On a couple of previous occasions, I've discussed the use of the Biological Species Concept (BSC) versus the Phylogenetic Species Concept* (PSC) - see posts here and here, the first of which defines the two terms if you're not familiar with them. One question that someone, somewhere will always bring up whenever I broach the subject is whether use of the PSC would require the division of humans into separate species, so this post is partially in response to that. From a more general perspective, I thought that some of you might be interested simply in knowing how a taxonomist recognises if they have a new species.
*The more I cover this topic, though, the less I like the term "phylogenetic species concept", because the PSC isn't determined solely, or even primarily, by phylogeny. I prefer the term "diagnostic species concept". Real, honest-to-goodness phylogenetic species concepts, where species are defined by holophyly, have been proposed before (e.g. Donoghue, 1985) and I think the term should be reserved for them. Such concepts have never been particularly popular, though, because frankly they're shit.
The first point, that I think I've said before but can never be stressed enough, is that whatever species concept you are using, a species represents a hypothesis. Sadly, specimens do not come with convenient labels saying to what species they belong. If they did, I'd probably be out of a job (not that I won't soon be out of a job - that thesis deadline looks like it's getting closer every day*). A taxonomist presented with an array of specimens will divide them up into what appear to be distinct species, but that division will always be subject to further revision. Intermediate specimens may be found that blur the boundaries between previously distinct morphologies. Two different "species" may hatch out of eggs laid by a single parent. That sort of thing. No taxonomic revision can ever be final.
What most taxonomists are looking for when distinguishing species are discrete, discontinuous characters. Whatever your favoured species concept, ultimately what you are trying to identify are populations that are genetically isolated from others. To give an example, Megalopsalis sp. A is found in Western Australia (I'm calling it "sp. A" here because 'tain't published yet). It differs from other species of Megalopsalis because the femur of the pedipalp (a little leg-like appendage in front of the legs that's used for feeding, visible in the photo below of Protolophus by Robert Pearson) has a row of spines running along the top (dorsal) edge. Over in Victoria and South Australia lives Megalopsalis hoggi, a closely related species. Specimens of M. hoggi never have spines on top of the pedipalp, but they may or may not have spines underneath it (ventrally).
You may be wondering why I regard presence or absence of dorsal spines as a character distinguishing species, but not presence or absence of ventral spines. After all, both are equally distinct. The difference is a matter of geography. In the specimens that I've seen of Megalopsalis sp. A, all of them have dorsal spines on the pedipalp, and I've not seen that character in Megalopsalis specimens outside the range of species A. Specimens with dorsal spines are geographically discontinuous from specimens without, indicating that they represent a distinct population. However, in the case of M. hoggi, specimens with ventral spines on the pedipalp are found in the same locality as specimens without ventral spines. The two are not discontinuous, suggesting that in this case that they are not genetically isolated.
It is possible that the ventrally-spined and ventrally-spineless individuals do represent separate species, but with overlapping distributions (and indeed, western specimens of M. hoggi tend to have spines while eastern specimens tend not to). If so, then study of other character complexes (such as population genetics) may reinforce their distinction. It is also possible that future collections may uncover overlapping variation in the Western Australian population that I've called "species A", and it may not be so distinct after all. But the recognition of two species, one variable and one not, is the most consistent with the information I have on hand so far.
Which leads up to my second point - pretty much any character could potentially distinguish a species, but pretty much any character could also potentially not distinguish a species. The ultimately important factor isn't the characters themselves - it's what they say about the gene flow leading to their current distribution. A single widespread species may encompass more variation within its distribution than a whole complex of closely-related species with restricted ranges. The difference is that character distributions will overlap in the former case but not in the latter.
So to get back to the question of whether use of the PSC would lead to recognition of separate species - personally, not having studied human variation to any great extent*, I don't actually know (really, it's exactly the same question as whether human races have any biological validity, just phrased using different terminology). I suspect that it wouldn't - while humans do vary considerably over their extraordinarily wide range, most of that variation is probably clinal rather than falling into discrete populations. It doesn't reflect underlying barriers to gene flow. This is especially the case in the modern world, where increased transportation and migration is breaking down what geographical barriers there may once have been.
*As organisms go, primates are mostly fairly dull.
As an unrelated postscript, in both of the earlier posts I've referred to a "tendency" of using the PSC to lead to recognition of more species than the BSC. Which got me thinking - are there any situations where the PSC would recognise less species than the BSC? Some possibilities that occured to me were situations involving polyploidy, or cases where interfertility was determined by the action of a single gene such as chirality reversals in gastropods. In these cases, diploid vs. polyploid or dextral vs. sinistral individuals might not be directly interfertile (which would mean they belonged to distinct biological species), but mutation from one state to the other might mean that there was still a reasonably steady gene flow going on (so they would not be distinct phylogenetic species). Any thoughts?
Donoghue, M. J. (1985). A critique of the biological species concept and recommendations for a phylogenetic alternative. Bryologist 88: 172-181.