· Many theories have been put forward by many scientists to explain the mechanism of evolution.
· Some of the important theories are given below.
1. Lamarckism or Lamarck's theory or Theory of Inheritance of Acquired characters
2. Darwinism or Darwin’s theory or Theory of Natural selection
3. Mutation theory of Hugo de Vries
4. Modern synthetic theory or Neo-Darwinism or Modern Concept of Evolution

1. Lamarckism or Lamarck's theory or Theory of Inheritance of Acquired Characters (1744-1829 AD)
· Lamarck, the great French biologist, put forward the theory of evolution for the first time.
· His famous book 'Philosophie Zoologique' (Zoological Philosophies) was published in 1809 AD (the same year Darwin was born.) in which he discussed his theory in detail.
· He coined the terms invertebrates and Annelida.
· The term Biology was given by Lamarck and Treviranus in 1802 AD.
· Main theme of Lamarck's theory is that changes in an environment lead to changing patterns in species.
· Lamarck’s theory was most severely criticized by Cuvier who greeted it by calling it ‘nouvelle folic’ (foolish).

A. Lamarck's Postulates
a. Tendency to grow
· Body parts of an organism always tend to grow up to a certain limit.
· The appearance of new structures is due to the inner will/wish/want/desire of an organism.
· It is also called the doctrine of desires or appetency.
b. Environmental Effect
· Environment influences modifications or creations of organs directly.
· Environment is changeable, so organisms have to be changed according to the environment.
· For this, they have to exert some special efforts like change in habits and behaviour.
· Example: Migration in fishes/birds etc.
c. Use and disuse of organs
· Development of an organ is directly proportional to its use and vice-versa.
· Newly changed environment demands the maximum use of some organs, so they get more developed, active, stronger and efficient, while some organs not used more, become vestigial.
· Example: long neck of giraffe, long and cylindrical body of snakes, webs on ducks leg is due to maximum use and vestigial organs due to no use.
d. Inheritance of Acquired characters
· Change in characters in the lifetime of an individual is called acquired character.
· Example: the muscular body of wrestlers/blacksmith, reduction in wing size in Ostrich, long neck of Giraffe.
· Acquired characters appear due to the use and disuse of organs.
· Acquired characters are accumulated in descendants and they become different from original stock over many generations.

B. Examples to support Lamarckism
· Present-day long-necked Giraffes were formed as a result of regular stretching of the neck to reach the higher levels of trees due to scarcity of grass. 
Actually, Giraffes who lived in African deserts had no vegetation except a few trees.
· Long and cylindrical body of present-day snakes is due to the regular stretching of their body in narrow caves. Loss of limbs is due to hindrance of limbs while crawling in burrows. They had to do it to be escaped from predatory mammals.
· Development of webs in hind limbs of ducks and frogs-active swimming.
· Reduction in wings of Ostrich and Kiwi- disuse of organs.
· Reduction of digit number from 4 to 1 in modern horse.
· The ancestors of carnivorous mammals like lions, tigers had normal claws for tearing the flesh of the prey. Preys gained in running, so they had to run faster for which the claws were a hindrance. Therefore they developed retractile claws.
· The submerged leaves of Ranunculus aquatilis are dissected while the emergent ones are simply lobed. When the plant is grown out of the water, all the leaves are undissected. In the submerged environment, all the leaves are dissected.
· Reduction of eyes in moles since they live underground and presence of vestigial organs due to no use.

C. Criticism on Lamarckism
· First postulate tendency to grow has no ground as there is no vital force in organisms that increases their body part.
· Regarding the second postulate, though the environment can affect animals it is doubtful that a new needs form a new structure.
· Third postulate is correct to some extent.
· Fourth postulate is disputed.
· German Biologist August Weismann, proposed the theory of continuity of germplasm in 1892AD. According to this, germ cells transmit the characters but somatic cells do not. Acquired characters have no effects on genes that will be lost after the death of an organism.
· Weismann cut off the tails of rats for about 22 generations but no single mouse was born without a tail. He kept Drosophila in total darkness for 76 generations but at last no, flies were born without eyes.
· Piercing of ear-lobe and boring of nostrils among Hindu women has been practised for many centuries but their children are never born with bored ear-lobe and nose.
· Wrestlers’ powerful muscles are never inherited.
· Eyes that are constantly and continuously used develop defects instead of being improved.
· Chinese women use iron shoes, but their babies have normal feet.
· Dull progeny of Noble Prize winners.

