meiosis. This brings about genetic variations in the individuals of the same species and leads to heritable variations.

4. Natural selection does not produce any genetic variations but once such variations occur it favours some genetic changes while rejecting others (driving force of evolution).

5. Reproductive isolation helps in preventing interbreeding between related organisms.

6.5.5 Evolution by Anthropogenic Sources — Natural Selection (Industrial Melanism)

Natural selection can be explained clearly through industrial melanism. Industrial melanism is a classical case of Natural selection exhibited by the peppered moth, Biston betularia. These were available in two forms, white winged and black winged. Before industrialization, peppered moth — both white and black coloured were common in England. Pre industrialization witnessed more white winged moths than dark winged moths because of the dense growth of white coloured lichens on trees. They escaped from predators due to the white background.

Post industrialization, the tree trunks became dark due to smoke and soot let out from the industries. The lichen population also decreased because they could not grow in polluted areas. The white winged moths were easily identified by their predators as the background turned black due to soot and smoke while the black winged moths camouflaged with dark background. Hence the dark coloured moth population was selected and their number increased when compared to the white moths. Nature offered positive selection pressure to the black coloured moths. The above proof shows that in a population, organisms that can adapt will survive and produce more progenies resulting in increase in population through natural selection.

Artificial selection is a byproduct of human exploitation of forests, oceans and fisheries or the use of pesticides, herbicides or drugs. For hundreds of years humans have selected various types of dogs, all of which are variants of the single species of dog. If human beings can produce new varieties in short period, then "nature" with its vast resources and long duration can easily produce new species by selection.

6.5.6 Adaptive Radiation

The evolutionary process which produces new species diverged from a single ancestral form becomes adapted to newly invaded habitats is called adaptive radiation. Adaptive radiations are best exemplified in closely related groups that have evolved in relatively short time. Darwin's finches and Australian marsupials are best examples for adaptive radiation. When more than one adaptive radiation occurs in an isolated geographical area, having the same structural and functional similarity it is due to convergent evolution.

Darwin's finches: Their common ancestor arrived on the Galapagos about 2 million years ago. During that time, Darwin's finches have evolved into 14 recognized species differing in body size, beak shape and feeding behavior. Changes in the size and form of the beak have enabled different species to utilize different food resources such as insects, seeds, nectar from cactus flowers and blood from iguanas, all driven by Natural selection. Genetic variation in the ALX1 gene in the DNA of Darwin finches is associated with variation in the beak shape. Mild mutation in the ALX1 gene leads to phenotypic change in the shape of the beak of the Darwin finches.

Marsupials in Australia and placental mammals in North America are two subclasses of mammals — they have adapted in similar way to a particular food resource, locomotory skill or climate. They were separated from the common ancestor more than 100 million years ago and each lineage continued to evolve independently. Despite temporal and geographical separation, Marsupials in Australia and placental mammals in North America have produced varieties of species living in similar habitats with similar ways of life. Their overall resemblance in shape, locomotory mode, feeding and foraging are superimposed upon different modes of reproduction. This feature reflects their distinctive evolutionary relationships. Over 200 species of marsupials live in Australia along with many fewer species of placental mammals. The marsupials have undergone adaptive radiation to occupy the diverse habitats in Australia, just as the placental mammals have radiated across North America.

6.6 Mechanism of Evolution

Microevolution (evolution on a small scale) refers to the changes in allele frequencies within a population. Allele frequencies in a population may change due to four fundamental forces of evolution such as natural selection, genetic drift, mutation and gene flow.

6.6.1 Natural Selection

It occurs when one allele (or combination of alleles of differences) makes an organism more or less fit to survive and reproduce in a given environment. If an allele reduces fitness, its frequencies tend to drop from one generation to the next. The evolutionary path of a given gene — that is, how its allele's change in frequency in the population across generation — may result from several evolutionary mechanisms acting at once. For example, one gene's allele frequencies might be modified by both gene flow and genetic drift; for another gene, mutation may produce a new allele that is favoured by natural selection.

There are mainly three types of natural selection:

1. Stabilising Selection (centripetal selection): This type of selection operates in a stable environment. The organisms with average phenotypes survive whereas the extreme individuals from both the ends are eliminated. There is no speciation but the phenotypic stability is maintained within the population over generation. For example, measurements of sparrows that survived the storm clustered around the mean, and the sparrows that failed to survive the storm clustered around the extremes of the variation showing stabilizing selection.

2. Directional Selection: The environment which undergoes gradual change is subjected to directional selection. This type of selection removes the individuals from one end towards the other end of phenotypic distribution. For example, size differences between male and female sparrows. Both male and female look alike externally but differ in body weight. Females show directional selection in relation to body weight.

