What Defines a Species?

That question confuses even the experts. Even Darwin couldn’t do it. In 1856 he wrote, concerning the definition of species, “It all comes, I believe, from trying to define the indefinable”.

Traditionally, biologists loosely define a species as a population whose members breed mostly among themselves under natural conditions. Generally this means that they are genetically distinct. But now that we can sequence DNA, the concept of “genetically distinct” itself is problematic - exactly how many genes or base pairs have to be different? As one species evolves to become two, when are they different enough? It’s not just a philosophical exercise; how a particular species (or group of individuals within a species) is defined may determine whether it is an “endangered species” or not, and thus worthy of protection under the law. It can mean its very survival.

For a good recent article on the subject, see “What is a Species?”, Scientific American, June 2008, pp. 72-79.