Learning to Adapt
Michael Sorenson: Evolution of a Con Artist
Evolutionary biologists traditionally thought that for new species to evolve, groups of animals had to be geographically separated long enough that they became unable to interbreed. This classic view is known as allopatric speciation. The possibility that new species might develop in the same geographical area, known as sympatric speciation, is less accepted among biologists, but may be possible if groups become reproductively isolated by physiology, genetics, or behavioral quirks.
The case for sympatric speciation is made stronger by the findings of biologist Michael Sorenson, who sees evidence in the fascinating behavior of indigobirds. Sorenson and his graduate students travel to Tibati, Cameroon, to study indigobirds that display brood parasitism, a phenomenon that he says floored him the first time he learned of it. The birds lay their eggs in the nests of other bird species. The young indigobirds look and act like the hosts’ chicks, so the hosts are tricked into raising the imitators as their own. As the birds become adults, they learn to sing the hosts’ song and use both it and their native indigobird songs to attract mates.
About ten species of indigobirds display brood parasitism, Sorenson explains, and each species lays its eggs in the nests of a specific host. Because the birds will only mate with other indigobirds that were raised in nests of their own hosts, they become reproductively isolated from indigobirds with different hosts.
For the theory of sympatric speciation to hold weight, behaviors, like song preferences, must lead to reproductive isolation, and ultimately an inability for the birds from two different groups to interbreed at all. Along with his collaborator and former advisor Robert Payne, a University of Michigan biologist, Sorenson has been studying two “races” of the Cameroon indigobird that differ only in their hosts. One parasitizes the African firefinch, the other the black-bellied firefinch. He explains, “We think they may represent two reproductively isolated populations in the very beginning stages of speciation.”
To test the overall model of sympatric speciation in this group, Sorenson and former student Chris Balakrishnan designed an experiment to play back the mimicked host songs to the males of each group. As expected, the indigobirds that grew up in the nests of African firefinches were agitated by hearing mimicked African firefinch songs, responding as if to a competitor for female interest. More importantly, if the mimicked black-bellied firefinch song was played, the males showed no interest, perceiving no threat. “In indigobirds sympatric speciation is definitely plausible,” Sorenson says, “and we have just about all the pieces of the puzzle.”
Such behavioral reproductive isolation may eventually lead to enough genetic changes over time to turn the races into recognizably different species, but the evolution of a new species is not immediate. “You might have to come back in 20,000 years or so,” Sorenson says. “But we know it has happened in the past, because we have ten species of indigobirds today, each with different hosts.” Sorenson is now expanding his observations to indigobird populations in East Africa. The driving force of this research, he says, is to understand how evolution has produced the diversity of life on this planet. “Indigobirds are just a unique and fascinating example of things working a little bit differently than the usual.”
This research appears in the May/June 2006 issue of Behavioral Ecology.
For more information, visit http://people.bu.edu/msoren.
— by Leah Eisenstadt