Blackcap migration routes are genetically determined, and the population studied by Schaefer has historically wintered in Spain. Those that flew north couldn’t find food in barren winter landscapes, and perished. But during the last half-century, people in the U.K. put so much food out for birds that north-flying blackcaps could survive.These are the first steps toward anagenetic speciation, a process that has been observed in other species1. As the authors point out, this is fascinating not just because it may be an example of speciation in action, but that it is speciation influenced by humans.
About 30 percent of blackcaps from southern Germany and Austria now migrate to the United Kingdom, shaving 360 miles from their traditional, 1,000-mile Mediterranean voyage. Because they’ve less distance to travel, they tend to arrive home first in the summertime and to live in prime forest-edge spots. All this makes the U.K. migrants more likely to mate with each other than with their old-fashioned brethren.
From these groupings, subtle differences are emerging. The U.K. birds tend to have rounded wings, which sacrifice long-distance flying power for increased maneuverability. Now that they don’t need wide bills to eat Mediterranean olives in winter, their bills are becoming narrower and better-suited to summer insect diets. They’re also slightly darker.
1Ayala, F. J., M. L. Tracey, D. Hedgecock & R. C. Richmond (1974) Genetic differentiation during the speciation process in Drosophila. Evolution, 576-592.
Pfosser, M., G. Jakubowsky, P. M. Schlüter, T. Fer, H. Kato, T. F. Stuessy & B. Y. Sun (2005) Evolution of Dystaenia takesimana (Apiaceae), endemic to Ullung Island, Korea. Plant Systematics and Evolution, 256, 159-170.
Stebbins, G. L. & D. L. Hartl (1988) Comparative evolution: latent potentials for anagenetic advance. Proc. Natl. Acad. Sci. USA, 85, 5141-5145.
Stuessy, T. F., G. Jakubowsky, R. S. Gómez, M. Pfosser, P. M. Schlüter, T. Fer, B. Y. Sun & H. Kato (2006) Anagenetic evolution in island plants. Journal of Biogeography (J. Biogeogr.), 33, 1259-1265.
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