2. Neo- Lamarckism
· The modified and updated form of Lamarckism by T.H. Morgan, Nageli, Packwards, Gadow, Cope, Dali, MC Dougall and others.
· Neo-Lamarckism shows the direct and indirect effects of physical and chemical factors on the somatic as well as germ cells of living organisms.
· Due to changing environment, there is a change in the habits and habitat of organisms that makes organisms acquire new characteristics. These new characters are transmitted to their offspring and new species evolve over many generations.
· The main theme of Neo-Lamarckism is “Universality of adaptation”.
· Only those variations are passed on to the offspring which also affect germ cells or where somatic cells give rise to germ cells.
· According to Neo-Lamarckism, the body character is not a single attribute but the result of the interaction of environmental conditions and heredity.
· Environment plays an important role in the development of a particular character and may lead to well-marked changes in an organism.
· Of the many changes in the organisms, only a few (those which affect germ cells or somatic cells giving rise to germ cells) happen to be transmitted to the offspring.

Differences between Lamarckism and Neo-Lamarckism
Original theory given by Lamarck.Modified and updated version of Lamarckism.
It has stress on environmental effect, use and disuse of organs and inheritance of acquired charactersIt does not give stress to these factors.
The environment brings about conscious reactions in animals.It stresses the direct effect of changing environment on the organism.
Acquired characters are transferred to the next generation.Normally only those modifications are transferred to the next generation which influences germ cells or where somatic cells give rise to germ cells.

C. Darwinism or Darwin's Theory of Natural selection (1809-1882 AD)
· Charles Darwin, a popular naturalist of England, was born at Shrewsbury on twelfth Feb. 1809AD.
· He got an opportunity to take part in a voyage called HMS, Beagle in 1831AD commissioned by British Admiral for surveying around the southern hemisphere. The voyage (sea journey) lasted for five years (1831-1836AD) collecting flora and fauna of South America and the Galapagos Islands.
· The two books by which Darwin was greatly impressed were ‘Principles of Geology’ published in 1832AD by a British geologist, Charles Lyell (1797-1875AD) and 'An essay on the principles of population' (1799AD) by British political economist Thomas Robert Malthus (1766-1834AD).
· He observed many kinds of plants and animals especially finches and gigantic tortoises in different islands of the Archipelago (present Indonesia).
· Alfred Russel Wallace, another English naturalist, investigated the flora and fauna of South America and South-East Asia. He published his ideas in a paper entitled 'On the Tendencies of Varieties to Depart Indefinitely From Original Types' and the concept was similar to Darwin. It was sent to Darwin for its critical review.
· Darwin presented a joint paper entitled ‘Origin of species' but his monumental work was published in November 1859 AD in his book 'on the origin of species by means of natural selection.

A. Darwin's postulations or Salient features of Darwinism
a. Overproduction or rapid multiplication or enormous fertility, organisms reproduce in a geometric ratio but their total number remains almost constant.
· Capacity of organisms to reproduce in a large number is called overproduction. It is compensated by great destruction, so their total number almost remains constant.
· Examples include:
· A female rabbit gives birth to 6 young ones in one litter and produces four litters in a year. A six-month-old rabbit is capable of reproduction.
· Elephant matures at the age of 30, survives till 90 and produces 6 offspring and if all of them survived, in 750 years, a single pair would produce about 19 million elephants.
· Paramecium divides 3 times in 48 hours by binary fission under favourable conditions. Single Paramecium can produce 280 million offspring in a month. The mass would be equal to 10,000 times that of the earth at the end of the 9000th generation.
· Single female houseflies lay 6 batches of eggs and each batch contains 120-150 eggs. Such eggs hatch in about 10 days. After 14 days of hatching, young flies again start laying eggs. So, if all eggs, larvae and pupae survive, the offspring of a single pair would reach up to 191 
 10^18 within a short period.
· A single Salmon produces 28 ✖ 10^6 eggs. If the entire eggs hatch, the seas would be unable to hold them.

