4 Main Theories of Genetic Evolution | Cell Biology

The following points highlight the four main theories of genetic evolution. The theories are: 1. Lamarckism 2. Darwinism 3. Mutation Theory 4. Synthetic Theory.

Genetic Evolution: Theory # 1. Lamarckism:

This theory was propounded by Jean Baptiste Lamarck (1744-1829), a French biologist, through his numerous writings.

There are four postulates of this theory as listed below:

i. Living organisms and their organs tend to increase in size during evolution.

ii. New organs develop in an organism if this development is needed for its survival.

iii. Those organs that are in frequent use will develop more and more, as their constant use will make them more developed, while the organs not being used, will tend to become more weak.

iv. The modifications thus produced, due to the use and disuse, in the traits of an organism will be inherited and would accumulate with time.

In short, his theory is termed as Theory of Use and Disuse. The theory may be explained with the help of following example. Giraffes feed on tree leaves. As the easily accessible leaves within the reach are exhausted, they will have to stretch their necks higher and higher in order to reach the level of the leaves.

This stretching will cause some increase in the length of their necks, which will be passed on to the next generation. In each generation, therefore, their necks will be subjected to this type of stretching; its effect will accumulate over generations giving rise to the present day long necks of giraffes.

The theory of Lamarck was seriously criti­cised and is not accepted. Several experiments showed the ‘theory of inheritance of acquired characters’ to be incorrect. Many followers of Lamarck introduced various modifications of his theory, one of the important variations being that changes in the traits of organisms are induced by the environment (in place of the ‘need’ postu­lated by Lamarck).

Genetic Evolution: Theory # 2. Darwinism:

The theory of origin of species through natu­ral selection was proposed by Charles Darwin. Darwin visited several islands of the Atlantic Ocean near the coast of South America, and of South Pacific Ocean. He collected a vast amount of data on the biological entities in these islands.

His theory visualizes:

i. Normal reproduction by the members of a species, increases their number at a geo­metric rate. However, there are only limited food supplies and other environmental opportunities.

ii. As a result, members of the same species compete with each other for survival, which Darwin called, the struggle for existence.

iii. Members of a single species show variation for various traits, and Darwin assumed these variations to be hereditary. He suggested that the ‘fittest’ members of the species will ‘survive’ the ‘struggle for existence’; which he termed as ‘the survival of the fittest,’ and the factor responsible for this phenomenon was called natural selection.

Darwin, along with A. R. Wallace, conclu­ded that natural selection coupled with a change in the environment in which a species lives, bring about the evolution of a new species different from the parental species.

Thus the two main features of the Darwinian theory of evolution are:

(a) Origin of hereditary variations in a species, and

(b) Selective multi­plication of those variations which make the species more adapted to the prevailing environ­ment through natural selection.

According to this scheme, natural selection is the force which determines the direction in which evolution in a species would proceed, and the direction of this influence would depend on the prevailing environment. Thus natural selection is central to evo­lution, and this constitutes the Darwinian theory of evolution.

Natural selection can operate only when genetic variation exists in a population (individuals of a single species).

Darwin recog­nized two types of variation in living organisms:

(i) Continuous variations (quantitative in nature) and

(ii) Discontinuous or discrete variations (comparable to qualitative variation). Darwin visualized continuous variation as the basis for evolution. Several scientists have demonstrated the existence of considerable genetic variation within a species and attempted to elucidate the modes of origin of variations and selection of the variations so generated.

Genetic Evolution: Theory # 3. Mutation Theory:

This theory was put forth by Hugo de Vries (1840-1935), a Dutch botanist, who described a large number of discrete variations in Oenothera lamarckiana. He used the term mutation for the phenomenon of appearance of sudden heritable changes; the various mutations were called new and separate species by him.

He suggested that new species could arise in a single step due to mutation. Further, since mutations are random, it was presumed that evolution is random, and does not proceed in a definite direction.

The theory of natural selection presumes that populations are expected to become pro­gressively more and more adapted to their envi­ronment, whereas mutationists consider that populations are pre-adapted and adaptations do not necessarily arise due to natural selection.

De Vries theory of mutation though sup­posed to be considered as gene mutation in Oenothera lamarckiana, was later shown to be due to translocation of chromosome segments.

Genetic Evolution: Theory # 4.Synthetic Theory:

Modern understandings in cytology, gene­tics, cytogenetics, population genetics and evo­lution gave a way for the formulation of a cohe­rent theory called modern synthesis around 1930s by S. Wright, H. J. Muller, Th. Dobzhansky, R. B. Goldschmidt, J. S. Huxley, R. A. Fisher, J. B.S. Haldane, Ernst Mayr and G. L. Stebbins.

With the advances in the understanding to chromosome behaviour and aberrations and their consequent effects, Stebbins discussed the ‘Synthetic Theory’ recognizing the following fac­tors:

(i) Gene mutations

(ii) Changes in structure and number of chromosome

(iii) Genetic recombination

(iv) Natural selection

(v) Reproduc­tive isolation

The first three is to provide the genetic variability and the last two directing the evolutionary process. There are additional fac­tors namely migration of individuals from one population to another and hybridization between races, species or even related genera, increasing the genetic variability available to the popula­tions undergoing the process of evolution. Thus mutation, genetic recombination and natural.