Incipient speciation in blackcaps?

Thanks to Otter and Greg Mayer for alerting me to a BBC report on a dividing gene pool in blackcap warblers, which led me to the original paper in Current Biology.

What the authors have demonstrated is that within the last thirty years, the blackcaps (Sylvia atricapilla), which formerly overwintered in Spain and returned to central Europe in spring for breeding, have now divided into two groups that use different overwintering sites.  One group continues its winter journey to Spain, but the other now overwinters in England.  The authors speculate,  probably correctly, that a few decades ago some blackcaps found a rich feeding site in the UK, where the bird-loving populace puts out plenty of food for the native species.

Since then, blackcaps in Europe go to either Spain or to England.  Back at the breeding grounds, you can identify which bird went where by looking at the ratio of deuterium isotopes in a small snip of the bird’s claws. (These isotope ratios identify what a bird has been eating: those overwintering in Spain eat mostly fruit, while the UK migrants live on seeds and other tasty comestibles provided by aviphilic Brits.)

The authors examined both genetic markers and morphology of these two groups of migrants when they co-mingled on their common breeding grounds in Germany. There are two main findings:

1,  There was a very slight but significant difference between the groups in their genetic markers, showing that they were not completely mixing their DNA on the breeding grounds each year. In other words, the two groups showed genetic evidence for weak reproductive isolation, which is the stuff of speciation. (Two groups attain species status when they diverge so much that there is no mixing of genes between them. )  This isolation probably stems from assortative mating of the two types:   birds returning to Germany from the UK have to travel less distance, and show up on the breeding grounds earlier than their Spanish counterparts. That means that individuals who migrate to a given locality tend to pair up with others from that locality, simply because that’s who is around at mating time.

2.  The two groups also differed in five morphological traits:  wing shape, head color, body color, bill color, and beak shape.  These differences may reflect strong natural selection that differentiated the birds since they began overwintering at different sites. (Selection for beak shape differences, for example, may reflect the different types of foods that each group has to handle in winter.)  If these differences are genetic (and we don’t know for sure), then this also confirms some reproductive isolation between the two groups of birds.

So what does it mean?  Well, it seems to show that in a very short period — only a few dozen blackcap generations — the gene pool of the birds has undergone some sundering, although that splittting is by no means complete.  Moreover, this division has happened without any geographic barriers keeping the two types apart.  Both overwintering groups breed in the same place, and the “barrier” leading to genetic difference was caused by a behavioral change in the birds themselves, a change that may be genetic. (The authors’ other papers, which I haven’t yet read, suggest that the two groups differ by a single gene affecting the direction of migration. I have trouble believing this!)  This is a very interesting result, and was well worth publishing in Current Biology.

What it does not mean is that speciation in this group is underway, despite one author’s suggestion to that end in the BBC piece.  Speciation may be starting, but it seems likely that, given the degree of interbreeding that does occur in central Europe, the differentiation between the groups will reach some kind of equilibrium, with the two types remaining as races (morphologically differentiated groups) rather than full species (groups that have no gene flow between them).   In other words, they may wind up like human races — populations that differ in some of their traits but which can and do interbreed, so that selection driving them apart is balanced by interbreeding pulling them together.

This should not detract from the work or the phenomenon, which is really interesting.  In this spirit I offer three more caveats:

a.  British bird lovers may not have caused this phenomenon. Although the BBC article touts the phenomenon as showing that “feeding birds changes evolution,” we’re not sure if the largesse of seeds and suet is what really prompted the new population of overwinterers. In the BBC article, Oxford biologist Joseph Tobias speculates that the main drive may have been increasingly warmer winters in the UK.  However, without food I’m not sure the birds would have survived well enough to return.

b.  The morphological differences may not be genetic.  A genetic basis seems likely, but to ensure that those five traits were really due to evolutionary change and not simply to effects of the environment itself (e.g., diet), it would be good to rear the offspring of these birds under constant conditions in the lab, seeing if they retain their parental traits. My bet is that they will.

c.   If the morphological differences are genetic, they (and the DNA differences) might reflect a sampling artifact in the original UK migrants rather than post-migration differentiation/selection. Again, I don’t think this is likely, but if those birds who migrated to the UK were a nonrandom sample of the original population, the differences between them and the others might reflect a sampling event rather than differences that evolved after the split in wintering grounds.

The main fault of the BBC article, as opposed to the paper, was its failure to point out that the situation may reflect stable genetic differentiation rather than incipient speciation.  After all, the vast majority of populations that differentiate fail to become full species.  What is clear, though, is that these two populations of blackcaps have attained some level of reproductive isolation, which is the stuff of speciation.

If the blackcap populations do become full species, this would not, however, demonstrate reflect sympatric speciation, for the division did not initially occur in the presence of free gene flow between the speciating populations. (The initial tendency to go to different overwintering grounds  immediately reduces gene flow between the two types of birds because it yields, as a byproduct, assortative mating based on different arrival times at the breeding grounds.)

Fig. 1.  A cute blackcap. Photograph by Will Forrest from the Yorkshire Birders website.

Rolshausen, G., G. Segelbacher, K. A. Hobson, and H. M. Schaefer. 2009. Contemporary evolution of reproductive isolation and phenotypic divergence in sympatry along a migratory divide. Current Biology 19:1-5.