4 September 2008
New findings challenge long-held assumptions about flightless bird evolution
by Kate Melville
University of Florida zoologists say that large flightless birds such as the ostrich and emu do not share a common flightless ancestor as once believed. Instead, each species individually lost flight after diverging from ancestors that did have the ability to fly.
Edward Braun and his fellow researchers began closely studying this group of flightless birds (known collectively as ratites) after they noticed that the ratites did not form a natural group based on their genetic makeup. Rather, they belonged to multiple related but distinct groups that contained another group of birds, the tinamous, with the ability to fly.
Previously, the ratites were used as a textbook example of vicariance, a term that describes the geographical division of a single species, resulting in two or more very similar sub-groups that can then undergo further evolutionary change and eventually become very distinct from one another.
Scientists assumed that a single flightless common ancestor of the ratites lived on the supercontinent of Gondwana, which slowly broke up into Africa, South America, Australia and New Zealand; once divided, the ancestor species evolved slightly in each new location to produce the differences among the present-day ratites, Braun explained. But in light of this new information, he said it's more likely that the ratites' ancestors distributed themselves among the southern continents after the breakup of Gondwana, which began about 167 million years ago, in a much more obvious way. They flew.
The study, appearing in the Proceedings of the National Academy of Sciences, has several important implications, says Braun. He contends it means some ratites, like the emus, are much more closely related to their airborne cousins, the tinamous, than they are to other ratites. And second, it means the ratites are products of parallel evolution - different species in significantly different environments following the exact same evolutionary course.
Although these new revelations teach evolutionary scientists a great deal, they also pose a great many new questions. For example, why did these birds evolve into such similar organisms in such different environments? "To know for sure, we'll have to go into the lab and really study the genetics underlying the ratites' developmental pathway," Braun said. "But nobody would have asked that question without the type of data we've collected, which raises the question in the first place."
Source: University of Florida