Long-held assumption about emergence of new species questioned

speciation_wideDarwin 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. Their study is scheduled for online publication in the Proceedings of the National Academy of Sciences on Sept. 2.

“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. Edited from Long-held assumption about emergence of new species questioned.

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2 Responses to Long-held assumption about emergence of new species questioned

  1. Phil Krause says:

    First of all lets clear up the myth that mules are infertile and cannot produce offspring. Horses have 64 chromosomes and donkeys have 62. Mules and hinnies have 63 chromosomes. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos. So far, there are no recorded cases of fertile mule stallions. However, occasionally a female mule produces offspring when mated with a purebred horse or donkey. Therefore, a small percentage of mules are viable.

    This study found that there is no evidence to suggest that new species are more likely to form when populations are separated by physical barriers. A physical barrier is not enough to cause a change in separated populations, they also need to have ecological differences between these populations. “Ecological speciation” in which new species arise as a result of populations adapting to different environments is a real driving force for genetic adaption to these environments. If a physical barrier separates two populations of the same species and the environments are sufficiently different from each other, I think the study would have more chance of proving positive. However, in my opinion, hybridization plays a much bigger role in speciation than is given credit for and maybe even the major driving force behind it.

    • Deskarati says:

      Studies like these are great for breaking down historically accepted ideas and will hopefully lead to more thorough examinations. But, computer models always leave me with a suspicion that a theory bias may be inadvertently programmed in.

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