INTRODUCTION: Living beings originated on earth. But when did this life originate and how? These questions are answered in this chapter. So, to study history of life, two events are studied: -
(i) Origin of Life
(ii) Evolution of Life
(1.1) ORIGIN OF LIFE:Life was present on earth about 3.9 billion years ago because some rocks which are 3.9 billion years old have carbonates which resulted from life processes only. The oldest rock (4.3 billion years ago) didn't show any signs of life. There are many theories on origin of life.
(1) Theory of Special Creation:Proposed by Spanish monk Father Suarez. It states that life originated on earth from some supernatural force of God in the same form as they exist today. Life was created by God in six days. 1st day- earth & Heaven. 2nd day- sky and water. 3rd day- Lands & plants. 4rth day- sun, moon, stars. 5th day- fish & birds. 6th day-man (Adam), woman (Eve) and other animals.
(2) Theory of Spontaneous generation: (Theory of Abiogenesis) –Proposed by Anaximander (611-541 B.C.) and Aristotle (384-322 B.C.). It states that life originates spontaneously from non-living matter like rain, mud, air, dung etc. But Louis Pasteur (1862) proved that new microorganisms arise from pre-existing microorganisms.This rejects Abiogenesis theory and states that new life arises from pre existing life only.
(3) Cosmozoic Theory : ( Interplanetary Theory/ Theory of Panspermia): Given by Richter(1865) and Helmholtz(1884). Supported by Arrhenius (1908). It states that life originated in the universe and reached the earth from some other heavenly body in the form of seeds (sperms) or spores in meteorites or spaceships. On earth, these found fertile soil and grew into organisms.But this theory is not accepted because when spores travel from universe to earth, these cannot survive extreme conditions of interplanetary space like extreme cold, dryness, intense cosmic and u.v. radiations.
(4) Modern Theory/ Naturalistic Theory/Theory of Chemical origin/ Terrestrial origin
Chemosynthetic Theory :Formulated by Haeckel. Supported by Oparin (1923) and Haldane (1928). Most favoured theory. It states that life originated from inorganic substances through series of reactions. Organic molecules collected and formed life. This means chemical evolution precedes biological evolution and this process involves following events: -
(a) Origin of Earth: -To study origin of life, origin of earth must be known. According to Big-Bang theory, there was a big thermonuclear explosion of a dense entity about 15 billion years ago. Universe originated from that. About 4.5 billion years ago, solar system was formed from gaseous cloud Solar Nebula. Nebula kept on collapsing till it became flat disc of atoms and particles. Its central part became very hot & formed sun. Atoms and dust moving around the center collected to form clumps. Clumps further aggregated with other particles, grew in size and became planets. Earth had solid part-Lithosphere and gaseous part- Atmosphere. Liquid part-Hydrosphere was formed when earth cooled to temperature below 100 degrees centigrade.
(b) Chemical origin of life: -Given by Oparin (1923) and Haldane (1928). It states that organic molecules and life were formed from simple compounds through series of chemical reactions in ocean about 4200 million years ago. This theory could be tested and explained, so it was the widely accepted theory. Successive steps of chemical origin of life are:
(i) Formation of early molecules: -
-First living things evolved from non-living things.
-Spontaneous generation of life under present environmental conditions i.e. oxidizing
is not possible. So, at that time environmental conditions at that time were reducing type (without free oxygen) and not oxidizing type.
-In primitive atmosphere, oxygen was not present in free gaseous form (O2) but occurred as bound in water or as oxides of metal (pyrites, uranites etc.)
-The early atmosphere was rich in hydrogen occurring as CH4, NH3 and H2O.
-Atmospheric water vapour condensed into water drops and fell as rain. It rolled down on surfaces of rocks, accumulated at a place and formed pools and oceans. During this, minerals washed from rock surfaces into pools and oceans and dissolved in it.
(ii) Formation of simple organic molecules: -
-- Solar radiations, heat radiations from lightening, heat from volcanic activities, cosmic rays provided energy for chemical reactions.
-- Accumulated water was termed as “Hot Dilute Soup” (Haldane) / Warm Little Pond (Darwin)/ primordial soup (Oparin). It was hot, alkaline (pH=8) and rich in amino acids fats, sugar, and nitrogenous bases. It acted as stage for combination of chemical elements and setting of chemical reactions.
