Theories of Lamarckism and Darwinism

1.0Lamarckism/ Theory of inheritance of acquired characters

Lamarckism is an evolutionary theory first proposed by a French naturalist Jean Baptiste Lamarck (1744-1829).

Lamarck published a series of books. Of these Philosophie Zoologique, published in 1809 most clearly states Lamarck’s theories of evolution.  

Basic concepts of Lamarckism

Internal vital forces: Due to the presence of some internal vital forces all organisms have the tendency to increase in size of their organs or entire body.

Effect of environment and new needs: Environment influences all type of organisms. Changing environment gives rise to new needs. New needs or desires produce new structures (doctrine of desire/appetency) and change habits of the organism.

Use and disuse of organs: If an organ is constantly used over generations, it would be better developed whereas disuse of organ results in its degeneration (vestigial organs).

Inheritance of acquired characters: During the life time of an organism, new characters develop due to internal vital forces, effect of environment, new needs or use and disuse of organs.

All these acquired characters are inherited from one generation to another. By continuous inheritance through several generations, the variations are accumulated up to such an extent that they can give rise to new species.

Examples:

1. Long neck and forelimbs of giraffe:       

Lamarck gave the example of Giraffes who in an attempt to forage leaves on tall trees had to adapt by elongation of their necks. As they passed on this acquired character of elongated neck to succeeding generations, Giraffes, slowly over the years came to acquire long necks.

Diagram showing elongation of neck in giraffe according to Lamarckism

2. Aquatic birds stretched their toes and developed web.

3. Snakes lost their legs.

Snakes have evolved from lizard like ancestors which had two pairs of limbs. The continuous creeping through holes and cervices made snakes body elongated and due to continuous disuse of limbs because they hinder while creeping in burrows result in loss of their limbs.    

Lamarck had said that evolution of life forms had occurred but driven by use and disuse of organs. Nobody believes this conjecture any more.

Criticism of Lamarckism

(1) Weismann's theory of Continuity of Germplasm:

Weismann’s experiment on rats

He repeated this for 22 generations, but offspring still had a long tail 

Cut tails and mate

• Weismann disproved Lamarck's theory, inheritance of acquired characteristics.

• He cut off the tails of rats for as many as 22 generations and allowed them to breed, but tailless or reduced tailed rats were never born.

• On the basis of this experiment Weismann proposed the theory of continuity of germplasm.

• According to this theory -

Two types of protoplasms are present in an organism, germplasm and somatoplasm.

There is a continuity of germplasm and the variations influencing the germ cells are only inherited but the somatoplasm is not transmitted to the next generation, hence it does not carry variations to the next generation.

(2) Boring of ear pinna and nose in Indian women is never inherited to the next generations.

(3) Chinese women used to wear iron shoes in order to have small feet, but they still have normal feet.

2.0Neo-Lamarckism

According to Neo Lamarckism, "Changing environment may create some physical and chemical changes in somatoplasm of organisms, which may affect their germplasm also and such acquired characters can inherit."

Many experiments were done to support this but they were not convincing and satisfactory.

3.0Darwinism - Theory of Natural Selection

• Charles Robert Darwin was born on 12^th Feb. 1809 in England.
• Darwin travelled by H.M.S. Beagle ship, which left on 27 Dec. 1831 and returned on 02 Oct. 1836 through S. America, S. Africa, Australia & Galapagos Islands.
• Darwin was influenced by two books-

(i) "Principles of population" of Malthus

(ii) "Principles of geology" of Charles Lyell

• This theory was later on explained by Darwin in his book 'On the origin of species by means of Natural selection' (1859).

Basic concepts of Darwinism - 

Branching Descent and Natural Selection are the two key concepts of Darwinian Theory of evolution.

Natural selection is based on certain observations which are factual.

(i) Over production:

∙ All organisms have the capability to produce enormous number of offspring or organisms (multiply in geometric ratio).

∙ Hence, theoretically population size will grow exponentially if everybody reproduced maximally (this fact can be seen in a growing bacterial population) but the fact is that population sizes in reality are limited.

(ii) Struggle for existence:

∙ Natural resources are limited and populations are stable in size (except for seasonal fluctuation) means that there had been competition for resources. Only some survived and grew at the cost of others that could not flourish. This is called struggle for existence.

