One of the arguments commonly levelled against the idea of speciation is that we can never see it happening. That argument is simply incorrect, & some time soon I guess I should at least give you some links to evidence that supports my statement. But in the meantime, I've just come across another apparent example of speciation in the here-&-now. (I've written before about speciation in indigobirds.)
Flycatchers are small birds found on many Pacific islands (& elsewhere). In an on-line article in ScienceNOW Daily News, Elizabeth Pennisi describes how biologists studying two subspecies of monarch flycatchers in the Solomon Islands have found that a single genetic changes turns a small, brown-bellied bird black, possibly leading it to mate with like-coloured birds - and setting it on the road to becoming a new species. (I did wonder on first reading this whether there should also be a 'perhaps' in front of 'setting'...) Certainly plumage colours do send strong social signals to flycatchers (eg Filardi & Smith, 2008).
By the way, there are some lovely images of Pacific flycatchers here on Living the Scientific Life - together with a discussion of a paper on the molecular phylogenetics of this species.
There are at least 5 sub-species of the Monarch flycatcher (Monarcha castaneiventris) in the Solomons. In the study described by Pennisi (a forthcoming publication by Uly et al. 2009), scientists decided to check out the differences between two of the sub-species, which live on separate islands. One is iridescent blue-black on its head & back, with a red-brown belly. The other is completely blue-black. The team knew that black pigs & sheep have a mutation in the gene that controls the amount of black pigment cells produce, & wondered if something similar was operating in the birds.
They isolated & sequenced part of the gene - which codes for the melanocortin-1 receptor - from birds from both subspecies. And they found that the key difference between the two was a mutation in a single nucleotide, leading to a single amino acid change in the melanocortin-1 protein. That was all it took to have cells producing mostly black pigment, rather than brown.
Did this matter to the birds themselves? Well, yes, apparently it does. Flycatchers are strongly territorial towards birds of their own species. But decoys of the black subspecies were attacked by black flycatchers, yet evoked little interest from the brown-bellied variety, and vice versa. Filardi & Smith (2008) had already demonstrated that, in monomorphic flycatcher species where the sexes look alike, both males & females show a strong aggressive response to interlopers, so presumably in these cases both sexes use colour to determine who's a member of their own species/subspecies.
So, if the birds use the same cues to choose potential sexual partners as they do to identify territorial intruders, this one mutation could be enough to set the flycatchers on the route to full reproductive isolation & ultimately the formation of a new species. (I was right to want that 'perhaps'!)
C.E.Filardi & C.E.Smith (2008) Social selection and geographic variation in two monarch flycatchers from the Solomon Islands. The Condor 110(1): 24-34. doi: 10.1525/cond.2008.110.1.24
E.Pennisi (2009) On the road to a new species. ScienceNOW Daily News, http://sciencenow.sciencemag.org/cgi/content/full/2009/615/1
J.A.Uly, R.G.Moyle, C.E.Filardi & Z.A.Cheviron (2009) Difference in plumage color used in species recognition between incipient species is linked to a single amino acid substitution in the melanocortin-1 receptor. The American Naturalist 174(August)