3. Disruptive Selection (centrifugal selection): When homogenous environment changes into heterogenous environment this type of selection is operational. The organisms of both the extreme phenotypes are selected whereas individuals with average phenotype are eliminated. This results in splitting of the population into sub population or species. This is a rare form of selection but leads to formation of two or more different species. It is also called adaptive radiation. Example: Darwin's finches beak size in relation to seed size inhabiting Galapagos islands.

Group selection and sexual selection are other types of selection. The two major group selections are Altruism and Kin selection.

6.6.2 Gene Flow

Movement of genes through gametes or movement of individuals in (immigration) and out (emigration) of a population is referred to as gene flow. Organisms and gametes that enter the population may have new alleles or may bring in existing alleles but in different proportions than those already in the population. Gene flow can be a strong agent of evolution.

6.6.3 Genetic Drift or Sewall Wright Effect

Genetic drift is a mechanism of evolution in which allele frequencies of a population change over generation due to chance (sampling error). Genetic drift occurs in all population sizes, but its effects are strong in a small population. It may result in a loss of some alleles (including beneficial ones) and fixation of other alleles. Genetic drift can have major effects, when the population is reduced in size by natural disaster due to bottle neck effect or when a small group of population splits from the main population to form a new colony due to founder's effect.

6.6.4 Mutation

Although mutation is the original source of all genetic variation, mutation rate for most organisms is low. Hence new mutations on an allele frequencies from one generation to the next is usually not large.

6.7 Hardy Weinberg Principle

In nature, populations are usually evolving — such as the grass in an open meadow, wolves in a forest and bacteria in a person's body are all natural populations. All of these populations are likely to be evolving some of their genes. Evolution does not mean that the population is moving towards perfection; rather the population is changing its genetic makeup over generations. For example, in a wolf population, there may be a shift in the frequency of a gene variant for black fur than grey fur. Sometimes, this type of change is due to natural selection or due to migration or due to random events.

Hardy of UK and Weinberg of Germany stated that the allele frequencies in a population are stable and are constant from generation to generation in the absence of gene flow, genetic drift, mutation, recombination and natural selection. If a population is in a state of Hardy Weinberg equilibrium, the frequencies of alleles and genotypes or sets of alleles in that population will remain same over generations. Evolution is a change in the allele frequencies in a population over time. Hence population in Hardy Weinberg is not evolving.

Suppose we have a large population of beetles (infinitely large) and appear in two colours — dark grey (black) and light grey, and their colour is determined by 'A' gene. 'Homozygous dominant allele AA' and Heterozygous allele 'Aa' beetles are dark grey and homozygous recessive allele 'aa' beetles are light grey. In a population, let's say that dominant gene recessive 'A' allele has frequency (p) of 0.3 and 'a' allele has a frequency (q) of 0.7. Then p + q = 1.

If a population is in Hardy Weinberg equilibrium, the genotype frequency can be estimated by Hardy Weinberg equation:

(p + q)² = p² + 2pq + q²

p² = frequency of homozygous dominant alleles AA

2pq = frequency of heterozygous alleles Aa

q² = frequency of homozygous recessive alleles aa

p = 0.3, q = 0.7

p² = (0.3)² = 0.09 = 9% AA

2pq = 2(0.3)(0.7) = 0.42 = 42% Aa

q² = (0.7)² = 0.49 = 49% aa

Hence the beetle population appears to be in Hardy-Weinberg equilibrium. When the beetles in Hardy-Weinberg equilibrium reproduce, the allele and genotype frequency in the next generation would be: Let's assume that the frequency of dominant 'A' and recessive 'a' allele in the pool of gametes that make the next generation would be the same, then there would be no variation in the progeny. The genotype frequencies of the parent appear in the next generation (that is 9% dominant alleles AA, 42% recessive alleles Aa and 49% aa). If we assume that the beetle's mate randomly (selection of male gamete and female gamete in the pool of gametes), the probability of getting the offspring genotype depends on the genotype of the combining parental gametes.

Hardy Weinberg's Assumptions

• No mutation: No new alleles are generated by mutation nor the genes get duplicated or deleted.
• Random mating: Every organism gets a chance to mate and the mating is random with each other with no preferences for a particular genotype.
• No gene flow: Neither individuals nor their gametes enter (immigration) or exit (emigration) the population.
• Very large population size: The population should be infinite in size.
• No natural selection: All alleles are fit to survive and reproduce.

If any one of these assumptions were not met, the population will not be in Hardy-Weinberg equilibrium. Only if the allele frequencies changes from one generation to the other, evolution will take place.

6.8 Origin and Evolution of Man

Mammals evolved in the early Jurassic period, about 210 million years ago (mya). Hominid evolution occurred in Asia and Africa. Hominids proved that human beings are superior to other animals and efficient in making tools and culture.