-- Amino acids, fats, sugar and nitrogenous bases combined to form proteins, nucleic acids polysaccharides etc.
Evidence in support of Oparin-Haldane Theory-
--Miller and Urey Experiment- Harrold C. Urey (1893-1981) and his student Stanley Miller (1953) successfully experimented Oparin-Haldane theory. They simulated it. Miller put H2O, CH4, NH3 and H2 in a spark chamber. For source of energy- they used two ele-ctrodes to provide electrical energy. For evaporation- He connected the spark chamber to another flask with arrangement for boiling water. For rain and Haldane's soup-He connected the other end of spark chamber to a trap by a tube that passed through condenser .The trap was then connected with a flask for boiling water to provide circulation.
'Control'- Control experiment was set up which contained everything except energy source.
Results- After 18 days, good amount of organic compounds like amino acids, other monomers appeared in the aqueous sample whereas in Control apparatus insignificant amount of organic compounds were seen.
(iii) Formation of complex organic molecules (Polymerization): -
The small and simpler molecules combined to form larger and complex molecules. e.g.
Simple sugars formed complex sugars (starch), amino acids formed Proteins, fatty acids and Glycerol formed Fats.
--These chemicals occurred in 'hot dilute soup' or 'broth'.
--For polymerization to go on, two requirements should be met
a) Adequate source of energy (must have been provided from sun, lightening, ATP)
b) Removal of water from surface of reactants so that they can concentrate and polymerise further. It also might have been provided by evaporation from solar heat.
(iv) Formation and interaction of macromolecules: -
Miller's experiment suggests that prebiotic molecules must have formed under the
conditions present at that time. But formation of molecules doesn't mean origin of life.
--Three conditions should be fulfilled for origin of life
a) Continuous supply of self producing molecules.
b) Replicators should mutate.
c) Supply of free energy and partial isolation from general environment is must.
--T.Cech and Atman proposed RNA as first macromolecule as it can act as a substrate & an enzyme.
--S.Fox (1957) suggested Protenoid (formed from heating of amino acids) as first macro-molecule.
(v) First cell like structures/ Protocells: -
These self-replicating molecules are called Protobionts/Ebionts.
--These showed mutations because of high temperature in early earth atmosphere.
--Two main Protobionts are Coacervates and Micro spheres.
--Coacervates are the colloidal aggregates formed by chemicals under proper temperature 'pH and ionization. Oparin (1924) observed that when a mixture of large protein and a polysaccharide is shaken, coacervates are formed. Their interior has protein, polysaccharide and water. It is separated from outer aqueous solution, which has very less Protein and polysaccharide. But these coacervates don't have lipid membrane and cannot reproduce. So these cannot be considered as precursors of life.
--When artificially produced organic compounds are mixed with cool water, micro spheres are formed. If mixture has lipids, then micro spheres formed have lipid membrane. Sydney Fox (1950) heated mixture of dry amino acids between 130-180 degrees centigrade and then cooled it in water. Protenoid micro spheres were formed (1-2 micron meter in diameter). These can form new Protenoid by budding induced artificially. But again, these have limited diversity so cannot be wholly regarded as precursors of life.
--First cell like structures were self-duplicating nucleic acids, water and other organic or inorganic molecules.
--Protobionts had chromophores (molecules that absorbed light), which made them energy transducing devices.
--Early life (Protolife) was mainly driven by solar energy.
--Proteins acted as enzymes to form DNA genes.
--First cells used RNA as genetic material.
--DNA evolved from RNA template.
(vi) First Living Beings: -
--First cell acquires reproducing characters and becomes first living being.
--First living beings had naked DNA and RNA as genetic material.
--The earliest living organisms were Anaerobes, which depended upon existing organic molecules for their nutrition. So these were Heterotrophs.
--As gradually, supply of organic molecules diminished, heterotrophs would have changed into autotrophs.
(vii) Development of Chemoautotrophs: -
--Heterotrophs consumed food (chemical compounds). This resulted in formation of org-anisms, which synthesized their own food. They obtained energy for food synthesis from degradation of organic compounds. Such organisms were called Chemoautotrophs.
--Chemoautotrophs synthesized their own food by process called-Chemo synthesis.
