"Where we have sexual reproduction a species can be objectively defined as a group of organisms which reproduce sexually amongst themselves but don't reproduce with members of other species.

Where we don't have sexual reproduction - as in asexual species, or in fossils where we have no idea of how they reproduced - then there is no objective definition of the species, and the species just becomes like the genus, the family or the class. It's subject to arbitrary human decision."

Richard Dawkins