The term species, in the most general sense, refers to the various kinds of living things. Thus, species are generally recognized as distinct, fully differentiated groups of organisms. However, most modern definitions of species also recognize them as reproductive communities and acknowledge that mating occurs between members of each species but does not occur (or occurs only rarely) among members of different species. Species are therefore generally recognized as genetically differentiated, reproductive communities within which there is a pattern of ancestry and descent among organisms. Although most scientists accept this general definition, there are two somewhat different criteria that are often employed in the recognition of species, and the application of the two criteria does not always lead to the same conclusions.

The Role of Interbreeding

The first major criterion for the designation of species is the actual occurrence of interbreeding among the various organisms and populations within...