An example of speciation in action?

The Blackcap, Sylvia atricapilla, is a small songbird that is quite common in Britain through spring and summer. In autumn it migrates south, spending its winter in Iberia and north Africa.In the 1960s and 1970s, people started noticing Blackcaps around in midwinter. Sightings increased over the years and soon it was clear that they were overwintering in quite sizeable numbers. They were noticed as much by ordinary people as by birdwatchers, because they frequently visited gardens of all sizes and fought for their turn at the bird table alongside the usual finches etc. This is in contrast to their habitat preference in summer, when they don’t generally visit gardens but prefer good-sized woodlands.Ringing/banding studies on Blackcaps have shown that the birds we’re seeing in Britain in winter are not the same ones that are here in summer. Our summer birds' behaviour hasn't changed - they still go off to Iberia and north Africa for winter. The birds that come to Britain in winter originate from central Europe, mainly Germany, and they go back to Germany in spring.Now, here’s the (really) interesting bit. Most German Blackcaps overwinter in the same areas as the British birds, reaching them by migrating in a south-westerly direction. However, a growing proportion (about 20% now) of the German Blackcap population sets off west-north-west rather than south-west at migration time, and ends up in Britain. Breeding studies with captive Blackcaps have shown that the migratory direction is genetically determined. Two WNW-migrators produce WNW-migrating offspring, the same goes for the SW-migrators. Pair one of each, and the offspring set off to migrate in an intermediate direction. These results indicate that the migratory direction is determined by just one or a few genes.However, in the wild pairing of one WNWer with one SWer doesn’t happen frequently at all, because — with their shorter journeys — the WNWers are all back home first. So they pair with each other, leaving the SWers to pair among themselves later. Reproductive isolation is achieved — not in space but in time. That the WNWers get the pick of the best breeding territory and the SWers have to make do with what’s left may well explain why the WNWers have increased so much in such a short space of time.The two groups of birds are also starting to look slightly different. The WNWers have shorter, rounder wings than the SWers — not so good for distance travel but better for manoeuvrability. They also have longer and thinner bills and browner plumage, traits which could be adaptive to their more wooded winter grounds and the food available to them there. The differences are subtle but consistent.I’d suggest that here could be a good example of a mutation that turned out to be beneficial causing a rapid evolutionary change in a wild bird population. Now that reproductive isolation has occurred, is there any reason to think that the different selective pressures facing the two populations would not lead to further differentiation? This looks to be a speciation event in progress.Sources: http://www.biologie.uni-freiburg.de/...lshausen-curbio09.pdf, http://jeb.biologists.org/content/199/1/49.full.pdf. And here's one of our local Blackcaps, photographed last spring. Edited by Admin, : Change title, was "Blackcaps in Britain - reproductive isolation in action"

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