Speciation Theory Questioned
September 3, 2013

Mating Barrier Theory Of Speciation Called Into Question

Brett Smith for redOrbit.com - Your Universe Online

While scientists have catalogued millions and millions of species, there is still no agreement on how exactly new species form. However, a new study published this week in Proceedings of the National Academy of Sciences could deal a major blow to one prevailing theory - namely, that new species arise when a barrier prevents reproduction between populations.

Species-creating barriers could be physical, such as a mountain range, or they could be genetic differences that prevent the birth of a viable offspring. For example, when a horse mates with a donkey a sterile mule is born.

"Most research on the formation of species has assumed that these types of reproductive barriers are a major cause of speciation. But our results provide no support for this, and our study is actually the first direct test of how these barriers affect the rate at which species form," said study co-author Daniel Rabosky, a biologist at the University of Michigan.

Over the past several years, geneticists have been focused on the discovery of a handful of "speciation genes" that represent the genetic reproductive barriers between species. In 2008, the journal Nature published a paper on the first speciation gene in a mammal.

However, Rabosky and his co-author Daniel Matute of the University of Chicago, posited that if genetic barriers to reproduction can cause a new species to arise, then groups of organisms that are prone to accumulate those genes should also exhibit high rates of species formation.

They tested that theory through the comparison of speciation rates to the prevalence of genetic barriers in birds and fruit flies, two organisms with extensive data sets on interspecies breeding experiments. The genetic markers were compared to evolutionary tree-based estimates of the speciation pace for nine fruit fly groups and two-thirds of known bird species.

"We found no evidence that these things are related. The rate at which genetic reproductive barriers arise does not predict the rate at which new species form in nature," Rabosky said.

"If these results are true more generally — which we would not yet claim but do suspect — it would imply that our understanding of species formation is extremely incomplete because we've spent so long studying the wrong things, due to this erroneous assumption that the main cause of species formation is the formation of barriers to reproduction.”

"To be clear, reproductive barriers are still important on some level,” he added. “All sorts of plants and animals live together in the same place, which couldn't happen without reproductive barriers. But our results question whether genetic reproductive barriers played a major role in how those species formed in the first place."

While the study was somewhat limited in scope, the role of speciation genes may only be minimal if the team’s findings apply more widely to other organisms, they said.

"The whole enterprise of finding 'speciation genes' is potentially irrelevant to understanding the origin of species," Rabosky said. "But our study is certainly not going to be the final word on this. If anything, our results indicate that a lot more data will be needed before we can conclusively link the mechanisms we usually study in the lab to the patterns of species formation we see in the natural world."

Another theory suggests that it is fairly easy for one species to diverge into new species, but that extinctions prevent the majority of new species from leaving a lasting evolutionary  impression.