Feeding birds in winter is a most innocent human activity, but it can nonetheless have profound effects on the evolutionary future of a species, and those changes can be seen in the very near term. That’s the conclusion of Schaefer et al. In their report which shows that what was once a single population of birds known as blackcaps (Sylvia atricapilla) has been split into two reproductively isolated groups in fewer than 30 generations, despite the fact that they continue to breed side by side in the very same forests.
How did this speedy evolution come about?
The split that the researchers observed followed the recent establishment of a migratory divide between southwest– and northwest-migrating blackcap 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. A first step to species differentiation which can lead to species separation in the end.
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. 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.
Although, it is not known if those birds will end up as separate species eventually, it goes without saying that it provides input to the long-standing debate about whether geographic separation is necessary for speciation to occur.