The earliest fossils of the prehistoric man like Ramapithecus and Sivapithecus lived some 14 mya and were derived from ape like Dryopithecus. Dryopithecus and Ramapithecus were hairy and walked like gorillas and chimpanzees. Ramapithecus is regarded as a possible ancestor of Australopithecus and therefore of modern humans. They were vegetarians.

Australopithecus lived in East African grasslands about 5 mya and was called the Australian ape man. He was about 1.5 meters tall with bipedal locomotion, omnivorous, semi erect, and lived in caves. Low forehead, brow ridges over the eyes, protruding face, lack of chin, low brain capacity of about 350 to 450 cc, human like dentition, lumbar curve in the vertebral column were his distinguishing features.

Homo habilis lived about 2 mya. Their brain capacity was between 650 to 800 cc, and was probably vegetarian. They had bipedal locomotion and used tools made of chipped stones.

Homo erectus the first human like being was around 1.7 mya and was much closer to human in looks, skull was flatter and thicker than the modern man and had a large brain capacity of around 900 cc. Homo erectus probably ate meat. Homo ergaster and Homo erectus were the first to leave Africa.

Neanderthal human was found in Neander Valley, Germany with a brain size of 1400 cc and lived between 34,000 to 1,00,000 years ago. They differ from the modern human in having semierect posture, flat cranium, sloping forehead, thin large orbits, heavy brow ridges, protruding jaws and no chin. They used animal hides to protect their bodies, knew the use of fire and buried their dead. They did not practice agriculture and animal domestication.

Cro Magnon was one of the most talked forms of modern human found from the rocks of Cro Magnon, France and is considered as the ancestor of modern Europeans. They were not only adapted to various environmental conditions, but were also known for their cave paintings, figures on floors and walls.

Homo sapiens or modern human arose in Africa some 25,000 years ago and moved to other continents and developed into distinct races. They had a brain capacity of 1300 to 1600 cc. They started cultivating crops and domesticating animals.

6.9 Isolating Mechanisms

Isolation is the separation of the members of a single population into sub populations so that genetic integrity of the subpopulation can be maintained. Closely related species living in the same area do not breed together; they are prevented by isolating barriers. An isolating barrier is any evolved character of the two species that stops them from interbreeding. Several kinds of isolating barriers are distinguished. The most important distinction is Prezygotic and Post-zygotic isolation.

Prezygotic mechanisms include those which prevent two species from coming into contact. This includes ecological, seasonal, ethological and morphological. Post-zygotic mechanisms are those which act after fertilization that include hybrid sterility, hybrid inviability and hybrid breakdown.

Prezygotic mechanisms:

1. Ecological isolation or habitat isolation: The members of the same population may be separated from one another by a difference in their habitat. For example, Rana areolata occupies burrows dug by mammals and tortoises during the day and breeds in grassy shallow ponds whereas Rana grylio breeds in deep waters. Due to the difference in their habitat the two species are able to maintain their respective species identities.

2. Seasonal isolation: In this type of isolation, difference in the breeding seasons prevents interbreeding. Example: Toad, Bufo americanus breeds much early in the spring; whereas Bufo fowleri breeds very late in the season. They are able to maintain their species identity because of the differences in the breeding seasons.

3. Sexual or ethological isolation or Behavioural isolation: Prevents mating due to the difference in their sexual behavior. The species are not separated from one another either in time or in space. The mating calls of two closely related species of frogs, Hyla versicolor (grey tree frog) and Hyla femoralis (pine wood tree frog) are different which prevents interbreeding.

4. Morphological isolation or mechanical isolation: This type of isolation is due to the differences in their external genitalia that is seen in two different species. The size difference between the toad species Bufo quercicus and Bufo valliceps prevents their interbreeding.

5. Physiological isolation: Though mating may occur, the gametes are prevented from fertilization due to mechanical or physiological factors. Example: The sperms of Drosophila virilis survive only for about a day when introduced into the sperm receptacle of Drosophila americana while the sperms of Drosophila americana live for a longer time.

6. Cytological isolation: Fertilization does not take place due to the differences in the chromosome numbers between the two species, the bull frog Rana catesbiana and gopher frog Rana areolata.

Post-zygotic mechanisms:

1. Hybrid inviability: In this type, the sperm enters the egg, fertilization occurs and the embryo develops into the adult but it dies before reaching maturity. In certain fishes, frogs, beetles, even if fertilization takes place between two species, due to genetic incompatibility they do not leave any surviving offspring.

2. Hybrid sterility: In this type, hybrids are formed due to inter specific crosses but they are sterile due to the failure of the chromosomes to segregate normally during meiosis. Example: Mule (inter specific cross between a horse and a donkey).