--E.g. of Chemoautotrophs are Bacteria, Sulphur Bacteria.
(viii) Development of Photoautotrophs: -
--Organisms developed Chloroplasts, which absorbed light energy.
--These then performed Photosynthesis and made their own food. So, these were called as Photoautotrophs.
--Photosynthesis released oxygen. So, the atmosphere became oxidizing type from reducing type.
--This change in atmosphere initiated evolution of life towards diversities.
(1.2) EVOLUTION :-
term 'Evolution' was first given by Herbert Spencer. It is a process of slow and gradual change. But observers tell that diverse forms have common characters pointing to common ancestory. E.g. Mammals & Amphibians have limbs for locomotion, birds have wings for flying, and Fish have fins for swimming.
According to Darwin, Evolution is Descent with modification. It may be 'Progressive' (Simpler to complex forms) or 'Retrogressive' (complex to simpler forms).
(1.3) IDEAS ON EVOLUTION: -
(i) Ancient Indian Thought: -Ancient Indians studied Evolution. Thoughts about Evolution are given in Manu Samhita/ Manu Smriti (200 A.D.). Ayurveda deals with origin of life.
(ii) Ancient Greek Thoughts: -First theories of Evolution came about 2000 years before Darwin.
--According to Plato (428-348 BC), God is perfect and everything on earth are His ideas. Each species was an unchanging ideal form (eidos).
--Aristotle (384-322 BC) put Plato's concept in a chain like series of form. He called it 'Scala Naturae' or 'Ladder of Nature'. This series was from imperfect to perfect species.
(1.4) EVIDENCES OF EVOLUTION:-
There are strong evidences of organic evolution due to which it is accepted. The evidences are obtained from studying different aspects of life of plants and animals.
(a) Biogeographical evidences: - Biogeography is the study of distribution of species. Some species are restricted to a particular geographical area and are found nowhere else. These are called 'Endemic'. Some species are found everywhere and are called 'Cosmopolitan'.
--Darwin studied evolution during his sea voyage in Galapagos Islands (group of 22 different islands, lies on equator, are 900 km. off west coast of south America). He observed unusual plants, insects, lizards, and giant tortoises, meter long Iguanas and seashells. There were no woodpeckers and insect eating warblers. But variety of Finches were present which were originally seed eating but became insect eating on island. These were different type of Finches with different type of beaks but all originated from a common ancestral seed eating stock of Finches. They radiated to different geographical areas, underwent adaptations in beaks and emerged as new species. This is called 'Adaptive Radiation'.
--Darwin studied marsupials in Australia. Some marsupials resemble other placental animals living in other continents, which have similar habitat as Australia. Natural Selection favoured those changes, which made these two groups alike. This is called 'Parallel' or 'Convergent' Evolution.
(b) Anatomical evidences: - The similarities in anatomical structure prove descent from a common ancestor.
(i) Homologous Organs: - are the organs having common origin but performing differentfunctions. Term 'Homologous organs' was given by Richard Owen (1804-1892).
E.g. Forelimbs of bat, humans, whales, birds have same structural plan.
E.g. Thorns of Bougainvillea and tendrils of Cucurbita are located in same axillary position. E.g. Brain in different vertebrates has homologous structure. E.g. Heart in all groups have same basic plan but it has been criticized also saying that heart has undergone evolution from mammals to fishes. Fish heart has two chambers (1 auricle, 1 ventricle).
When fish changed to amphibians (on land), mode of respiration also changed. Separation of oxygenated and deoxygenated blood became necessary. So heart became three chambered (2 auricles, 1 ventricle). Amphibians changed to reptiles, which have heart with two auricles and an incompletely divided ventricle. In birds and mammals, ventricle became completely divided and heart became four chambered.
(ii) Analogous Organs: - Organs with similar function but different structure and origin.
E.g.-Wings of birds and wings of butterfly, Eyes of octopus and eyes of mammals, Flippers of Penguin and Dolphin, Tuberous root of sweet Potato and underground stem of Potato.
(iii) Vestigial Organs: - Organs, which are, present in rudimentary form in the body but do not perform any function. Adaptations of organisms to new environment made these structures useless. E.g. Vermiform appendix in humans, Ear pinna, Jaw apparatus in reptiles, Nictitating membrane of eye, Wisdom tooth.
