How does one species become many?
Evolutionary biologists have long suspected that the diversification of a single species into multiple descendent species â that is, an âadaptive radiationâ â 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 âfitnessâ for a group of related species that recently diverged from a common ancestral species.
A global team of biologists led by ±«ÓăֱȄ have compiled nearly two decades of field data â representing the study of more than 3,400 Darwinâs finches in the GalĂĄpagos Islands â 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 âfitness landscapeâ to predict the likelihood of an individualâs 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.
âBiological 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,â said lead author Marc-Olivier Beausoleil, a doctoral researcher at ±«ÓăֱȄ supervised by Professor Rowan Barrett.
As a result, âthe diversity of life is a product of the radiation of species to specialize on different environments; in the case of Darwinâs finches, those environments are different food typesâ 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 âmountain,â suggesting that each species is not perfectly adapted to their food type. Whether such âperfectionâ will ultimately evolve remains to be seen.
About the study
 by Marc-Olivier Beausoleil et al. was published in Evolution.