If somebody asked me to write a short essay giving an overview of my favourite topic, the nature of species, I doubt that I could. I can write a long essay on it (in fact, several) but it would be excruciatingly hard to write a short one. For that, we need a real writer. Carl Zimmer is the guy. He has an essay on species in the current edition of Scientific American. And despite quoting some obscure Australian philosopher, it is a good summary of the issues. How he manages to get up on a topic like that amazes me. It took me a good five years.
There’s a connection with this blog. A while back I wrote on microbial species, which became a paper in History and Philosophy of the Life Sciences, after being a talk at a Microbiology conference in Exeter. As a result, Carl became aware that species conceptions was my thang, and so he called me up and we talked for an hour or so on his dime. I hope that I was helpful.
The essay has the following subhead:
The debate over species definition is far from over and is more than a mere academic spat. Proper classification is essential for designating the endangered list.
This is perhaps the most crucial aspect of the species concept debate, but it isn’t the most theoretically interesting. Biology, like most sciences, has a need for units of measurement. And like most sciences those units need to be grounded in the real world. So what species, the “rank” of biology that it is agreed on most sides is the most or only natural one in the Linnean hierarchy, are determines many measures of biology in fields from genetics to ecology. If, as a significant number of specialists think, the rank is a mere convention, then those measures become arbitrary and meaningless.
So, what sort of “unit” might a species be? I can think of three alternatives. One is that species are, in fact, simply a matter of convention, which is to say something that makes things convenient for us in communication, just as John Locke said in the Essay (although that was about the logical species, not the biological one). Instead, say researchers like Paris polychaete specialist Frederick Pleijel and Rutgers geneticist Jody Hey, we need to replace the notion species with something like a “least inclusive taxonomic unit” (LITU, Pleijel) or “evolutionary group” (Hey). There are other replacement concepts in the offing. And the so-called “phylogenetic species concept” is not really a concept of species, at least in one of the versions under that name, so much as something very like a LITU that gets called a species.
But there always have been replacement terms for “species” available. Even Linnaeus had a rankless taxonomic concept, cohort, and taxon itself was first of all a rankless concept. And always those terms end up being replaced by species anyway. Even “deme”, now a core term of population genetics (roughly meaning the local breeding population) was originally a rank-free term that got subverted. So I think that no matter what we think about ranks, we will continue to talk about species.
The second alternative is that species is a term that plays a theoretical role in biology, and this seems intuitively right: we talk about species as the units of evolution, so they are supposed to be required by evolutionary biology, and likewise in ecology, species are the unit that is crucial in defining the biodiversity of a region or ecosystem.
But if species are theoretical objects, we ought to find them as a consequence of theory, not as a “unit” that we feed into theoretical or operational processes, and so far as I can tell, this is not the case. Population genetics and evolutionary theory have populations, haplotypes, alleles, trophic nodes, niches and so on, but what they do not have are species. In every case where species are used in theory, they are primitives, or stand as surrogate terms for the things mentioned. Theory does not define species.
This might be challenged by adherents of Mayr’s biological species concept, or one of the derivative or related conceptions – a species is a protected gene pool, as Mayr said. This is certainly the view of Coyne and Orr in their Speciation book of a few years ago. But as Zimmer’s piece points out (and quotes me among others as pointing out) the vast bulk of life would not be in species if that were the case, and anyway, species were well described and identified long before genetics was developed, some two centuries before. So they must at least be things that can be observed in the absence of theory. Of course, some species are harder to identify than others, requiring techniques that are recent, but that still doesn’t make species theoretical objects.
This brings me to my third alternative: species aren’t theoretical objects at all; they are objects that have phenomenal salience. That is, we do not define species, we see them. Consider an analogous case: mountains. Mountains are hard to define, and they have a multitude of geological causes, ranging from uplift, subduction, vulcanism, differential erosion, and so forth. “Mountain” is not a theoretical object of geology – subduction zones, tectonic plates, and volcanoes are. A mountain is just something you see, although there are no necessary sets of properties (or heights) that mountains have to have, and it is often vague when differentiating between them. A mountain calls for an explanation, and the explanation relies on theory, but equally so do mesas, land bridges, and caves.
So my answer to the question Carl proposes: what is a species? is that a species is something one sees when one realises that two organisms are in the same one. They are natural objects, not mere conveniences, but they are not derived from explanations, but rather they call for them…