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"

Thread copied here from the An example of speciation in action? thread in the Proposed New Topics forum.

But they're still birds... Actually, this is interesting. I would be interested to see when they decide to actually classify them as new species, and who gets the right to name the WNWers.

Hi Perdition,I think that we're well away from the point where we could call these WNW birds a new species. I've seen one of these winter blackcaps and they look, sound and (save for migration) act just like the regular birds. It certainly is an interesting example of reproductive isolation though.Mutate and Survive

Hi aiki, thanks.I was just reviewing the small differences in the Asian Greenish Warbler ring species, and wonder if any field studies are being set up to track changes in plumage and song as these two populations diverge.It will likely take several generations for such changes to start cropping up, and so any field studies would have to be dedicated to the long term, like the Darwin Finch study by the Grants.Enjoy.

Another interesting aspect of this case is that human behaviour (putting out food for the birds in winter) is thought to be an important factor, helping to boost the overwinter survival of the WNWers. They can be remarkably aggressive to other species at the bird feeders - successfully defending a lump of suet from all comers is a sure way to get through those colder nights. If everyone in Britain decided to stop feeding the birds, the course of this story might take another twist.What this example highlights to me is the capricious nature of 'beneficial mutations'. When creationists use this term they often seem to have in mind something like bionic legs or a dramatic increase in intelligence. However, there can be a large chunk of chance involved as to whether the effects of a mutation are good or bad. A mutation that caused British-born Blackcaps to head WNW at migration time would send them not to a well-stocked garden bird feeder but to a watery grave halfway across the Atlantic.

General bird ringing work, especially Constant Effort sites, should yield a steady stream of data on biometrics. I don't know of any ongoing Blackcap-specific fieldwork, but then I wouldn't, necessarily, as I'm not a research biologist but just a fanatical birdwatcher I'll have a look around, though.A bit about Constant Effort: http://www.euring.org/...urope/euro_ces_guidelines210904.pdf

Fair enough. Who determines how much difference is needed, though? I know the definition of species is up to interpretation, but who is the arbiter? It certainly is. And Aiki says that the two populations can breed, but often don't because of the timing of their arrivals. Would a speciation event require them to be unable to interbreed, or is it enough to simply show that they tend not to?In the Green Warbler example RAZD has brought up, there seems to be multiple species that can interbreed with the ones near them, until the ring closes and the two "ends" can't. Or is there only one species...or is there two, with some undefined line somewhere in the middle? The picture, with the multiple colors shading into each other seems to indicate multiple, interbreeding species.

Well, as with any mutation, they need to grant the ability to exploit a food source or exploit an old one better, to be beneficial. The fact that the food source is left out by people is almost irrelevant. If the people stopped, the mutation would no longer be beneficial, but this is no different than any other environmental change that removes a food source.

Groups that could produce offspring with outsiders but that do not breed with them are sometimes called 'demes':

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In biology, a deme is a term for a local population of organisms of one species that actively interbreed with one another and share a distinct gene pool. When demes are isolated for a very long time they can become distinct subspecies or species.

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Edited by Modulous, : No reason given.

This depends on exactly how you choose to define a species. Some people consider pre-mating reproductive isolation, as in this case, to be sufficient while the hard liners go for post-zygotic reproductive isolation, i.e. hybrid sterility or inviability.TTFN,WK

Hi Perdition, They are currently classed as multiple varieties of the same species (hence the three part names with the first two being the normal species genus\species pair and the third being the variety name)There are also four hybrid zones between the five daughter variety populations. The hybrids do not appear to infiltrate back into either of the neighboring variety populations, which leads me to conclude that the varieties are demes (thanks Modulus):

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Message 10: Groups that could produce offspring with outsiders but that do not breed with them are sometimes called 'demes':

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In biology, a deme is a term for a local population of organisms of one species that actively interbreed with one another and share a distinct gene pool. When demes are isolated for a very long time they can become distinct subspecies or species.

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Each of the daughter variety populations meet the definition for deme as given. It is not known (at least by me) that the two northern varieties of greenish warbler cannot interbreed, just that they don't: their behavior blocks them from seeing the other population as potential mates.Similarly, with the blackcaps, we observe that they normally don't breed, due to timing (rather than behavior), BUT they also note that the migratory behavior is different for the few hybrids from captive breeding experiments: Thus any naturally occurring hybrids would possibly not survive their altered migration, and this would help solidify the genetic differences.As Wounded King (Message 11) points out, there is variation on where biologists draw the line. There are some that are "lumpers" and some that are "splitters" with different opinions on how many individuals fall into the different species categories. Indeed, this is an example of natural selection operating on the different varieties\behaviors, and if the opportunities and challenges change it is possible that the populations would merge, or that one or the other would die out.Enjoy.ps - also avid birder

Thanks to everyone that responded, especially for the new term (demes).I knew it was sort of a personal choice for when to split the two into different species, but demes seems to be a very good middle ground.

There is no arbiter, nor any universally agreed on definition. That's why different works will claim different species. Regardless of what people may claim their standard of speciation is, I think you'd be very hard pressed to find an actual list of species written by anyone which truy obeyed the rule that two species should be biologically incapable of breeding. For most species, I doubt anyone really knows, since I don't think all closely related species are routinely interbred to see if it produces fertile offspring. In cases where experiments have been done, intentionall or accidentally, some very distantly related species have been shown to be able to hybridise, and produce fertile offspring.
All sorts of fowl are capable of hybridising together, in many cases producing fertile offspring. There'd be far fewer species of grouse and duck if people were taking this concept seriously.Using two different definitions of species, a 1999 study counted either 101 or 249 endemic bird species in Mexico.

Presumably it's easier to drum up support to conserve a unique form if it is designated a full species rather than 'just' a subspecies - would we expect conservationists to be ardent supporters of 'splitting'?The small subset of birders whose main motivation is to amass a huge life list of species seen are fans of splitting too, for obvious reasons I know a few who'd be delighted if the WNW Blackcaps were split just because it would be another tick on their list, but as Granny says that's unlikely to happen any time soon.