∙ It is of three types -

(a) Intra specific struggle: It is competition among the individuals of same species for same needs like food, shelter and breeding. (Most acute type of struggle)

(b) Inter specific struggle: It is the struggle among the individuals of different species for food and shelter. It is the most potent force for organic evolution.

(c) Environmental struggle: This struggle is between the organisms and their environment. All organisms struggle with cold, heat, wind, rain, drought, flood etc.

(iii) Variations and heredity:

∙ Members of a population vary in characteristics (in fact no two individuals are alike) even though they look superficially similar i.e. population has built in variation in characteristics.

∙ Those characteristics which enable some to survive better in natural conditions (climate, food, physical factors, etc.) are called adaptive or useful variations while others are called as non-adaptive or harmful variations. 

∙ The novelty and brilliant insight of Darwin was, he asserted that variations, which are heritable and which make resource utilisation better for few (adapted to habitat better) will enable only those to reproduce and leave more progeny.

(iv) Natural selection/ Survival of the fittest:

Natural Selection

∙ Individuals with more adaptive variations are "better fit" than the individuals with less adaptive variations. Hence, those who are better fit in an environment would be selected by nature and leave more progeny than others. Darwin called it natural selection and implied it as a mechanism of evolution.

∙ Fitness is the end result of the ability to adapt and get selected by nature.

∙ The fitness, according to Darwin, refers ultimately and only to reproductive fitness.

∙ It is observed that all adult individuals of a population don't have equal chances of mating; some males with better phenotype are preferred by females. This is called Sexual selection.

(v) Origin of New species:

∙ As a result of heritable variations and natural selection there would be a change in population characteristic and hence new forms appear to arise.

Criticism of Darwinism

1. The main drawback of this theory is that Darwin didn't have the knowledge of genetics and he had no satisfactory explanation for the cause, origin and inheritance of variations.

2. This theory only explained the survival of fittest but was unable to explain the arrival of fittest.

3. Darwin was unable to explain why in a population only a few individuals develop useful variations and others have harmful variations.

4. Criticism of Darwinism was based on sexual selection. Why only females have the right of selection for mating?

5. Darwin couldn't explain the existence of vestigial organs.

6. Darwin was unable to differentiate the somatic and germinal variations.

Comparison of Lamarckism and Darwinism 

4.0Mutation Theory & Neo-Darwinism

Mutation theory-

∙ This theory was proposed by Hugo de Vries based on his work on evening primrose (Oenothera lamarckiana).

∙ Large differences arising suddenly in a population are called mutations. Actually, mutations are sudden changes of genetic material (DNA) and hence all are inheritable. 

∙ In addition to recombination, mutation is another phenomenon that leads to variation in DNA.

∙ Mutation is a discontinuous source of variations and provides raw material for evolution.

∙ According to Hugo de Vries it is mutation which causes evolution and not the minor variations (heritable) that Darwin talked about.

∙ Mutations are random and directionless while Darwinian variations are small and directional.

∙ Evolution for Darwin was gradual while de Vries believed mutation caused speciation and hence called it saltation (single step large mutation).

Criticism-

(i) Natural mutations are not very common as Hugo de Vries thought.

(ii) Mutations are normally recessive & harmful, while the characters taking part in evolution are usually dominant.

Neo-Darwinism/Modern synthetic theory of organic evolution-

• Neo-Darwinism is a modified form of Darwinism along with recent researches of Weismann, De Vries, Stebbins, Dobzhansky, Sewall Wright, Mayr etc.
• According to this theory following factors are responsible for formation of new species-

(i) Rapid multiplication

(ii) Limited food and space 

(iii) Struggle for existence

(iv) Genetic variations

1. Gene recombination - New combinations of genes which are usually caused by the crossing over during gametogenesis. It is continuous and common source of variation in a sexually reproducing population.
2. Mutation - Discontinuous source of variations
3. Hybridization - It is crossing of organisms which are genetically different in one or more traits.
4. Gene migration & Gene flow - When migration of a section of population to another place occurs, gene frequencies change in the original as well as in the new population. New genes/alleles are added to the new population and these are lost from the old population.
5. Genetic drift - If the change in gene frequency occurs by chance, it is called genetic drift. 

(v) Natural Selection: Natural selection is a process in which heritable variations enabling better survival are enabled to reproduce and leave greater number of progenies.

(vi)Isolation: Isolation is a segregation of populations by some barriers which prevent interbreeding. The reproductive isolation between the populations due to certain barriers (E.g. Geographical isolation) leads to the formation of new species.

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