I was just thinking the other day that I hadn't done a hard science post in a bit, because nothing had really struck me as interesting. And then I found an article in last week's edition of the Journal Science, and I'm cured! :}
Here, Seehausen et al (2008) discussed evidence for a controversial form of speciation known as sympatric speciation. Sympatric speciation is where you take one ancestral population, and from it form two distinct species without removing either group form contact with each other. This is a tricky process, because during the process the groups will be in contact with each other, and will have the possibility to pass genes. This geneflow erodes differences between groups, and forestalls species formation, if it doesn't outright prevent it.
There are a few ways you can pull this off, though. Often, it comes from a behavioural mating preference - one example is the apple maggot, which is in the throws of speciation because it breeds on apples, as opposed to other worms that breed on the hawthorn. Same species, for now, but they're on the path for divergence. Seehausen et al have another example of this controversial mode of species formation.
What Seehausen et al show a slightly different process is at work in the dynamic waters of Lake Victoria. In some cichlid fish, gradual transitions of light colour that comes from differences in turbidity between islands appears to be driving speciation by sensory bias. First, a word about the hypothesis.
There's natural standing variation for lots of things, including behaviour, and some of it is heritable. Some of the heritable variation can be variation in mating preferences for males (it's most frequently females who drive this sort of selection) with differences in attributes, such as morph, colour, shape, etc. Those females are likely to make with that subset of males - square loving females are likely to get with square males - and produce offspring with a good chance of having the trait (squareness) and preference for the trait. Over time, this results in square mice that like to mate with other square mice, which sort of closes the breeding group (or limits) geneflow from the ancestral group*.
Now, this probably isn't the most common mode of speciation. By far, it looks like geographic disruption is far more informative of speciation, but it looks good on paper. How's it end up working in practice?
Evidence is rare, and rarely good for any form of sympatry. But keeping that in mind, the evidence the authors presented is a good step forward. First we see that a) there's variation in male conspicuous colouration. The authors argue that it's because of water turbidity, and colours that remain conspicuous in in the turbid water. Specifically, they talk about a blue form and a red form. Blue forms are shallow waters with shallow light gradients, and red forms are from shallow waters with deep light gradients.
Next, they sequenced a gene (LWS), and found that numbers of alleles that were in the population really depended on how steep or shallow the light gradient was. Additionally, they found an allele that was especially common in red form cichlid. Further, the Fst value, a measure of fixation, was found to be very high between most islands.
However, when they attempted to look at neutral bits of DNA called microsatellites, they found that this fixation coefficient Fst was fairly low. Three fifths of their Fst values were significant at least the p=.05 level (a one in twenty chance that a data set was explainable by a random process), but they only averaged .016, which is a) reasonable, but not very high and b) definitely not as strong as the .6 to .8 values they were getting for the functional LWS locus. They say this is because LWS is under intense selection, leading to the rapid development of population structure, whereas the neutral loci have not yet been dragged along for the ride.
Finally, the paper discusses the female mating preference in more detail, outlining differences between islands in their male preference. They showed that the region around LWS wasn't responsible for the mating, which I would interpret as saying that there isn't a tight linkage between the mating preference and the preferred trait - this is relevant as when the genome gets scrambled up in recombination, before the DNA is passed on to the wee little fishes, there's a strong chance that the traits aren't both inherited by an offspring. Environment, not a physical linkage, is driving the pairing.
All in all, it's a neat example of sympatry in process. We don't often get to watch speciation as it happens, because speciation takes so long that it typically doesn't happen on a time scale humans can stick around for. But by knowing where to look, and how to watch it, we can find support and evidence for new species in the making.
... and now that I wrote that last sentence, I'm struck with a strange sense of deja-vu. Weird.
*This explanation is not exactly true. Especially with respect to this paper. But it is, as they say, good enough for government work.
Edit: Shoot! I forgot the citation! Here it is:
Ole Seehausen, Yohey Terai, Isabel S. Magalhaes, Karen L. Carleton, Hillary D. J. Mrosso, Ryutaro Miyagi, Inke van der Sluijs, Maria V. Schneider, Martine E. Maan, Hidenori Tachida, Hiroo Imai and Norihiro Okada. (2008) Speciation through sensory drive in cichlid fish. Nature (v. 455) pp. 620-626.