b. Struggle for Existence
· There is a struggle among organisms due to overproduction and limited food and area.
· Struggle may be intra-specific (within the members of same species) or inter-specific (among the members of different species).
· Intra-specific competition is more serious than the inter-specific competition.
· Struggle for existence checks up the size of the population i.e. natural check-up.
· Environmental struggle is the struggle between organisms and factors of environment like drought, heavy rain, earthquakes, diseases etc.
· Example: reptiles of the Mesozoic era were gigantic and led a successful life in that period but subsequently could not face changes in the environment and became extinct.

c. Variations and Heredity

· The difference between parents and offspring is called variation and similarity is heredity.
· Evolution will not occur without variation.
· Variations help to fit an animal into its surroundings and it will be better adapted to survive and reproduce and offspring will acquire favourable variations due to heredity.
· Variations bring new characters and heredity passes them onto the next generation.
· Variations may be continuous (raw material for evolution) or discontinuous (mutation).
· Variations appear due to the presence of alternative genes called alleles in chromosomes and relative proportions of different alleles.

d. Natural selection (Darwin) or Survival of the Fittest (Herbert Spencer)
· The term survival of fittest, used by Spencer, was used by Darwin as Natural Selection.
· Survival of the fittest is the automatic weeding out process of less perfect forms.
· Natural selects best-suited individuals to their environment and rejects less perfect forms.
· Best suited individuals only transmit their characters to offspring.
· Poorly developed forms are usually destroyed before they attain maturity.
· Natural selection occurs only after variation and it is a controlling factor of evolution but not a creating factor of evolution.
· Examples are:
i. Presence of long neck and forelimbs of giraffes

ii. Peppered moth Biston betularia has 2 forms- light coloured (Biston betularia typica) and black peppered moth (Biston betularia carbonara). Before the industrial revolution, light coloured moths were abundant as they camouflaged with the lichen-covered tree trunks. During the industrial revolution, black moths were abundant due to the deposition of smoke into tree trunks. With the development of industries, coal was replaced by oil and electricity, so again light coloured moths were dominant and black coloured rare. It proves that true black melanic forms arise by recurring random mutations.
iii. After the continuous exposure to DDT, some mosquitoes that developed resistance against it survived, reproduced and soon outnumbered DDT sensitive mosquitoes. Here DDT brought about a change in gene frequency in the mosquitoes population.
iv. Sickle cell anaemia - It is a disease of abnormal haemoglobin. RBCs become unable to transport O2 required so it causes the death of homozygous individuals. Heterozygous individuals bear both normal and sickle-shaped RBCs. These persons are highly resistant to malaria because Plasmodium can't survive in sickle-shaped RBC. Malaria is fatal to normal individuals. So, natural selection favours heterozygous individuals over normal ones. In east Africa and part of Asia, 40% of the population carries this sickle cell anaemic allele.

e. Origin of Species (Speciation)
· Due to a continuous struggle for existence leading to the survival of the fittest and the inheritance of successful variations, there is the origin of species called speciation.
· New species arise by natural selection over many generations, differing markedly from ancestors.

Summary of Darwin's concept of the Origin of Species by Natural selection
I. Organisms produce far more offspring than can surviveStruggle for existence.
II. The total number of individuals In a population almost remains constant.Struggle for existence.
III. Variations exist in the population.Survival of fittest or
Natural selection
IV. Individuals struggle for survival.Survival of fittest or
Natural selection
V. Survival of fittest.Origin of new species (Speciation)
VI. Continuous changing environment.Origin of new species (Speciation)

B. Criticism on Darwinism

i. It didn’t discuss mutation.
ii. It explained the survival of fittest but not the arrival of the fittest.
iii. It didn't explain vestigial organs, the origin of highly complicated organs like tusks of elephants, huge antlers of deers, electric organ of Torpedo etc.
iv. It didn't distinguish heritable variation from non-heritable variation.
v. It had no attempt to describe how life originated on the earth. He considered all variations are inheritable.
vi. To explain the transmission of characters from generation to generation, Darwin proposed ‘Pangenesis theory’ in 1868AD which stands completely discarded.

