What's a Species, and Why?

A good article by Scott LaFee for the San Diego Union-Tribune tackles these questions in some depth, probing the different mechanisms of speciation and offering examples from fruit flies to copepods. A species, biologist Dan Bolnick notes, remains hard to define by the classic criterion of an interbreeding population. “Even if you use the criterion of reproductive compatibility, how compatible is too much to consider groups separate? What if the interbreeding species hybridize 10 percent of the time? One percent? One hundred percent? The cutoff is arbitrary.”