Darwin referred to the origin of species as "that mystery of mysteries," and even today, more than 150 years later, evolutionary biologists cannot fully explain how new animals and plants arise.
For decades, nearly all research in the field has been based on the assumption that the main cause of the emergence of new species, a process called speciation, is the formation of barriers to reproduction between populations.
Those barriers can be geographic -- such as a new mountain, river or glacier that physically separates two populations of animals or plants -- or they can be genetic differences that prevent incompatible individuals from producing fertile offspring. A textbook example of the latter is the mule; horses and donkeys can mate, but their offspring are sterile.
But now a University of Michigan biologist and a colleague are questioning the long-held assumption that genetic reproductive barriers, also known as reproductive isolation, are a driving force behind speciation.
"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 Daniel Rabosky, assistant professor in the U-M Department of Ecology and Evolutionary Biology and a curator of herpetology at the Museum of Zoology.
Rabosky and Daniel Matute of the University of Chicago reasoned that if genetic barriers to reproduction are a leading cause of new species, then groups of organisms that quickly accumulate those genes should also show high rates of species formation.
They tested that idea by comparing speciation rates to genetic indicators of reproductive isolation in birds and fruit flies. They chose birds and fruit flies because extensive data sets on interspecies breeding experiments exist for both groups. The researchers used evolutionary tree-based estimates of speciation rates for nine major fruit fly groups and two-thirds of known bird species.
Rabosky and Matute created computer models to carry out the comparison, and the results surprised them. "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. 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 speciation is often defined as the evolution of reproductive isolation, the new findings suggest that a broader definition may be needed, Rabosky and Matute conclude.