The definition of species has been a long-standing problem in biology. Many biologists respond to the problem in the same way that Darwin himself did. In On the Origin of Species, Darwin wrote, “No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.” The difficulty of defining species has come to be called the species problem.

Here is the problem in a nutshell: The species, as an evolutionary unit, must by definition be fluid and capable of changing, giving rise through evolution to new species. The whole point of the Darwinian revolution is that species are not fixed. How, then, can we define something that changes over time, and, by the process of speciation, even gives rise to two species from one?