Is a Galapagos finch caught in a split?

One group of finches on Santa Cruz Island in the Galapagos may become a new textbook example of the way in which two species emerge from one while still living together.

Early ideas for explaining how species arise required a geographic barrier, such as a body of water. Physically separated populations grow increasingly different from each other.

From the cradle of ideas about speciation, Sarah Huber of the University of Massachusetts in Amherst reports signs in the Galapagos of a different process: populations diverging without a geographic barrier. In recent years, other biologists have seen traces of this process in such creatures as cichlid fish and Rhagoletis flies (SN: 7/21/01, p. 42). Huber studies the species called medium ground finches (Geospiza fortis). Biologists have noted that these and 13 other modern Galapagos finch species have beaks that differ with the types of seeds they eat.

All the G. fortis birds that Huber analyzed had beaks within the lengths and depths expected for the species. Yet her measurements tend to cluster toward two extremes, a large-beaked group and a small-beaked one, instead of occurring in an even distribution.

Huber also found that the birds tend to choose mates with beaks close in size and shape to their own. Her genetic analysis showed unusually low gene flow between the large- and small-beaked groups.

These finches are demonstrating the kinds of changes that could lead to a split into two new species, Huber says. She says that she doesn't yet know what's driving the divergence, but the birds may eat mostly seeds of different sizes.

Huber compares these inland birds and their Spartan environment with those only 12 kilometers away, at Academy Bay on Santa Cruz Island. They're described in the Aug. 7 Proceedings of the Royal Society B by Andrew P. Hendry of McGill University in Montreal. Old records show that the bay birds used to have two clusters of beak sizes too. Now the clusters have blurred together. The authors suggest that an influx of people has brought lots more kinds of food for the birds, wiping out any need for specialized beaks and thus reversing a natural evolutionary trend.