Abstract:

This is a mini-review on the historical changes in the concept of species. Biologists use different methods or criteria to discriminate species, leading to the formation of different species concepts, e.g. biological species, morphospecies, ecological species, evolutionary species, phylogenetic/cladistic species, or their combinations. These concepts respectively reveal a specific profile of the species’ attributes, as well as reflecting the objective existence of these creatures as different species, but not being satisfied with everyone. For eukaryotes, reproductive isolation (incapable of reproducing fertile offspring) should be the key for two populations to differentiate into two different species, no matter how much they differ morphologically. The mechanisms underlying such isolation might be geographical, behavioral, or otherwise. Reproductive isolation is certainly accompanied by some morphological or genetic changes that are often used as criteria by taxonomists or molecular evolutionary biologists to distinguish species, although these attributes may not be associated with reproductive isolation itself. Extinct species known only from fossils are impossible to be classified taxonomically according to reproductive isolation. The exact definition of the term “species” is still controversial, as a species concept based on reproductive isolation is usually not applicable, but a usable species definition (e.g. morphospecies) is regarded to be artificial.

Key words: species concept, biological species, morphospecies, reproductive isolation