Theories of organic evolution

Lamarckism and Neo-Lamarckism Jean Baptiste Lamarck (1744-1829), who proposed the 'theory of inheritance of acquired characters' or popularly known as 'Lamarckism', was a French biologist. Three of the numerous writings by Lamarck, namely "Recherches Surle Organisation des Corps Vivant (1902)", "Philosophie Zoologique (1809)" and ''Historie Naturelle des Animaux sans Vertebres (1815-1822)" are particularly important in connection with theory of evolution, in which he had postulated that environment influenced the morphological characters and that these changes were inherited to next generation. His four postulates which formed the basis of his theory, however, attracted much criticism even during his life time. The postulates and the objections against them were : (i) Living organisms and their component parts continuously tend to increase in size. Examples however, show that more highly evolved flowering plants are really much smaller in size although the evolutionary trend in certain groups can be associated with increase in size, (ii) New organs will be produced due to new need and movement, which the individual develops and maintains due to such a need. A rather crude example, however, would show its inadequacy; in case we need to fly, we will have wings in course of time,

(iii) The development and degeneration of organs will be based on use and disuse, respectively. He illustrated this postulate with the help of examples like long neck of giraffe and limblessness of snakes. However, such a postulate may be correct as far as the growth of an organ witnin the life time of an individual is concerned but its inheritance to next generation could not be proved experimentally, (iv) The modifications which are produced due to above three postulates would be inherited and consequently would accumulate in course of time. All serious experiments performed to test this postulate failed to prove it. These experiments included those of Weismann and McDougall, the details of which can be found elsewhere (Gupta—Cytology, Genetics and Evolution, 5th edition).

Future followers of Lamarck tried to modify Lamarckism in order to make it acceptable, based on the idea of adaptation and an intimate direct and causal relationship between structure, function and environment. Prominent Neo-Lamarckians were Giard (1846-1908), a French and Cope (1840-1897), an American. They thought that although the necessity may not be logical, yet there would be a causal relationship between structures, function and the environment. This emphasis on structure, function and environment in Neo-Lamarckism definitely suggested that forces of evolution must integrate these three aspects.

The major drawback in Lamarckism and Neo-Lamarckism is that both assumed the inheritance of acquired characters. While in Lamarckism, these acquired characters result from needs, according to Neo-Lamarckism these will be induced due to environment and habit.

Darwinism and Neo-Darwinism Charles Robert Darwin (1809-1882), an English biologist proposed the theory of natural selection, after he had the privilege of going on voyage of exploration on the famous ship HMS Beagle with Dr. Henslow, visiting many islaads of Atlantic Ocean, some coasts of S. America and some islands of South Pacific. He took extensive notes and collected a lot of material during the voyage and published many book's on returning after five years. Darwin's ideas on natural selection resulted from an essay, 'Malthas on Population' in 1838 which suggested that a struggle for existence among plants and animals would ultimately lead to natural selection of those which are fittest. In 1858, he received a short essay entitled "On the tendency of varieties to depart indefinitely from the original type" from a young English naturalist, Alfred Russel Wallace (1823-1913), who sent it for his comments and suggestions. Darwin's work was jointly published with Wallace's paper in the Proceedings of the Linnean Society in 1859. Late in the same year he published his famous book 'Origin of Species'.

The three important postulates which formed the basis of Darwin's theory were (i) multiplication of individuals of a species in a geometric proportion, (ii) existence of variations and (iii) operation of natural selection on the existing variability in order to select best fitted variants. The tendency of a population to increase by normal methods of reproduction, combined with the fact that most populations are stationary, was termed 'struggle for existence' by Darwin. However, this struggle did not necessarily mean a battle between and among the organisms but more simply implied that the death could occur by any means preventing all offspring of a species from surviving to reproductive age. With so many variations in populations of species, the struggle for existence results in 'survival of the fittest', a term coined by Herbert, Spencer. The survival of the fittest is a direct result of the selection and proliferation of only those organisms which were most suitably adapted to the environment and most successful in mating. Darwin recognized two principal types of variations : (i) continuous variations showing the whole range of variation in a particular character, and (ii) discontinuous variations which appear all of a sudden and show no gradation. Darwin called the second type of variation as 'sports' for which the term mutation was coined by Hugo de Vries. Later, Darwin regarded continuous variations to be more important, since discontinuous variations being mostly harmful would not be selected again.

The major achievement of Darwin was to recognize one of the major factors in adaptation i.e. natural selection. Except the feature involving natural selection, other factors of Darwinism had to be either abandoned or modified in the modern Synthetic Theory of Evolution. However, Darwin was not clear about the sources of variations and the mechanisms involved in 'natural selection'. For instance, Darwin thought the natural selection operates through death of unfit rather than through differential reproduction, which we now know is the main pathway of natural selection. Further, he thought that the natural selection operates on variations already available and did not consider the possibility of origin of new hereditary variations. In fact, Darwin thought only the possibility of the survival of the fittest and could not see the possibility of the arrival of the fittest. The later concept, as proposed in Mutation Theory of de Vries, is a very important concept, because now we know that a fully formed useful character may appear due to mutation in one single generation.

Neo-Darwinism is a modified form of Darwinism. Weismann and his followers rejected Darwinism except the principal element of natural selection. These Neo-Darwinians, although distinguished between germplasm and somatoplasm, yet they could not appreciate the role of discontinuous variations or mutations in evolution. Darwin thought that adaptations result mainly due to a single source i.e. natural selection. However, Neo-Darwinians thought that they result from multiple forces and natural selection is one of the many forces. It was also realized that the characters as such are not inherited; instead there are character determiners which control the development. The ultimate character will result due to the interaction between determiner's reaction of the organism and the environment during its development.

As emphasized earlier in this section, knowledge about sources of variability and the mechanism of evolution remained obscure until after rediscovery of Mendel's laws. Now we know that a change in the genetic constitution of a population forms the basis of evolution. Considerable genetic variability exists within a population as well as between different populations of the same species. It is these genetic differences, caused by mutation and recombination, and acted upon by selection and other factors like migration, that contribute to evolution. Keeping in view the above facts, the modern 'Synthetic Theory' was proposed to explain the evolution.