What are the problems faced today in defining species and new taxa? For example, are wolves and dogs the same species or not?

– Teresa Lopes, High school teacher


Author: Dr. Aniek Ivens

Postdoctoral Fellow

The Rockefeller University, New York


If a cat were to have sex with a dog, no kittens or puppies or some mix of those would be born: cats and dogs clearly belong to different species. However, when a female horse mates with a male donkey, they do get a hybrid baby (a mule). Still, horses and donkeys are considered different species, because most mules are sterile: a mule mating with mule won’t produce another mule. This is the classic way (termed the biological species concept) to determine whether two organisms belong to the same species: if they can get fertile babies, they are, otherwise not. Following this definition, dogs and wolves actually belong to a single species: they can successfully interbreed.

There are, however, cases, in which this all gets a bit hazy. What if there is no way for us to test whether two organisms can interbreed, just because they don’t feel like it in the lab? Or what if we show in a zoo that they can reproduce, but in practice they never do because there is, say, an ocean between their populations and they just never meet, and have never done so many years. This is the case for the European Red Deer and the American Wapiti: even though they can successfully interbreed in captivity, they are considered separate species, because in practice they never do. A third problem with this classic species boundary is that the distinction between sterile and fertile is actually not so black/white as previously thought. Hybrids can sometimes actually reproduce but do so at lower rates than non-hybrids, because hybrids are generally less fit. A fourth and last problem is posed by organisms that don’t have sex at all, such as clonal plants or aphids, or bacteria. To them the classic criterium cannot be applied, but that doesn’t mean they do not have species!

In all these cases, the classic definition of species no longer holds, and scientists have to decide on a (somewhat arbitrary) limit of how much genetic material species have to share and exchange (termed geneflow) before we call them distinct species. Recent advances in genetics now allow us to determine gene flow with higher precision than before, thereby facilitating the task of calculating whether we are dealing with multiple species or not. Yet, over the years, scientists have developed multiple other species concepts and so far they have not yet reached consensus on which is the best one. In fact, perhaps the most important unanswered question on this topic is not whether we will ever be able to determine a unifying species concept that can be applied to all organisms, but whether we actually need one. As long as the people studying a particular organism can distinguish between its various species, we have all we need.