3. Mutation theory of Hugo de Vries
· According to mutation theory introduced by a batch botanist in 1901 AD, Hugo de Vries, mutants occur from time to time with totally new characters from their ancestors. These characters are transmitted to successive generations as they are caused by changes in genes.
· While experimenting with evening prime rose (Oenothera lamarckiana) which grew wild in Holland, he found such unusual plants that were sufficiently unlike to be said as new species.
It is briefly described as follows.

Hugo de Vries found 4 types of plants.
a) Progressive: having additional or new characters.
b) Retrogressive: having a reduction in one or more characters.
c) Degressive: Weak and had low survival.
d) Inconstant: That resembled parents and at times proved variants.

Salient features of Mutation Theory
a) Mutants form the raw material for evolution.
b) Mutants appear all of sudden and produce their effects instantly.
c) Mutants are totally different from the parents and there are no intermediate stages between the two.
d) Same mutations can appear in several individuals of a species.
e) Same mutation appears again and again to increase the chance of selection by nature.
f) Mutation can appear in all directions.
g) Mutations are inheritable.
h) A single mutation may produce a new species.
i) Natural selects beneficial mutations and eliminates lethal ones.
j) Evolution is a discontinuous or a jerky process.

4. Neo-Darwinism (Modern Synthetic Theory of evolution or Modern Concept of Evolution)
· This is the modified and updated version of Darwinism in the light of genetic research by Sewall Wright, R.A. Fisher, Ernst Mayer, H.J. Muller, Julian Huxley, T. Dobzhansky, G. L. Stebbins and others.
· It is the most accepted and recent theory.
· It considers both Mendelian and post-Mendelian laws of heredity which Darwin didn’t do.
· Initially, Darwin believed that only natural selection brings the origin of species (speciation).
· Modern concept believes that the origin of species is the combination of natural selection and mutation.
· Thus the modern concept of evolution is the synthesis of Darwin’s and Hugo de Vries’ theories.
· Dobzhansky (1937AD) in his book ''Genetics and Origin of Species'' provided the initial basis of synthetic theory.

Modern Concept of Evolution is:
1. Mutations
· Mutations are sudden heritable changes or discontinuous variations.
· According to Hugo de Vries, evolution is caused by the mutations but not by the minor variations that Darwin talked about.
· According to Darwin, evolution is gradual but according to de Vries, evolution is discontinuous.
· Mutations are of two types: chromosomal mutations and gene mutations.

A. Chromosomal mutations
· These are due to changes in structure and changes in the number of chromosomes.
a. Numerical changes in chromosomes
· These mutations are caused by changes in the chromosome number.
· They are of two types.
i. Aneuploidy: It is the numerical change in the chromosomal number of the genome like monosomy (2N-1), trisomy (2N+1), tetrasomy (2N+2), nullisomy (2N-2).
ii. Euploidy: It consists of genomes containing chromosomes that are multiples of some basic number.
· The euploids are the organisms that contain a balanced set or sets of chromosomes in any number.
· The number of chromosomes in a basic set is called the monoploid number, x. Thus, 1x is monoploid, 2x is diploid and greater than 2 are called polyploids (3x, 4x, 5x, 6x) and so on.
· The haploid (n) refers strictly to the number of chromosomes in gametes.

b. Structural changes in chromosomes (chromosomal aberrations)
· When the change occurs in the morphology of chromosomes, it is called chromosomal aberration.
· These are of four types:
  1. duplication (doubling of a segment),
  2. deficiency (deletion of a segment),
  3. translocation (passage of segment of a chromosome to a non-homologous chromosome) and
  4. inversion (reversal in the order of genes).

B. Gene mutation
· When the changes are in the gene structure and expression due to addition, deletion, substitution or inversion of nucleotides, these are called gene mutations.
· It can be induced by radiations and mutagens.
· Gene mutations that involve substitution, deletion or insertion of a single nitrogen base are called point mutations.
· The gene mutations which involve more than one nitrogen base or the entire gene are called gross mutations.
· It occurs due to change in the chemical nature of nucleotide base pairs of DNA.
· Any addition or reduction of one or more nucleotide base pairs or their rearrangements in DNA changes the sequence of amino acids, so changing the nature of proteins; hence there is a change in phenotype.
· Gene pool is the sum total of genes in any population.
· Evolution occurs through the accumulation of genetic variation in the population. 

