Sexual reproduction, which involves meiosis helps in recombination of genes during gametic fusion. This leads to differences in the phenotype of the offspring from its parents. These differences are called variation.
Variation is the difference found among individuals of the same species and the offspring of the same parent. Variation is the raw material which plays an important role in evolution. Evolution would not be possible without variation.
Somatic variation: These are the variations which affect the body (somatic) cells of the organisms. They are not heritable. They occur due to environmental factors.
Germinal variation: These variations are produced in germ cells of an organism. They are inherited. They may be present in ancestors or may occur suddenly. They are classified into two types:
1. Continuous variation
2. Discontinuous variation
Continuous variation: These are small variations which occur among individuals of a population. They are also called as fluctuating variations. They occur by gradual accumulation in a population. e.g. skin colour, height and weight of an individual, colour of eye, etc.
Discontinuous variation: These are sudden changes which occur in an organism due to mutations. They do not have any intermediate forms. These large variations are not useful for evolution. e.g. short legged Ancon sheep, six or more digits (fingers) in human, etc.
Discontinuous variation form the basis for Mutation theory proposed by De Vries.