Linnaeus' tercentenary and species

Carl Linné, or as we know him from his Latinised name, Carolus Linnaeus, turns 300 this year, on May 23rd. And Nature has a series of articles on the famous Swede in this week's edition, as well as a slew of other interesting papers. I don't know, nothing blogworthy comes along for months, and then they hit you with too much to do properly...

OK, there is a piece on his legacy to taxonomy in the age of molecular systematics; one on the role and problems of amateurs in systematics and how they may resolve some of the problems of insufficient professionals; Linnaeus' raccoon named Sjupp (not Rocky, alas!); genomes and systematics; information age systematics; also here; how sex sold Linaneus' system; but the piece I want to discuss today is this one - "Species and the specious", by Emma Maris.

The nature of species is a crucial issue in conservation biology, in part because a definition based on genetic diversity was incorporated in the United States' Endangered Species Act. So it is debated is polar bears are a species or a subspecies of brown bears, or if the red wolf is a coyotes-wolf hybrid or a distinct species in it's own right.

Hybridism is a crucial issue. A paper by Jim Mallet in the same edition of Nature summarises the increasing evidence that hybridism is a major form of speciation, in plants, of course, where botanists have known this for over a century, but also increasingly found in animals. According to Mallet, around 10% of animal species are known to hybridise, and 25% of plants. He has a nice illustration of how this might allow a novel species to find its way to a new adaptive peak [subscription required]:

So the traditional, sometimes called "Linnaean" notion of species is under attack. Not that there ever was such a definition - in Linnaeus' system species were just the bottom category. He neither defined the term, nor gave much guidance how to distinguish them. He simply asserted that species are now as created and that they are natural not artificial.

Recently, a diagnostic notion of species called the "phylogenetic" conception (people call them concepts, but they aren't - they are definitions or conceptions of a concept "species") has become operationally useful, and there are a slew of computer programs available to analyse and classify specimens on the basis of their traits, phenotypic or molecular. A result of this has been "taxonomic inflation" - the older "Linnaean" species are turning out to be much more divisible than before on the basis of molecular data. So now we have arguments about what it is we want to save...

Of course, biodiversity also measures genetic diversity, so it is unclear whether this is a substantive debate or one based on practical and political grounds. Maris' article indicates a strong political component, but one in which the species definition is at issue based on theory.

Since I report on this I have to give an opinion. It is based on my view of what the concept species is all about - marking differences over evolutionary time (this is not an evolutionary species "concept" though). A species is, in my view, a lineage of organisms that is held distinct from other such lineages by both endogenous and exogenous forces. If the persistence of a subspecific variety or unit is such that it encroaches on "evolutionary time", then it is a species. What "evolutionary time" is is moot, but I will arbitrarily put it on the minimum of 10^5 generations, to be safe. This leaves us with some problem cases, like the Israeli Naked Mole Rat which has had distinct chromosomal groups for around 3 million years, but overall it is good enough. I don't think there needs to be exceptionless categories in biology.

But it is true, as Joel Cracraft has said, that no more facts will decide on the conceptual issues here. We won't be able to "find out" what a species is simply by gathering more data. We can only defend various acts of recognition of patterns. In my view, there is no rank "species", just "species phenomena", so defining the concept in detail is a lost cause. We explain after recognising species, we don't have a theory-driven conceptual bucket into which phenomena can be neatly placed.