3. Hybrid breakdown: F1 Hybrids are viable and fertile, but F2 hybrids may be inviable or sterile.

6.10 Speciation

The process by which one species evolves into one or more different species is called speciation. A.E. Emerson defines species as a 'genetically distinctive, reproductively isolated natural population'. Speciation is a fundamental process in evolution. Evolution of a new species in a single lineage is called anagenesis or phyletic speciation. If one species diverges to become two or more species it is cladogenesis or divergent evolution.

6.10.1 Sympatric Speciation or Reproductive Isolation

It is a mode of speciation through which new species form from a single ancestral species while both species continue to inhabit the same geographical region. Two or more species are involved. New species formed due to genetic modification in the ancestor that is naturally selected can no longer breed with the parent population. Sexual isolation is strongest. Phenotypic plasticity has emerged as potentially important first step in speciation initiated within an isolated population. A well studied example is the adaptation of Apple maggots that feed on apples in North America. When the apple trees were imported to North America, Apple maggot flies (Rhagoletis pomonella) a parasitic insect that normally laid its eggs in the fruit of wild hawthorns until one subset of population began to lay its eggs in the fruit of domesticated apple trees (Malus domestica) that grew in the same area. This small group of apple maggot flies selected a different host species from the rest of the population and its offsprings became accustomed to domesticated apples.

Do You Know?

Phenotypic plasticity is the ability of single genotype to produce more than one phenotype. When this plasticity is expressed seasonally in planktons, it is referred to as cyclomorphosis.

6.10.2 Allopatric Speciation or Geographical Speciation

It is a mode of speciation that occurs when biological populations of similar species become isolated from each other that prevents gene flow. One species becomes two species due to geographical barriers — hence new species is evolved. Example: Darwin's finches. The barriers are land separation, migration or mountain formation. When barriers occur between species, change in ecological conditions and environment leads to adaptations that produce differences. If there are no adaptations, they will not survive. Sexual isolation is weakest.

6.11 Extinction of Animals

Extinction was common if not inevitable because species could not always adapt to large or rapid environmental changes. The impact of extinction can conveniently be considered at three levels.

Species extinction eliminates an entire species, by an environmental event (flood, etcetera) or by biological event (disease or non-availability of food resource half or more).

Mass extinction eliminates half or more species in a region or ecosystem, as might occur following a volcanic eruption. Five major mass extinctions occurred since the Cambrian period. This mass extinction is often referred to as K-T extinction.

Table 6.2: Details of the five major mass extinctions since the Cambrian period

Extinction Period	Date (Mya)	Estimated % of Marine Animal Extinction (Genera)	Estimated % of Marine Animal Extinction (Species)
End of the Ordovician	440	61	85
End of the Devonian	365	55	82
End of the Permian	245	84	96
End of the Triassic	208	50	76
End of the Cretaceous	65	50	76

Do You Know?

K-T Extinction refers to the German word Cretaceous and Tertiary periods.

Global extinction eliminates most of the species on a large scale or larger taxonomic groups in the continent or the Earth. Snow ball Earth and extinction following elevation in CO₂ levels are examples.

Extinction events opens up new habitats and so can facilitate the radiation of organisms that survived the mass extinction.

Summary

Evolutionary Biology is the study of history of life forms on Earth which originated on Earth millions of years ago. How Earth originated, how life originated, what is the place of man in the universe are all general questions. This chapter deals with several theories to explain the life on Earth. Evidence from the fossil record and many other areas of biology like embryology, anatomy and molecular biology indicates a common ancestry. The theories advanced by Lamarck, Darwin, Hugo de Vries explained the intricate evolutionary process. Geological time scale with different eras, periods and epochs gives an idea about the dominant species in those days. The mathematical distribution of gene and genotype frequencies remains constant in a small population was contributed by Hardy and Weinberg in 1608. Natural Selection and gene pool are the important factors those affect Hardy Weinberg equilibrium. Human evolution states that humans developed from primates or ape like ancestors. The emergence of Homo sapiens as a distinct species from apes and placental mammals in brain size, eating habit and other behavior proves that 'Ontogeny recapitulates Phylogeny'. Separation of a single population into its subunits preserves the genetic integrity. Types of isolating mechanism and speciation prevent interbreeding of different species. Various causes of extinction of animals and the levels of impact are dealt with in the lesson.

