In songbirds, evolution moves in fits and starts, a new study suggests. The study argues that most evolutionary change results from the formation of new species, and not from gradual change within species, but it has rankled some researchers who question its approach.
Biologists have long had difficulty determining how much evolutionary change comes from new species being formed and how much from gradual changes that accumulate within species over time. The first view was popularized by Stephen J. Gould and Niles Eldredge in their much-debated theory of punctuated equilibrium.
Hoping to shed some light on the issue, evolutionary ecologist Robert Ricklefs of the University of Missouri, St. Louis, turned to passerine birds, the group of 5700 species that perch and sing, which account for over half of all birds species. Working with museum specimens, he measured the body, wings, tail, legs, and bill of 1018 species representing all major passerine groups. Then he borrowed information on genetic relatedness and age of bird groups from the famous study of bird relationships done by Charles Sibley and Jon Ahlquist. Ricklefs's statistical analysis revealed that groups with a history of generating lots of new species showed the most size and shape variation in their modern descendants. And to his surprise, older groups showed no more variation than younger groups. Taken together, the results suggest that species formation, not gradual change within species, produces most morphological variation in songbirds, Ricklefs concludes in a paper published 15 July in Nature.
Some experts are not impressed, however. Because morphological variation is a key criterion used to recognize species, explains avian systematist Joel Cracraft of the American Museum of Natural History in New York City, groups diverse in size and shape would be expected to have more species, making Ricklefs's findings "virtually tautological." Meanwhile, Storrs Olson, Curator of Birds at the Smithsonian Institution, points out that scientists know of many counterexamples to the assertion that species formation is the major cause of diversity. For example, Olson says, Bermuda land snails show great diversity but have formed few species, whereas some flycatcher groups show little diversity while forming many species. Ricklefs says such cases show that species formation and morphological diversity are independent, thus validating the need for his study.