THEORIES OF EVOLUTION
To understand and explain the process of evolution various theories have been put forward by various scientists such as Lamarck, Darwin etc.
Lamarck's theory
popularly known as 'The Inheritance of Acquired Characters in Organisms'. It can be defined as 'the changes in structure or function of any organ acquired during the life-time of an individual in response to changes in the surrounding environment are inherited by its offsprings and keep on adding up over a period of time'.
These changes lead to the origin of new species. It comprises four propositions or assumptions: internal vital force, effect of environment and new needs, use and disuse of organs and inheritance of acquired characters.
The theory of natural selection was announced on June 30, 1858 by the English naturalist Charles Darwin (1809 1882) and Alfred Russel Wallace (1823-1913) in the paper Origin of Species by Means of Natural Selection. This theory is also known as Darwin - Wallace theory. In 1859, Darwin published his observations and conclusions. This theory is based on the following five propositions:
(a) Overproduction or enormous fertility
Living beings have an innate ability of producing their own progeny for the continuity of race. It has been observed that more individuals of each kind are produced than could
possibly survive.
(b) Struggle for existence
According to Darwin, individuals multiply in geometric Neo ratio whereas space and food remain almost constant. Therefore, there is an intense competition and struggle to ensure living and to obtain a maximum amount of food and suitable land. As a result of struggle for existence, variability and inheritance, generations become better adapted to the environment.
(c) Variations and heredity
The everlasting competition among the organisms has compelled them to change according to the conditions so that they can utilize the natural resources and can survive successfully. Therefore, it is difficult to find any of the two individuals alike. Even the progeny of the same parents are not exactly alike in all respects. These differences are known as variations.
Certain Variations Appeared in the parent generation continue to appear in the progeny generation. These variations are known as heritable variations and form the raw material for evolution.
(d)Survival of the fittest or natural selection
During the struggle for existence only those individuals could survive which exhibit such variations that are proved to be more beneficial in facing the hardships and rigours of the environment or which change to adapt themselves to the changing conditions. It has been called natural selection by Darwin and survival of the fittest by Herbert Spencer.
From above observations Darwin made certain conclusions and summarised them in the book 'Origin of Species by Natural Selection' as follows:
As a result of struggle for existence, variability and inheritance, the successive generations tend to become better adapted to their environment. These Adaptations, get preserved in the individuals of the species and ultimately lead to the origin of new species from the old ones.
The environment is ever changing and it leads to further changes and the appearance of new adaptations in the organisms. As natural selection continues, the latter descendants after several generations become markedly distinct from their ancestors.
Furthermore, certain members of a population with one group of variations may become adapted to environmental changes in one way, while others with a different set of variations may become adapted in a different way. As a result two or more species may arise from a single ancestral species.
Drawbacks of Darwin's theory
- He considered minute fluctuating variations as principal factors which are not heritable and are not part of evolution.
- He did not distinguish between somatic and germinal variations and considered all variations as heritable.
- Darwinism explains the 'survival of the fittest' but not the arrival of fittest".
- He could not explain over specialization of particular structure which has led to the extinction of its possessors, e.g.. Saber toothed tiger.
- He proposed artificial selection which never lead to permanent variations.
Neo - Darwinism
- Neo-Darwinism is refinement of the original theory of natural selection tore move objections .According to Neo-Darwinism both mutations and natural selection are responsible for evolution.
- The chief Neo-Darwinians are Weismann, Mendel and de Vries. Neo-Darwinians believe that the main causes of difference in members of a species are differences in genetic pattern; and influence of different environments.
Mutation theory
- The mutation theory was put forward in 1901 by Hugo de Vries, a Dutch botanist, to explain the mechanism of evolution.
Features of mutation theory are following:
- Mutations from the raw material for evolution.
- Mutations appear suddenly and produce their effect immediately.
- Mutants are markedly different from the parents and there are no intermediate stages between the two
- . All mutations have a genetic basis and are, therefore, inheritable.
- A single mutation may produce a new species.
- Nature selects beneficial mutations and eliminates lethal mutations.
- Mutations are discontinuous variations, called 'sports' by Darwin and 'saltatory variations' by Bateson.
- Evolution for Darwin was gradual while de Vries believed mutation caused speciation and hence called it saltation (single step large mutation).
Modern Synthetic Theory
• Dobzhansky (1937) in his book 'Genetics and Origin of Species' provided the initial basis of synthetic theory. 'Modern Synthetic Theory of Evolution' was designated by Huxley in 1942.
• Some of the important workers who have contributed to the modern synthetic theory are: Dobzhansky, R.A. Fischer, J.B.S. Haldane, Sewall Wright, Ernst Mayr and G.L. Stebbins.
• According to synthetic theory there are five basic factors involved in the process of organic evolution. These are:
-Genetic variation in population
-Heredity
-Natural selection.
- Reproductive isolation
-Speciation