Differences between chromosomal mutation and gene mutation
Chromosomal mutationGene mutation
It occurs in chromosomes.It occurs in genes.
It changes the number and structure of chromosomes.It changes genetic makeup and genetic composition.

2. Genetic drift

· The theory of genetic drift was developed by Sewall Wright in 1930AD.
· Genetic drift is the change in the number and frequency of genes in a small isolated population due to intensive inbreeding causing permanent fixation of some alleles, disappearance of a number of alleles and frequency of others.
· Its effect is most marked in a very small isolated population.

Two examples of genetic drift are:
i. Founder effect (Founder principle)
· It occurs in the human population.
· It is said that when a small group of persons called founders, leave their homes to find a new settlement, the population in a new settlement may have different genotype frequencies from that of the parent population.
· Formation of a new group in a new settlement is the founder effect.
· Sometimes they develop into new species.

ii. Bottleneck effect
· It was introduced by Stebbins for annual and biennial cycles of increase and decrease of a size of a population.
· When a population is declining, the number of individuals may reduce to such extent that the small group of population constituting the population becomes isolated and restricted in distribution. Then they are exposed to random genetic drift resulting in the fixation of certain genes.
· Thus the population reestablishes its former richness.
· Such reduction in allele frequency is called the genetic bottleneck effect which prevents species from population or extinction.

3. Non-random mating
· Repeated mating between individuals of certain traits changes the gene frequency.
· The selection of more brightly coloured male bird by a female bird may increase the gene frequency of bright colour in the next generation.

4. Genetic Recombination 
· Exchange of genetic materials in sexually reproducing animals provides a chance of a new combination of genes resulting in a new character in offspring.
· Rearrangement of genes occurs due to crossing over, random fusion of gametes, independent assortment of genes during gamete formation, formation of new alleles and dual parentage.

5. Gene migration (Gene flow)
· If the migrating individuals interbreed with the members of the local population, these may bring many new alleles into the local gene pool of the host population. This is called gene migration.
· If the inter-specific hybrids are fertile, then these may initiate a new trend in evolution which lead to the formation of new species.

6. Hybridization

· Hybridization is the crossing of organisms that are genetically different in one or more traits (characters).
· It helps in the intermingling of genes of different groups of the same variety, species and sometimes different species.

7. Heredity

· The transmission of characters or variations from parent to offspring is called heredity which is an important mechanism of evolution.
· Organisms possessing hereditary characteristics that are helpful, either in the animal’s environment or in some other environment, are favoured in the struggle for existence.
· Thus, the offspring are able to benefit from the advantageous characteristics of their parents.

8. Natural selection
· Natural selection occurs due to genetic and chromosomal variability.
· It also occurs through differential reproduction in successive generations.
· Differential reproduction is that some individuals produce abundant offspring, some only a few and still others none.
· When it continues for many generations, genes of the individuals which produce more offspring will become predominant in the gene pool of the population.
· Natural selection does not produce genetic changes, rather controls these changes by favouring some gene recombination, rejecting others and constantly modifying gene pool of a population that guides population towards the new adaptation.

9. Artificial selection
· Artificial selection is the isolation of natural populations and the selective breeding of organisms having desired useful characteristics.
· Or in other words, it is the process of interbreeding and selection exercised by man to improve the breeds or races of the domestic animals and plants.
1. It is conducted by the man on a limited scale in specific labs.
2. Traits selected for improvement are beneficial to humans.
3. This process of inbreeding when repeated for many generations produces a new breed with desired traits.
a. Production of improved varieties of several domestic animals such as dogs, horses, pigeons, poultry, cows, sheep etc from their wild ancestors. The dairy scientists choose the cows that produce more milk. Their calves are chosen for inbreeding and after a few generations of selective breeding, a population of high milk yielding cows is obtained.
· Artificial selection may also give rise to new breeds or varieties which differ from the original population.
· The enormous differences and variations produced in pigeons due to selective breeding can be traced back to the ancestor wild rock pigeon.
· Artificial selection is also practised for the production of better varieties of crop plants. Cabbage, Kohlrabi and cauliflower are the descendants of a common ancestor, Colewort.
· Different varieties each with distinctive characteristics is obtained by artificial selection.
b. Production of improved varieties of wheat, rice, sugarcane, cotton, vegetables, fruits and ornamental plants.

