The species question

Razib has two interesting posts up at Gene Expression (1,2) that touch on something related to my own research: the question of whether or not "species" are really "real" biological entities, or just artificial groupings that humans use to make the world easier to understand. Razib has, he admits, "serious issues with the idea of species as such," and believes that focusing on things like the best way to define "species" can get in the way of understanding what is really going on.

I certainly can't argue with the second point. Focusing on how to define "species" can certainly get in the way of understanding. There's not a chance in hell that we're ever going to find a definition of "species" that will always permit us to clearly determine whether or not two populations are part of the same species. Evolution is to blame for that. Populations keep changing in nature, and new species are produced by changing old species. A week or so ago, I posted a real-life example of the kind of things you see when you look at populations that are in the process of splitting into new species. (It's a three part item, found here: part 1, part 2, and part 3).)

While I don't disagree with Razib on the matter of the confusion caused by the (necessary) vagueness of the term "species," I do disagree with him on the question of the biological reality of "species." I think that species are very much a real part of nature. We may not always be able to erect objective markers that can be used to separate one species from another, but that does not mean that species are necessarily "subjective." We, as a species, have recognized species distinctions in nature since the dawn of history. Intuition my not always (or even usually) be a good indicator of what's actually going on, but in this case I think it just might be.

Razib thinks of species in genomic terms:

I am inclined to take a very instrumentalist view toward this issue and don't dismiss phenetics or cladistics. Certainly species have more of a reality than genera, but their utility is contingent upon the branch of the tree of life you are examining. Myself, I tend to think of species as populations which exhibit a tightly linked correlation of alleles across a broad swath of loci. In other words, there are no idealized Platonic species, they are simply working classifications which allow us to make sense of the genetic architecture of a population.

This isn't a bad definition of species. Species are either single populations or linked populations, and they usually do have a high degree of genetic similarity across their entire genome. Ultimately, though, the genomic focus misses something important.

In nature, humans aren't the only ones who are able to tell the difference between two species. The organisms themselves are usually able to make the distinction. In fact, they're usually better at it than we are - we've got to do a lot of looking to figure things out; they "just know" the difference. That's a gross oversimplification, of course, and there are lots of exceptions. (In one of his posts, Razib defined evolution as "the science of exceptions to generalizations." That's such a great definition that it's probably going to make its way to the quote list above my desk.)

The example of speciation in progress that I presented on my old blog (linked above) shows that species aren't formed instantly, and that it takes some time for the organisms involved to work out all of the differences. That process isn't always smooth and even, and it can last quite a while in evolutionary terms, with different mechanisms for separating the species working at different rates. The study of each of the mechanisms involved when populations separate is important, but so are the interactions of the individual mechanisms with each other, and so is the role of ecological and environmental factors in differentiating the populations. Even without a clean definition of "species," speciation is still a real process that can (and should) be investigated.

I hope so, anyway. If it's not, I could be in for a long few years while I try to do just that.