(c) Embryological evidences: - These indicate that embryos of all vertebrates have similarity. They have many features, which are absent in adults. Similarities are-
(i) Similar early development- development in all starts from Zygote →Morula→ Blastula→ Gastrula → Adults.
(ii) Resemblance in Embryo- Early embryos of all vertebrates resemble in shape and str-ucture. They have same head with eye/ear rudiments, visceral arches, embryonic tail etc. Embryos of closely related vertebrates resemble more. This shows that all vertebrates have evolved from a common ancestor.
(iii) Recapitulation Theory –von Baer (1826) gave von Baer's Law. Later, Ernst Haeckel called it as Biogenetic law (1866). This law states that Ontogeny (the ancestral sequence) is recapitulation/repetition of Phylogeny. Von Baer disapproved the Biogenetic law. He noted that embryos never pass through ad-ult stages of other animals. But recapitulation has importance.E.g. Seedlings of Acacia tree first develop simple leaves, which later change into compound leaves.E.g. Modern day Oaks of southern U.S.keep their foliage throughout year but Oaks of northern U.S. shed their leaves in winter.
(d) Palaentological evidences: - Paleontology is the study of fossils. Fossils are the pres-erved remains of the organisms that lived in the past. Organism or body part of organism gets preserved when it buries under mud or rocks where decay doesn't take place. Foot-prints, skeletal remains, organic molecules serve as fossils. Age of fossil can be found out by finding age of the rock where it is present. Rocks in deeper strata are older than rocks in upper strata. Amber is best material for fossilization. Absolute Dating is the method in which rocks are dated by measuring the degree of decay of radioactive isotopes, which were inside the rock when it was formed. By Dating, Geological Time Scale was prepared. It has five eras, which are subdivided into periods that refer to stages of evolution of life. When we arrange fossils (from older to younger), we get evidences for successive changes in organism. First, there were Fish fossils, then amphibians, then reptiles, next mammals and birds. All major group of vertebrates are linked. Sometimes older fossils are linked to modern species by transitional forms. Vascular plants have left very few fossils, which have gaps also. There are few lineages also that appear to arise suddenly in fossil records and stayed as such for hundreds of million of years like sporangia (contain spores) are present at tip of the plant in primitive Psilopsida. These are placed at the base of leaf in modern Lycopsida. These are arranged on whorls on top of plant in Sphinopsida
This arrangement has been there for more than 350 million years.
A Lineage is evolutionary sequence arranged in linear order from ancestral group to descendent group. The continuous change of characters within an evolving lineage is 'Ev-olutionary Trend'. The number of trends in any lineage = number of characters evolving.
A trend can be 'Progressive' (increase in organ size) or 'Retrogressive' (decrease or degeneration of organ). E.g. Evolution of horse reveals number of trends. Evolutionary trend among horses was not universal. Direction and speed of evolution are subject to variation. Major evolutionary trend of horses are-
-Increase in size, loss of toes, lengthening of toes, lengthening of limbs, increase in height, brain enlargement, change in Dental pattern.
(e) Morphological Evidences: -There is similarity in external structure of organisms, which shows a common ancestory. E.g. In plants, there is structural similarity in having roots, stem, leaves, flowers etc.
(1.5) THEORIES OF EVOLUTION: -
Many evidences have proved that evolution occurs but how does it occur or what is the mechanism of evolution has been explained by four main theories.
(1) Lamarckism:-Put forth by French Biologist Lamarck (1744-1829). He published it in his book'Philosophique Zoologique'in 1809. Lamarck arranged fossils in chronological order (from older to younger form) and he could see various lineages also. According to his theory-
(i)Living organisms and their body parts continually increase in size.
(ii) New organs develop due to new needs and are maintained too.
(iii) Continuous use of an organ leads to its development and disuse causes its reduction.
(iv) Acquired characters due to use and disuse of an organ are passed to the offsprings.
Explanation of theory: - This theory was explained by certain examples like-
(i) Giraffe: - Earlier, giraffe used to feed on ground but when grass fell and lower foliage disappeared, they had to stretch their neck and eat upper foliage. This resulted in their long neck, which was passed on to next generation.
(ii) Aquatic birds: - these have arisen from terrestrial ancestors. They had to go into water for food and protection. For that, they stretched their skin and spread their toes. Gradually web developed between toes.
