Darwin (1859) was first to observe that evolution is ‘descent with modification’, though the mechanisms of both the inheritance (descent) and nature of variation (modification) were unknown to him at that time.
Genetic variation is introduced into a population by either mutation or immigration. Once variation has been introduced, the total array of genotypes may be considerably increased by recombination (Lewontin 1974). Existing variation in the population can be affected by natural selection or random fluctuations of allele frequencies. Selection can promote constancy (normalising selection), direct continuous change (directional selection) or diversification (diversifying selection; Dobzhansky et al. 1977).
Variation in finite populations may be reduced by random loss of alleles. The evolutionary change can be more rapid if a large population breaks up into isolated local strains. The rate of evolutionary change depends on the effective size of the local populations, the amount of interchange of individuals between populations, and is limited by mutation rates (Wright 1930).