While describing normalising selection it was said that in uniform environments selection limits the variability of populations and evolves a genetic homeostasis. The type of selection process that is in progress in heterogeneous environments is known as diversifying selection.
Diversifying selection is the converse of normalising selection. Let us assume that a population occupying a certain environment, has two or more groups of genotypes (AA, Aa and aa) and meets two or more sub-environments or habitats. Among the two or more genotypes, let us say that a rare genotype (aa) that is well adapted to its habitat will be promoted by selection and the frequency of that genotype will increase as long as the habitat is not fully occupied. But when the habitat is saturated, no further increase in the frequency of that genotype an be observed. It is quite likely that the excess population could spill over to another sub-environment. The genotype may not be completely adapted to the new habitat and it is here that diversifying selection plays a role so as to establish a population consisting different genotypes. The phenomenon where two or more genotypes for a given trait exkt in a population is called genetic polymorphism. The different sub-environments co'me to be occupied by different genotypes and such an occupation would be as complete and efficient as possible.