(iii) Deer: - Speed of deer is due to continuous running for protecting itself from enemies.
Criticism of theory: - Lamarck's theory was successful in explaining evolution but it proved faulty later and was not accepted fully.
--First proposition of increase in size was not correct. Evolution shows reduction in size.
E.g. Trees are primitive and herbs/shrubs have evolved from trees.
--Second proposition is also false. E.g. wings cannot be developed if we wish to fly.
--Third proposition has truth and is correct. This can be explained by examples of Giraffe, Deer, Aquatic birds given above.
--Fourth proposition of inheritance of acquired characters is not correct. But there are evidences against it and favour of it.
Evidences against it were given by German Biologist August Weismann (1834-1914). He put forth his theory of continuity of Germplasm. He said that each animal has 'Germ Cells' and 'Somatic cells'. Any change in somatic cells is not heritable whereas changes in Germ cells are passed on to next generation. Use and disuse of organs effects Somatic cells only, which means acquired characters, reside in somatic cells. Thus these are not inheritable. He gave some examples also. e.g. In rats, he cut their tail for 80 generations but tailless rats were never born. E.g. Women have been piercing their external pinna for thousands of years but it has never been inherited.
Evidences in favour of it go against of Weismann's theory and show that acquired characters are inherited to some extent through somatic cells also. Packard, Spencer, Mcdougall removed objections against Lamarckism. This revived the discarded Lamarckism and modified 'Neo-Lamarckism' was formed. E.g. Mcdougall trained rats to escape routes. It was found that number of errors made by rats decreased generation after generation.
Conclusion: - There are more evidences against than in favour of Lamarckism. So, this theory was not accepted.
(2) Darwinism: - Charles Robert Darwin (1809-1882) gave this theory. He went on voyage for five years, came back and published a book 'Origin of Species' (1859). Idea of natural selection struck his mind after examining Malthus work who wrote 'Principles of Population' stating that food supply increases in arithmetical progression and population increases in geometrical progression.
The main postulates of Darwin are: -
(i) Rapid Multiplication: - Each species produces offsprings in large numbers, multiplying in geometrical progression i.e. doubles in Ist year, increases to four times in 2nd year, eight times in 3rd year and so on. E.g. Paramecium divides three times in 48 hrs, Plants produce thousands of seeds each year.
(ii) Limited food and Space: -Because there is limited food and space; the population of a particular species remains constant.
(iii) Struggle for existence: -As there is limited food and space, offsprings compete severely for requirements. This is called 'Struggle for Existence'. It can be'Intraspecific' (Between individuals of same species. It is toughest as requirements are similar.) Or 'Interspecific' (between different species). “Environmental struggle' is the struggle with changes in environment.
(iv) Variations: - Struggle for existence among organisms changes them according to the conditions. These changes are called 'Variations'. Variations, which are most suited to the organism, are transmitted to next generation while unsuited variations are eliminated.
(v) Survival of fittest: -The organisms, which adapt to changes, survive.e.g. In drought,
Only those plants will survive which can maximize absorption of water. The competition is so severe that only few out of millions survive.
(vi) Inheritance of useful variations: - The individuals, which get selected by nature in struggle for existence, pass these variations to next generation.
(vii) Formation of new species: -New species arise by gradual modification of older ones. In each generation, new favourable variations appear and complement the already inherited variations. After number of generations, there are so many variations in an indi-vidual that it becomes a new species.
Criticism of Darwin's theory: - Darwin's theory is the most recognized theory today but it has evidences going in favour of it and against it.
Evidences in favour:
-Excessive multiplication, limited food and space, struggle for existence, variations, natu-ral selection can be easily seen in nature.
- Artificial Selection, which is done by man in breeding experiment, shows support for
Natural selection done by nature. In Breeding experiments, man also selects individuals
with desired traits and allows them to mate for creating a new breed of animals. If man
can do so with limited resources, then nature can easily produce new species with its vast resources.
-Position of nectars in flowers is related to proboscis of insects, which pollinate them. This cannot develop suddenly but evolved gradually by natural selection.
Evidences against: -
-According to Darwin, evolution starts with small variations i.e. organs appear as small structures first and then grow. But in case of organs like wings of birds, If they were small initially then they were useless. Then how could these be passed to next generation when these were not advantageous?
