Intergradation

In zoology, intergradation is the way in which two distinct subspecies are connected via areas where populations are found that have the characteristics of both. There are two types of intergradation:

Primary intergradation:
Occurs in cases were two subspecies are connected via one or more intermediate populations, each of which are in turn intermediate to their adjacent populations and exhibit more or less the same amount of variability as any other population within the species. Adjacent populations and subspecies are subject to cline intergradation, and in these situations it is usually taken for granted that the clines are causally related (by natural selection) to environmental gradients.