Evaluation

Exercise I. Multiple Choice Questions

1. The first life on earth originated ___

a) in air b) on land c) in water d) on mountain

2. Who published the book "Origin of species by Natural Selection" in 1859?

a) Charles Darwin b) Lamarck c) Weismann d) Hugo de Vries

3. Which of the following was the contribution of Hugo de Vries?

a) Theory of mutation b) Theory of natural Selection c) Theory of inheritance of acquired characters d) Germplasm theory

4. The wings of birds and butterflies is an example of ___

a) Adaptive radiation b) convergent evolution c) divergent evolution d) variation

5. The phenomenon of "Industrial Melanism" demonstrates ___

a) Natural selection b) induced mutation c) reproductive isolation d) geographical isolation

6. Darwin's finches are an excellent example of ___

a) connecting links b) seasonal migration c) adaptive radiation d) parasitism

7. Who proposed the Germplasm theory?

a) Darwin b) August Weismann c) Lamarck d) Alfred Wallace

8. The age of fossils can be determined by ___

a) electron microscope b) weighing the fossils c) carbon dating d) analysis of bones

9. Fossils are generally found in ___

a) igneous rocks b) metamorphic rocks c) volcanic rocks d) sedimentary rocks

10. Evolutionary history of an organism is called ___

a) ancestry b) ontogeny c) phylogeny d) paleontology

11. The golden age of reptiles was ___

a) Mesozoic era b) Cenozoic era c) Paleozoic era d) Proterozoic era

12. Which period was called "Age of fishes"?

a) Permian b) Triassic c) Devonian d) Ordovician

13. Modern man belongs to which period?

a) Quaternary b) Cretaceous c) Silurian d) Cambrian

14. The Neanderthal man had the brain capacity of ___

a) 650 to 800 cc b) 1200 cc c) 900 cc d) 1400 cc

15. According to Darwin, the organic evolution is due to ___

a) Intraspecific competition b) Interspecific competition c) Competition within closely related species d) Reduced feeding efficiency in one species due to the presence of interfering species

16. A population will not exist in Hardy-Weinberg equilibrium if ___

a) Individuals mate selectively b) There are no mutations c) There is no migration d) The population is large Exercise 17. Short Answer Question

17. List out the major gases seen to be found in the primitive earth.

Exercise 18. Short Answer Question

18. Explain the three major categories in which fossilization occur?

Exercise 19. Short Answer Question

19. Differentiate between divergent evolution and convergent evolution with one example for each.

Exercise 20. Long Answer Question

20. How does Hardy-Weinberg's expression (p² + 2pq + q² = 1) explain that genetic equilibrium is maintained in a population? List any four factors that can disturb the genetic equilibrium.

Exercise 21. Long Answer Question

21. Explain how mutations, natural selection and genetic drift affect Hardy Weinberg equilibrium.

Exercise 22. Short Answer Question

22. How did Darwin explain fitness of organisms?

Exercise 23. Short Answer Question

23. Mention the main objections to Darwinism.

Exercise 24. Long Answer Question

24. Taking the example of Peppered moth, explain the action of natural selection. What do you call the above phenomenon?

Exercise 25. Long Answer Question

25. Darwin's finches and Australian marsupials are suitable examples of adaptive radiation — Justify the statement.

Exercise 26. Short Answer Question

26. Who disproved Lamarck's Theory of acquired characters? How?

Exercise 27. Short Answer Question

27. How does Mutation theory of De Vries differ from Lamarck and Darwin's view in the origin of new species?

Exercise 28. Long Answer Question

28. Explain stabilizing, directional and disruptive selection with examples.

Exercise 29. Rearrange the following in correct order of human evolution

29. Rearrange the descent in human evolution:

Australopithecus → Homo erectus → Homo sapiens → Ramapithecus → Homo habilis

Exercise 30. Short Answer Question

30. How does Neanderthal man differ from the modern man in appearance?

Exercise 31. Long Answer Question

31. Define isolating mechanism and explain its types with suitable examples.

Exercise 32. Long Answer Question

32. Define speciation according to A.E. Emerson and explain its types giving suitable examples.

Exercise 33. Long Answer Question

33. Give an account on the major causes for the extinction of a particular species on earth.

Exercise 34. Long Answer Question

34. Explain the three levels of impact of extinction of species.

Book-back Exercises — Questions & Answers

Answers generated strictly from the Samacheer Kalvi textbook content. Attempt each question first, then click "Show Answer" to check.

I. Choose the Correct Answer

1. The first life on earth originated

a) in air b) on land c) in water d) on mountain

Answer: c) in water

Explanation: According to the chapter, Haldane proposed that the primordial sea served as a vast chemical laboratory and the sea became a 'hot' dilute soup containing large populations of organic monomers and polymers, indicating life originated in water.

2. Who published the book "Origin of species by Natural Selection" in 1859?

a) Charles Darwin b) Lamarck c) Weismann d) Hugo de Vries

Answer: a) Charles Darwin

Explanation: Charles Darwin explained the theory of evolution in his book 'The Origin of Species by Natural Selection'.

3. Which of the following was the contribution of Hugo de Vries?

a) Theory of mutation b) Theory of natural Selection c) Theory of inheritance of acquired characters d) Germplasm theory

Answer: a) Theory of mutation

Explanation: Hugo de Vries put forth the Mutation theory. He carried out his experiments in the Evening Primrose plant (Oenothera lamarckiana) and observed variations in them due to mutation.

