Species Concepts: Past, Present, and Future

No matter what your study system, whether you're a theoretician or an experimentalist, whether you work with genes or whole organisms – if you do research in ecology or evolution, chances are you're going to have to decide what species concept you're going to employ. When I started my PhD last year, I had to undergo the difficult process of moving from one species concept to the other. I did my MSc in a lab that focused heavily on taxonomy and systematics, and so tended to use the phylogenetic species concept (PSC). In my thesis, I sought to confirm that a species of sea slug (Haminoea japonica), found on the west coast of North America and in Europe, was the same as one found in Japan. Using three gene fragments and the PSC, I determined that the American/European species was indeed the same as that in Japan (the slug had invaded from Japan, most likely with shipments of oyster spat). In this study, the PSC was the most appropriate species concept to use, as we were primarily interested in using the genetic relationships of one population to another for identification purposes, and using the biological species concept (BSC) would have been unfeasible (importation of live invasive molluscs and captive breeding/mating trials of sea slugs both being difficult, if not impossible, tasks) and also irrelevant.

Haminoea japonica

Photo credit: Ángel Valdés

Fast forward to August 2012 and my entry into the stickleback world at the 7th International Conference on Stickleback Behavior and Evolution in Seattle. Of course a major theme at this conference was ecological speciation – the stickleback being a well-studied model of the process – and the implied species concept used was the BSC. This makes sense, of course; those who study ecological speciation aren't interested in defining the species, but rather in the process that creates those species. Using the PSC isn't useful here – just knowing that two things are different doesn't help you understand how they became different. And so, after arguing with Andrew about the merits and demerits of each species concept, I came to accept the BSC.

What I should actually say is that in the context of the work I'm doing now (parallel ecological speciation in threespine stickleback), I accepted that the BSC is the most appropriate species concept. But that's not to say I think that one species concept is inherently more correct than another; it just depends on the question you're trying to answer. Borrowing imagery from the "adaptive peak" metaphor for selection and adaptation, imagine a large mountain with twin peaks. Should such a formation be conceptualized as two mountains, or one? As with the species concept, I maintain that it all depends on your question. If you were stranded on one of the peaks, and were radioing for help, you would want to use a "mountain concept" that classified the two peaks based on their unique characteristics, such as geographic coordinates (analogous to the PSC). On the other hand, if you were a geologist trying to understand why there were two peaks instead of one, you would be most interested in a "mountain concept" that defined the processes by which peaks are made, such as uplift and erosion (analogous to the BSC).

Two mountains or one?

Photo credit: Wendenburg

I don't think I'm saying anything especially significant here, and in any case, books and articles defending one species concept over the other will continue to be published in prodigious quantities, just as they have been in the past.  But maybe for now, in the spirit of the holiday season now upon us, we can learn to accept each other's species concepts as context-dependent, and live in peace and harmony!