10. Reproductive isolation
· Reproductive isolation is the isolation of individuals of a species by geographical, genetic, ecological or physiological barriers so preventing gene flow (interbreeding) among the breeding populations maintaining their hereditary integrity by checking hybridization.
· Example: Mule.
· Each group can grow into different species by accumulations of genetic variations in a population and it is a must for evolution.
Reproductive isolation is of two types:
1. Pre-mating isolation mechanism
· It occurs before mating and is of many types.
a) Geographical isolation: It is the isolation of groups of related organisms by physical barriers such as sea, mountain, desert, river etc.
b) Genetic isolation: It refers to inter-specific sterility.
c) Ecological isolation: It refers to the occupation of different habitats in the same geographical area by 2 species. It prevents interbreeding.
d) Temporal isolation or seasonal isolation: It is the failure of 2 species to interbreed because they breed at different times in a year.
e) Behavioural (Ethological) isolation: Failure of 2 species to interbreed due to differences in courtship and mating behaviour.
f) Physiological isolation: It is the functional incompatibility in mating or in the production and survival of gametes.
g) Mechanical isolation: Different in size of genitalia between species that make interbreeding difficult or impossible.
h) Gametic isolation: Many animals shed gametes in water. The gametes of different animal species fail to fertilize.

2. Post-mating isolation mechanism
· It occurs after mating and is of the following types.
a) Incompatibility: Inter-specific mating may occur but fertilization or development fails to occur.
b) Hybrid inviability: Inter-specific mating may occur and produce an F1 hybrid but later is not viable and dies at some stage.
c) Hybrid sterility: Inter-specific mating may produce a viable F1 hybrid but the latter is sterile. This fails to reproduce further.
A mule (Hybrid) is produced by the mating of the male donkey and a mare (female horse). Similarly, hinny (hybrid) is produced by the mating of the male horse (stallion) and a female donkey.
d) Hybrid breakdown: Inter-specific mating produces vigorous F1 hybrids, which by backcross give rise to F2 hybrids that have lost vigour or fertility or both.

11. Speciation
· It is the total effect of the above activities over a long period.
· Speciation is the phenomenon of the formation of one or more species from the existing one by evolutionary means.
· The populations of a species are present in different environments.
· They are separated by geographical and physiological barriers.
· They accumulate variations (genetic differences) due to mutations, recombination, hybridization, genetic drifts and natural selection.
· These populations, so, become different from each other morphologically and genetically and are reproductively isolated, forming new species.
· Besides this, those individuals with new traits which can adjust to the changing environment are selected by nature and new traits spread by differential reproduction.
· Accumulation of such useful traits in the population, alter the individuals in structure and function and develop into a new species.

Speciation is of following three types:
1. Multiplicative speciation
· A parent species continues its race and tends to exist.
· It is called multiplicative as it increases the number of species.
· It is of further 2 types- Gradual and instant.
a. Gradual speciation: It occurs over many generations.
· It is of 2 types - Allopatric and sympatric.
i. Allopatric speciation
It occurs due to geographical isolation. It involves the separation of the original population (genetically related) due to geographical barriers such as mountain or land, bridge or ocean.
ii. Sympatric speciation
It occurs by ecological or other isolations. It produces new species within a single population. It may take place by polyploidy. A small segment of the original population becomes isolated reproductively.
b. Instant speciation: It occurs in a single generation by hybridization.

2. Phyletic speciation
· It is the replacement of one species by another without an increase in the number of species.
· Example: Eohippus evolved into Mesohippus and it became extinct.

3. Speciation by fusion 
· Occasionally 2 distinct species merge into one new species by the breakdown of previously existing reproductive isolation.
· It leads to a reduction in the total number of species

Differences between Darwinism and Neo-Darwinism
Put forward by Darwin and Wallace to explain the mechanism of speciation.Modified and updated form of Darwinism
Does not explain the cause of variation.Explains
It considers all favourable variations are inheritable.It considers only genetic variations are inheritable.
The unit of evolution is individual.The unit of evolution is population.
Does not consider reproductive isolation.Considers
Does not consider Mendelian and Post- mendelian laws of heredityConsiders

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