4. The wings of birds and butterflies is an example of

a) Adaptive radiation b) convergent evolution c) divergent evolution d) variation

Answer: b) convergent evolution

Explanation: Organisms having different structural patterns but similar function are termed as analogous structures — for example, the wings of birds and insects are different structurally but perform the same function of flight that brings about convergent evolution.

5. The phenomenon of "Industrial Melanism" demonstrates

a) Natural selection b) induced mutation c) reproductive isolation d) geographical isolation

Answer: a) Natural selection

Explanation: Natural selection can be explained clearly through industrial melanism. Industrial melanism is a classical case of Natural selection exhibited by the peppered moth, Biston betularia.

6. Darwin's finches are an excellent example of

a) connecting links b) seasonal migration c) adaptive radiation d) parasitism

Answer: c) adaptive radiation

Explanation: Darwin's finches and Australian marsupials are best examples for adaptive radiation. Darwin's finches have evolved into 14 recognized species differing in body size, beak shape and feeding behavior, driven by Natural selection.

7. Who proposed the Germplasm theory?

a) Darwin b) August Weismann c) Lamarck d) Alfred Wallace

Answer: b) August Weismann

Explanation: Weismann proved his germplasm theory that change in the somatoplasm will not be transferred to the next generation but changes in the germplasm will be inherited.

8. The age of fossils can be determined by

a) electron microscope b) weighing the fossils c) carbon dating d) analysis of bones

Answer: c) carbon dating

Explanation: Absolute dating is used to determine the precise age of a fossil by using radiometric dating to measure the decay of isotopes.

9. Fossils are generally found in

a) igneous rocks b) metamorphic rocks c) volcanic rocks d) sedimentary rocks

Answer: d) sedimentary rocks

Explanation: Fossilization is the process by which plant and animal remains are preserved in sedimentary rocks.

10. Evolutionary history of an organism is called

a) ancestry b) ontogeny c) phylogeny d) paleontology

Answer: c) phylogeny

Explanation: Ernst Von Haeckel propounded the biogenetic law which states that the life history of an individual (ontogeny) briefly recapitulates the evolutionary history of the race (phylogeny).

11. The golden age of reptiles was

a) Mesozoic era b) Cenozoic era c) Paleozoic era d) Proterozoic era

Answer: a) Mesozoic era

Explanation: Mesozoic era (dominance of reptiles) is called the Golden age of reptiles, and is divided into three periods namely Triassic, Jurassic and Cretaceous.

12. Which period was called "Age of fishes"?

a) Permian b) Triassic c) Devonian d) Ordovician

Answer: c) Devonian

Explanation: The Devonian period is called the Age of fishes; during this period many types of fishes such as lung fishes, lobe finned fishes and ray finned fishes appeared.

13. Modern man belongs to which period?

a) Quaternary b) Cretaceous c) Silurian d) Cambrian

Answer: a) Quaternary

Explanation: According to the geological time scale, the Quaternary period (Pleistocene epoch) is the Age of Human beings, and Homo sapiens or modern human arose during this era.

14. The Neanderthal man had the brain capacity of

a) 650 to 800 cc b) 1200cc c) 900cc d) 1400cc

Answer: d) 1400cc

Explanation: Neanderthal human was found in Neander Valley, Germany with a brain size of 1400 cc and lived between 34,000 to 1,00,000 years ago.

15. According to Darwin, the organic evolution is due to

a) Intraspecific competition b) Interspecific competition c) Competition within closely related species d) Reduced feeding efficiency in one species due to the presence of interfering species

Answer: b) Interspecific competition

Explanation: Darwin described struggle for existence including inter specific struggle with different species for food and space, and natural selection acting on these variations leads to organic evolution.

16. A population will not exist in Hardy-Weinberg equilibrium if

a) Individuals mate selectively b) There are no mutations c) There is no migration d) The population is large

Answer: a) Individuals mate selectively

Explanation: Hardy-Weinberg assumptions include random mating — every organism gets a chance to mate randomly with no preferences for a particular genotype. If mating is selective (not random), the population will not be in Hardy-Weinberg equilibrium.

II. Answer Briefly

17. List out the major gases seen to be found in the primitive earth.

Answer: The primitive earth had no proper atmosphere, but consisted of ammonia, methane, hydrogen and water vapour. The temperature of the earth was extremely high. UV rays from the sun split up water molecules into hydrogen and oxygen. Ammonia and methane combined with oxygen to form carbon dioxide and other gases.

18. Explain the three major categories in which fossilization occur.

Answer: The three major categories of fossilization are: (1) Actual remains – original hard parts such as bones, teeth or shells are preserved as such in the earth's crust (e.g., Woolly Mammoth preserved in frozen Siberia). (2) Petrifaction – original parts of the body are replaced molecule by molecule by minerals such as silica, calcium carbonate, resulting in a mineral replica. (3) Natural moulds and casts – after disintegration, body impressions left on soft mud harden into stones (moulds); cavities filled by hard minerals form casts. Hardened faecal matter (coprolites) also occur as fossils.

