Role of Variation on Evolution:
No two organisms or parts of an organism are precisely alike. We experience a particular man is different from others. A particular cow, sheep, or goat can be indentified in a herd. Thus, individuals of the species vary from one another in some way or other. Organisms have tendency to vary.
Variations can be classified in different ways:
(i) Variation in number is called meristamic variation while that in shape, size, colour etc is called substantive variation.
(ii) Variations may be of continuous or discontinuous nature.
(iii) Again variations may be somatic or somatogenic and germinal or blastogenic concerning the organs or cells in which they occur.
The somatic variations are caused due to direct influence of the environment upon the body or some of the organisms. These are never heritable. While the germinal variations occur in the germinal cells which produce gametes. Hence, these are heritable and play important role in evolution.
Causes of Variations:
Variations may occur due to following reasons:
(i) Due to new combination of germplasm by biparental parentage,
(ii) Due to defective meiosis
(iii) Due to secretions from endrocrine glands and physiology,
(iv) Due to mutations and
(v) Due to tendency to vary.
Effect of variation on evolution:
In the modern synthetic theory for evolution, ‘variation’ has been accepted as one of the prime causes of evolution. In natural selection, variation plays a major role. Variations may be beneficial, harmful and neutral. Beneficial variations make the individuals more suitable to survive in the struggle for existence while harmgul variation causes its bearer unfit in the race and hence they are deprived of the chance to survive and reproduce.
Darwin had considered small and continuous fluctuations were important for evolution. But actually these may be temporary response to the environment. But recent studies have proved that mutation can bring about sudden, drastic heritable changes which may cause variation. Hence mutations are the ultimate source for evolutionary changes.