-Vestigial organs have no function, then why these exist generation after generation
-Tusks of elephant, antlers of deer have overdeveloped causing hindrance, then why these were selected.
- According to Darwin, process of evolution is linked with inheritance. But he could not explain the basis of variations and how these variations are passed to next generation. In 1868, he gave Theory of Pangenesis according to which every organ produces minute particles'Pangenes'which are carried by blood and collect in the gametes. But Weismann's Germplasm theory (1892) states that germ cells separate from somatic cells early in development. So changes in somatic cells cannot be inherited. This disapproves Darwin's theory.
(3) DeVries Mutation Theory :-Put forth by Hugo deVries in1901. Darwin couldn't explain origin of variations. Hugo deVries proposed Mutation Theory to explain it. His observations were on wild variety of Evening Primrose (Oenothera lamarckiana). He states that new species arise from preexisting ones suddenly because of 'mutations', which are discontinuous and inheritable variations. So, this is discontinuous and sudden process instead of continuous and gradual as told by Lamarck and Darwin. DeVries said mutations are raw materials for evolution. Natural selection selects useful mutations and eliminates harmful ones.
Features of deVries Theory: -
-New species originate as a result of discontinuous variations that appear suddenly i.e. mutations are raw material for evolution.
-Same mutations can appear in several individuals of a species.
-Mutations reoccur. Same mutations appear again and again and increase their chances of selection.
-Mutations can occur in all directions.
-Nature selects beneficial mutations and eliminates useless mutations.
-Evolution is discontinuous and jerky process.
Criticism: -This theory has also, evidences in favour and against it.
Evidences in favour: -
(i) MacDougal and Shull repeated deVries experiment & obtained same results.
(ii) Mutations occur in same way as told by deVries.e.g. Short-legged Ancon sheep appeared from ordinary sheep all of a sudden in 1891. It was beneficial to farmers, as it could not jump over fences. E.g. Hornless cattle were produced all of a sudden in 1889.
(iii) Oenothera lamarckiana has 14 chromosomes but its mutant varieties have 16, 20, 22,24,28,30 chromosomes. This shows that mutants are independent species.
(iv) Mutations are inheritable.
Evidences against: -
(i) Mutations occur rarely, so how could variety of animals and plants appear by variations.
(ii) Mimicry, Camouflage, relationship of nectar position with insect proboscis cannot appear suddenly.
(iii) Mutations are recessive but evolution occurs due to inheritance of dominant mutations.
Significance: - deVries Theory was accepted. Mutations were found to be inheritable. But Evolution cannot occur just because of mutations. There are other factors also.
(1.6) MODERN CONCEPT OF EVOLUTION: -Modern concept of evolution is modification and elaboration of Darwin Theory. So, it is also called as 'Neo-Darwinism'.
There are changes and diversification (evolution) of organism through geological time.
This is called 'Macroevolution'. It results in changes (evolution) in hereditary material i.e. at genetic level and this is called as 'Microevolution'. Modern view says that populations are units of evolution and natural selection is the mechanism of evolution.
Population Genetics: - Population is the group of individuals belonging to the same species in the same area at a particular time. Study of frequency of genes in a population is known as 'Population Genetics'. When relative frequencies of different DNA in a population change over time, evolution takes place.G.H.Hardy and W.Weinberg (1908) gave Hardy-Weinberg equilibrium/principle, which was a simple mathematical relationship to study gene frequencies. It is:
dominant genotype.
p + q = 1
(p + q)² = p² + q² + 2pq
where
p² = frequency of dominant genotype
p = dominant gene
q² = frequency of recessive genotype
q = recessive gene
2pq = frequency of heterozygous dominant genotype
Both scientists studied genetic structure in a population, which didn't show evolution.
They noted that under certain constant conditions, the gene frequencies also remain cons-tant in a population through generations. So, when this Hardy-Weinberg Eqbm. disturbs, Evolution takes place.
Sources of Variations :-
Individuals in a population are similar in many ways but they are different also in some ways. If a population has to evolve, then it should have variations. Variations can be at two levels: 
Natural Selection acts on genetic variations. Then it expresses itself in a phenotype. So,Evolution requires genetic variation. Evolutionary agents change the genotype frequency in a population and cause deviation from Hardy-Weinberg Eqbm. There are five basic processes that affect Hardy-Weinberg Eqbm. And cause variations.