19. Differentiate between divergent evolution and convergent evolution with one example for each.

Answer: Divergent evolution refers to structures which are similar in origin but perform different functions (homologous organs), e.g., forelimbs of vertebrates (humerus, radius, ulna) which have the same basic bone plan but serve different functions in different animals. Convergent evolution refers to organisms having different structural patterns but performing similar functions (analogous organs), e.g., wings of birds and insects are structurally different but both are used for flight.

20. How does Hardy-Weinberg's expression (p² + 2pq + q² = 1) explain that genetic equilibrium is maintained in a population? List any four factors that can disturb the genetic equilibrium.

Answer: Hardy and Weinberg stated that allele frequencies in a population are stable and constant from generation to generation. If 'p' is the frequency of dominant allele 'A' and 'q' is the frequency of recessive allele 'a', then p + q = 1. The genotype frequencies are: p² (AA) + 2pq (Aa) + q² (aa) = 1. As long as the assumptions hold, frequencies remain unchanged, meaning the population is not evolving. Four factors that disturb genetic equilibrium are: (1) Mutation, (2) Gene flow (migration), (3) Genetic drift, (4) Natural selection (also: non-random mating).

21. Explain how mutations, natural selection and genetic drift affect Hardy Weinberg equilibrium.

Answer: Mutations generate new alleles or alter gene frequencies, disturbing the equilibrium. Though mutation rate is low, it is the original source of all genetic variation. Natural selection favours certain alleles, causing their frequencies to increase while reducing others over generations, thus shifting allele frequencies away from equilibrium. Genetic drift causes random changes in allele frequencies especially in small populations (bottle neck effect and founder's effect), leading to loss of some alleles and fixation of others, thereby disturbing Hardy-Weinberg equilibrium.

22. How did Darwin explain fitness of organisms?

Answer: Darwin believed that the struggle for existence resulted in the survival of the fittest. He noted that no two individuals are alike and useful variations help organisms overcome the struggle for existence. Nature acts as the most powerful selective force — organisms with useful variations become better adapted to the changed environment, survive, and pass on these variations to the next generation, while unfit organisms perish.

23. Mention the main objections to Darwinism.

Answer: The main objections raised against Darwinism were: (1) Darwin failed to explain the mechanism of variation. (2) Darwinism explains the survival of the fittest but not the arrival of the fittest. (3) He focused on small fluctuating variations that are mostly non-heritable. (4) He did not distinguish between somatic and germinal variations. (5) He could not explain the occurrence of vestigial organs, over-specialization of some organs like large tusks in extinct mammoths and oversized antlers in extinct Irish deer.

24. Taking the example of Peppered moth, explain the action of natural selection. What do you call the above phenomenon?

Answer: The peppered moth Biston betularia exists in two forms — white winged and black winged. Before industrialization, more white-winged moths existed because white-coloured lichens on trees camouflaged them from predators. Post-industrialization, tree trunks became dark due to smoke and soot, and lichen population decreased. White-winged moths became easily visible to predators, while black-winged moths were camouflaged against the dark background. Nature offered positive selection pressure to black coloured moths, increasing their population. This phenomenon is called Industrial melanism.

25. Darwin's finches and Australian marsupials are suitable examples of adaptive radiation. Justify the statement.

Answer: Darwin's finches: Their common ancestor arrived on the Galapagos about 2 million years ago. They evolved into 14 recognized species differing in body size, beak shape and feeding behavior, adapting to different food resources such as insects, seeds, nectar and blood — driven by Natural selection. This exemplifies adaptive radiation from a single ancestor. Australian marsupials: Over 200 species of marsupials have undergone adaptive radiation to occupy diverse habitats in Australia, just as placental mammals radiated across North America, despite being separated from a common ancestor more than 100 million years ago.

26. Who disproved Lamarck's Theory of acquired characters? How?

Answer: August Weismann disproved Lamarck's Theory of acquired characters. He conducted experiments on mice for twenty generations by cutting their tails and breeding them. All mice born in every generation had tails. This proved that acquired characters (tail removal) are not inherited. Weismann proved through his germplasm theory that changes in the somatoplasm will not be transferred to the next generation; only changes in the germplasm will be inherited.

27. How does Mutation theory of De Vries differ from Lamarck and Darwin's view in the origin of new species?

Answer: According to De Vries, sudden and large mutations (saltation) were responsible for the origin of new species. He believed mutations are random and directionless. In contrast, Lamarck and Darwin believed in gradual accumulation of all variations as the causative factors in the origin of new species. Darwin believed variations are small and directional. De Vries also stated that mutations produce no intermediate forms and are fully fledged, while Darwin's view involved continuous, slow change through natural selection over generations.

