The split that the researchers observed followed the recent establishment of a migratory divide between southwest- and northwest-migrating blackcap (Sylvia atricapilla) populations in Central Europe after humans began offering food to them in the winter. The two groups began to follow distinct migratory routes — wintering in Spain and the United Kingdom — and faced distinct selective pressures. Under that pressure, the two groups have since become locally adapted ecotypes. (Ecotypes represent the initial step of differentiation among populations of the same species, the researchers explained. If ecotypes continue down that path, they can ultimately become separate species.)
“The new northwest migratory route is shorter, and those birds feed on food provided by humans instead of fruits as the birds that migrate southwest do,” Schaefer said. “As a consequence, birds migrating northwest have rounder wings, which provide better maneuverability but make them less suited for long-distance migration.” They also have longer, narrower bills that are less equipped for eating large fruits like olives during the winter.
Schaefer says it isn’t clear whether the ecotypes will ever become separate species; in fact, he doubts they will because the habits of humans will tend to change over time. Even so, the findings do speak to the long-standing debate about whether geographic separation is necessary for speciation to occur. In particular, it had been contentious whether selection could act strongly and consistently enough in sympatry to separate a united gene pool.
“In highly mobile organisms such as birds, the consensus is that sympatric speciation is extremely rare, mainly because it is difficult to envisage how gene pools could be kept separate until speciation has occurred,” Schaefer said. “Our results now show that the initial steps of speciation can occur very quickly in a highly mobile, migratory bird,” because divergent selection during the overwintering phase leads to the evolution of reproductive isolation.
“This is a nice example of the speed of evolution,” he added. “It is something that we can see with our own eyes if we only look closely enough. It doesn’t have to take millions of years.”
Ed Yong explains further on his blog Not Exactly Rocket Science:
If the prospect of spending winter in Britain rather than Spain seems odd to you, you’re not alone. Indeed, blackcaps were hardly ever ventured across these shores before the 1950s. But since then, the birds have taken advantage of the glut of food left out on bird tables by animal-loving Brits. These banquets, along with the luxury of not flying over the Alps, have made Britain an increasingly popular holiday destination for wintering blackcaps. And that has set them down the path towards becoming two separate species.
The mystery of Britain’s winter blackcaps was solved in a classic series of experiments by Peter Berthold (awesome beard) in 1992. Berthold found that chicks from the two populations (those that fly to Britain and those that fly to Spain) would always fly in the same direction as their parents even if they were raised in identical environments. This strongly suggested that their travel plans were genetically set, and Berthold proved that by breeding birds from the two groups. Amazingly, their offspring migrated in a west-northwest direction, about halfway between the routes of their parents.
Berthold went on to show that the blackcaps’ inherited itineraries were the result of a handful of genes at most. And these initial differences have become magnified over time. When spring returns, the blackcaps fly home, they select mates and they form bonds that will last until the next year. But those returning from Britain have less distance to cover so they reach Germany first and they pair up with each other. When the stragglers from Spain get there, they only have each other to mate with.
Even though all of these birds spend most of the year in each others’ company, they are actually two populations separated by barriers of time that prevent genes from flowing from one group to another. Gregor Rolshausen from the University of Freiburg has found that their genetic separation is already well underway.
The one thing that is not explained but I would love to know, is how the slight change in beak morphology is correlated with a slight change in diet. Blackcaps don’t only eat fruits, they also eat insects. In these Blackcaps, what are the Brits feeding them (small fruits, I presume), has birdfeeder-related opportunism resulted in their eating insects less frequently?
The story was also discussed on NPR Science Friday.
- Image credit: Sergey Yeliseev, taken in Russia, Moscow region, Nagornoe, 07/15/2006. Flickr.
- Contemporary evolution of reproductive isolation and phenotypic divergence in sympatry along a migratory divide. Rolshausen, G., G. Segelbacher, K. A. Hobson, and H. M. Schaefer. 2009. Current Biology 19:1-5. DOI: 10.1016/j.cub.2009.10.061