W
hat is a species? When we try to define it, we generally start with a “mating group” -- the largest group such that any two individuals of the appropriate sex can reproduce. This turns out to be an insufficient definition for a number of reasons, such as ring species, geographic isolation, genetic "gradients", and so on. But most interesting to me is that species clearly have a “form” or essence to them which requires explanation.
What I mean is this: health and sickness are objective aspects of individual animals, which is a completely invisible concept within the "mating group" definition. Animal species clearly have a sort of “ideal”, roughly speaking, which certain animals can deviate from. Clearly, there are grey areas, such as lightly divergent phenotypes, helpful mutations, deviations that offer trade-offs of various kinds, etc. But even more obviously there are clear cases in which a mark of some kind has been missed. Birth defects, genetic diseases, physical mutilation -- these words take their meaning from the concept of a "healthy" animal. A sick animal is one that is lacking powers or qualities that it was intended to have. Everything else about it seems to "assume" the wholeness that it lacks.
Giving this quality of species a rigorous definition is extraordinarily difficult. Life is far more sophisticated than we often give it credit for.