28. Explain stabilizing, directional and disruptive selection with examples.

Answer: Stabilising Selection (centripetal): Operates in a stable environment. Organisms with average phenotypes survive while extreme individuals from both ends are eliminated. No speciation occurs but phenotypic stability is maintained. Example: Measurements of sparrows that survived a storm clustered around the mean. Directional Selection: Occurs in a gradually changing environment; removes individuals from one end towards the other end of phenotypic distribution. Example: Body weight differences between male and female sparrows. Disruptive Selection (centrifugal): When homogeneous environment changes into heterogeneous environment; extreme phenotypes are selected while average phenotypes are eliminated, resulting in new species. Example: Darwin's finches' beak size in relation to seed size on the Galapagos islands.

29. Rearrange the descent in human evolution: Australopithecus → Homo erectus → Homo sapiens → Ramapithecus → Homo habilis.

Answer: The correct sequence of human evolution is: Ramapithecus → Australopithecus → Homo habilis → Homo erectus → Homo sapiens. Ramapithecus lived about 14 mya, Australopithecus about 5 mya, Homo habilis about 2 mya, Homo erectus about 1.7 mya, and Homo sapiens arose about 25,000 years ago.

30. How does Neanderthal man differ from the modern man in appearance?

Answer: Neanderthal man differs from modern man in the following features: semi-erect posture, flat cranium, sloping forehead, thin large orbits, heavy brow ridges, protruding jaws and no chin. Neanderthal had a brain size of 1400 cc. Modern man (Homo sapiens) has a brain capacity of 1300 to 1600 cc, fully erect posture, a high forehead, a distinct chin and no heavy brow ridges. Neanderthals also did not practice agriculture and animal domestication, while modern man cultivated crops and domesticated animals.

III. Answer in Detail

31. Define isolating mechanism and explain its types with suitable examples.

Answer: Isolation is the separation of members of a single population into sub-populations so that genetic integrity of the sub-population can be maintained. An isolating barrier is any evolved character of two species that stops them from interbreeding. There are two major types:

Prezygotic mechanisms (prevent contact before fertilization): (1) Ecological isolation – members separated by different habitats; e.g., Rana areolata breeds in grassy shallow ponds while Rana grylio breeds in deep waters. (2) Seasonal isolation – difference in breeding seasons; e.g., Bufo americanus breeds early in spring while Bufo fowleri breeds late in season. (3) Ethological/Sexual isolation – difference in sexual behavior; e.g., different mating calls of Hyla versicolor and Hyla femoralis prevent interbreeding. (4) Morphological isolation – differences in external genitalia; e.g., size difference between Bufo quercicus and Bufo valliceps. (5) Physiological isolation – gametes prevented from fertilization; e.g., sperms of Drosophila virilis survive only one day in Drosophila americana. (6) Cytological isolation – differences in chromosome numbers prevent fertilization; e.g., Rana catesbiana and Rana areolata.

Post-zygotic mechanisms (act after fertilization): (1) Hybrid inviability – embryo develops but dies before maturity; e.g., certain fish and frog inter-specific crosses. (2) Hybrid sterility – hybrids formed but are sterile; e.g., Mule (horse × donkey). (3) Hybrid breakdown – F1 hybrids are viable and fertile but F2 hybrids are inviable or sterile.

32. Define speciation according to A.E. Emerson and explain its types giving suitable examples.

Answer: A.E. Emerson defines species as a 'genetically distinctive, reproductively isolated natural population'. The process by which one species evolves into one or more different species is called speciation. There are two major types:

Sympatric speciation (Reproductive isolation): New species form from a single ancestral species while

📝 Summary

1. Evolution refers to heritable changes in one or more characteristics of a population from one generation to the next, and the present state of mankind is the outcome of three kinds of evolution: chemical, organic, and social or cultural evolution.
2. The earth is estimated to be around 4.5 to 4.6 billion years old based on radiometric dating of meteorites, and the primitive earth had no proper atmosphere but consisted of ammonia, methane, hydrogen, and water vapour at extremely high temperatures.
3. Several theories explain the origin of life, including the Theory of Special Creation, Spontaneous Generation (Abiogenesis — coined by Thomas Huxley), Biogenesis (coined by Henry Bastian), and the Theory of Chemical Evolution proposed by Oparin in 1924.
4. Oparin proposed that organic compounds underwent a series of reactions leading to complex molecules that formed colloidal aggregates called coacervates in an aqueous environment, which are considered the first pre-cells that gradually transformed into living cells.
5. The chapter covers key topics including the geological time scale, evidences of biological evolution (anatomical, embryological, and geological), mechanisms of evolution, Hardy-Weinberg principle, isolating mechanisms, speciation, and the origin and evolution of man.