(i)Mutations: According to Hugo deVries, mutation is a random and sudden heritable change. Mutation rate is very slow but can cause considerable genetic variation.
Joshua Lederberg and Esther Lederberg (1952) performed experiment to show that there are Preadaptive mutations. Preadaptive mutations are advantageous and express themselves only when organism exposes itself to new environment according to which it has to adapt. So, these mutations already occurred but got selected later.
Experiment: They tookE.coli culture and spread it on master Agar plate. Each cell formed a new colony. They took imprints of these colonies on a velvet cloth. Then a replica plate with this velvet cloth was taken. A new plate containing Penicillin as medium was also taken and imprints from replica plate were touched on it. Certain colonies did not appear. The colonies, which appeared, were only those, which were resistant to antibiotic Penicillin. This means resistance to Penicillin (a mutation) had already occurred even before culture was exposed to Penicillin.
(ii)Recombination :It causes evolution. In meiosis, genes combine during crossing over and new combinations are produced. These new combinations occurring at different time and places recombine to form new alleles, which cause evolution.
(iii)Gene Migration: Members of a population sometimes migrate to another population. If they breed with them, new alleles are formed. This is called Gene Migration. If inter- breeding populations are closely related, fertile hybrids are produced. So, gene flow takes place when individuals enter or exit a population. This causes Evolution.
(iv)Genetic drift: is the elimination of certain genes when a section of population migrates or dies due to sudden natural calamity.
When genetic drift occurs in a population derived from small band of colonizers called 'founders', then huge changes occur and a totally different new species is formed. This is called 'Founder Effect'. Sometimes there is drastic reduction in allele frequency due to any chance event and this reduces population, which is not able to re-establish itself. Such reduction in allele frequencies is called 'Genetic Bottleneck'.
(v)Natural Selection: This is the method of differential reproduction. which means that some members of a population have genes, which enable them to grow up and reproduce faster. Thus their number of offsprings increases in next generation. If this keeps on going for many generations, then the genes of individual producing more offsprings becomes predominant in a gene pool of a population. This changes gene frequency. Only those organisms, which are best adapted to environment, have more surviving off springs. This is called 'Directional Selection' also and leads to evolution.
(1.7) GENETIC BASIS OF ADAPTATION :-
According to Darwin, Natural selection leads to evolution. Genetic variations occur in an organism. If those are favoured by natural selection, adaptation takes place. If there is no genetic variation, there is no adaptation and there will be no evolution.
Industrial Melanism (Example of Natural Selection): - Peppered MothBiston bitularia lives in England and shows cryptic colouration. There are two types of moths –Light (gray coloured) and Dark (black coloured). In the beginning, there were more light varieties but during 1850-1950, the population of dark varieties increased specially in polluted industrial areas like Birmingham. This was Evolution. Biologist's explanation to this was-
In the beginning light moths rested on pale tree trunks. Their light colour camouflaged with tree trunk colour and provided them protection from enemies. But in industrial areas
There was more pollution. Black soot deposited on tree trunks and light moths became easily visible on dark trunks. So, to protect themselves, these light moths adapted and changed to dark (melanic) moths. This evolution of darker forms in response to industrial pollution is called 'Industrial Melanism'.
Bernard Kettlewell tested it in 1950. He reared dark and light varieties of moths in equal numbers. Then he released one group in polluted area (Birmingham) and other in Unpolluted area (Dorset). The results were-
In Birmingham – 19% light moth, 40% dark moths were recaptured.
In Dorset - 12.5% light moths and 6% dark moths were recaptured.
Reverse Evolution –' Clean Air Legislation' was passed in Britain in 1956, which reduced pollution. As a result of this, melanic moths also decreased in number. This was reverse Evolution. But genes for light moths did not disappear completely.
