Study of Darwin's finches sheds light on how one species become many

Using data on four species of Darwin's finches on the Galápagos Islands, researchers led by McGill University have confirmed a long-standing hypothesis that species diversity evolves through adaptation to different resources.

Evolutionary biologists have long suspected that the diversification of a single species into multiple descendent species—that is, an "adaptive radiation"—is the result of each species adapting to a different environment. Yet formal tests of this hypothesis have been elusive owing to the difficulty of firmly establishing the relationship between species traits and evolutionary "fitness" for a group of related species that recently diverged from a common ancestral species.

A global team of biologists led by McGill University has compiled nearly two decades of field data—representing the study of more than 3,400 Darwin's finches in the Galápagos Islands—to identify the relationship between beak traits and the longevity of individual finches from four different species.

Recently selected as the Editor's Choice article for the December issue of Evolution, the study used data from four species, which all evolved from a single common ancestor less than 1 million years ago. The researchers constructed a detailed "fitness landscape" to predict the likelihood of an individual's longevity in relation to their beak traits.

They found that finches with the beak traits typical of each species lived the longest, whereas those that deviated from the typical traits had lower survival. In short, the traits of each species correspond to fitness peaks that can be likened to mountains on a topographic map separated from other mountains by valleys of lower fitness.

"Biological species are diverse in their shape and functions mainly because individual traits, such as beaks, are selected by the environment in which the species are found," said lead author Marc-Olivier Beausoleil, a doctoral researcher at McGill University, supervised by Professor Rowan Barrett.

As a result, "the diversity of life is a product of the radiation of species to specialize on different environments; in the case of Darwin's finches, those environments are different food types," adds Professor Andrew Hendry, who has been a part of the project for more than 20 years.

Perhaps surprisingly, the researchers also found that the different species of finches studied have not reached the top of their fitness "mountain," suggesting that each species is not perfectly adapted to their food type. Whether such "perfection" will ultimately evolve remains to be seen.