∞ generated and posted on 2016.09.11 ∞
Idea that two populations represent different species if individuals from each population are somewhat unable despite opportunity to mate to produce successful hybrid progeny.
The biological species concept is most applicable to species that are obligately sexual and if two populations are indeed separate species, then either mating is difficult to achieve, and this is true even given physical association (two organisms found in approximately the same place at the same time), conception is difficult to achieve, or resulting progeny are substantially less evolutionarily fit than their parents. That is, separate biological species will not tend to substantially share gene pools.
Complicating things, often separate biological species may share genes a low rates, that is, as a consequence of horizontal gene transfer (a.k.a., introgression).
Another problem with the biological species concept – though it serves basically as the foundation for ideas on how speciation occurs or indeed what a species is – is that it is difficult to apply to organisms which are not obligately sexual or which, for example, are extinct.