ABSTRACT
Making sense of the diversity of organisms alive today primarily means working out how to group the dierent types of organisms into logical categories. e rst task is therefore deciding on how to distinguish one group of organisms from other, similar groups, and it is generally agreed that the essential unit for eukaryotic, sexually reproducing organisms is the species. is is ideally dened by the interbreeding criterion: if the males and females from two dierent groups cannot interbreed to produce fertile ospring, they belong to dierent species; if they can, they are viewed as members of the same species. us, if members of two apparently dierent populations can interbreed to produce fertile ospring, they are subspecies of the same species. is functional test works well in the context of contemporary speciation and is particularly important in theoretical discussions on the evolution of novel species. is is because the reproductive isolation that derives from a species’ inability to breed with related species drives future diversity.