Natural Selection and Polymorphism :-
When two or more distinct forms are present in a population then that population is called 'Polymorphic'. E.g.1- ABO blood groups in humans. There are four forms-Type A, Type B, Type AB and Type O. e.g.2- Sickle Cell genes in humans. These genes cause sickle cell anaemia, which is lethal. Normal Haemoglobin gene is HbA, which mutates and changes to defective gene HbS. HbS makes RBCs sickle shaped and unable to carry oxygen, which causes death of the organism. Homozygotes with normal alleles (HbA/HbA) or with sickle cell alleles (HbS/HbS) suffer from malaria easily. Sickle shaped RBCs of heterozygous individuals (HbA/HbS) kill malarial parasite. Thus the individual heterozygous for defective haemoglobin fights malaria better than homozygous normal individual. Because of this, natural selection has favoured the occurrence of sickle cell genes in malaria-affected regions. Homozygotes (HbS/HbS) die but heterozygotes (HbA/HbS) reproduce and balance the loss. So, this is also called 'Balancing Selection'. It occurs rarely.
(1.8) SPECIATION: - Process of formation of new species from existing ones is called Speciation. It is of two types-
(a)Allopatric- When a population splits up into two or more populations because of geographical barriers like glaciers, ocean, mountain etc. e.g. Formation of new species of Darwin's Finches in Galapagos Islands.
(b)Sympatric- Formation of new species within a population without any geographical
Isolation. New species formed is reproductively isolated from parent species. This occurs when chromosomes fail to segregate during meiosis or replicate without under-going mitosis. E.g. Polyploidy- normal chromosome number multiplies.
Isolation:- Reproductive Isolation is the mechanism which stops the population of two different species from interbreeding. It maintains integrity of species by checking hybrid-isation. Factors, which prevent interbreeding between species, are calledIsolating Mechanisms'.
These are of two types-
(i)Pre-mating Isolating Mechanisms: - These operate before mating and are of many types depending upon the isolating mechanism involved.
a) Geographical- No interbreeding because of separation by physical barriers like sea, mountain, desert etc.
b) Genetic: -No interbreeding because of sterility.
c) Ecological: - Two species live in different habitat of same geographical area Thus, they cannot interbreed.
d) Temporal: - No interbreeding because of different breeding times in different species.
e) Behavioral: -No interbreeding due to different courtship and mating behaviours.
(ii) Post mating Isolating Mechanisms: -These operate after mating and are of following types.
a) Hybrid In viability-Mating between two species occurs, fertilization may occur, hybrid
may be produced but is not viable and dies later.
b) Hybrid Sterility – Mating occurs, fertilization occurs, hybrid is produced but is sterile.
So, no further reproduction takes place.
c) Hybrid Breakdown (F2): -Mating occurs, Fertilization occurs, vigourous F1hybrids are produced. Back Cross occurs. F2 hybrids are produced which are less fertile.
d) Incompatibility – Mating may occur but no fertilization or development takes place.
Significance of Reproductive Isolation: - It is essential for formation of new species. But hybrids can be produced in laboratory between species that do not interbreed in nature. E.g. Mule (sterile)- Male Donkey x Mare (female horse)
E.g. Hinny (sterile)- Stallion (male horse) x female Donkey
E.g. Tigons (fertile)- Asian Tigers x African Lioness.
(1.9) SPECIES CONCEPT:-
Species is the basic unit of classification. Taxonomists have defined species in many ways. According to Davis and Heywood, species is assemblage of individuals with morphological features in common and separable from other such assemblages by corre-lated morphological discontinuities in a number of features. Lumper combines population into single species or group. Splitter separates population into different species or group. According to many biologists, there is a biological species concept according to which a species can be defined as sexually interbreeding group of individuals separated from other species by reproductive isolation. Members of same species can interbreed and produce fertile offsprings. But members of different species cannot inter-breed.
Sibling Species: - two species, which are morphologically identical but cannot interbreed, are called as sibling species. E.g.Drosophila pseudoobscura andDrosophila persimilis.
Polytypic Species: - When different species which are morphologically and geographica-lly separated are put under single group/species based upon their cross fertilization then that group/species is called Polytypic species. E.g. various species of North American sparrows are united with multiple geographical races of song sparrows. But concept of polytypic species is not valid as some organisms are asexual also.
Evolutionary species concept: -Many biologists gave this concept and defined species on the basis of evolutionary isolation and not sexual isolation. According to George Simpson(1961), an evolutionary species is a lineage evolving separately from others and
With its own evolutionary role and tendencies. In this method, many factors like genetic, Morphological, behavioral, ecological are considered but still it does not provide solution to all problems.
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