Long-held assumption about emergence of new species questioned

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 that prevent incompatible individuals from producing fertile offspring. A textbook example of the latter is the mule; horses and 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 , 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 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 . They chose birds and fruit flies because extensive data sets on 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.

Over the last decade or so, the pursuit of the genetic underpinnings of speciation has led to reports on the discovery of a handful of "speciation genes," defined as genes that contribute to between species. The studies include a 2008 Nature paper that reported the first speciation gene in a mammal.

But if the findings of Rabosky and Matute prove to be more widely applicable to other organisms, speciation genes probably play a minimal role in the formation of species, they said.

"The whole enterprise of finding 'speciation genes' is potentially irrelevant to understanding the ," 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."

The researchers speculated that part of what is missing from speciation studies is, paradoxically, extinction. Some researchers have suggested that speciation might be limited primarily by factors associated with the persistence of new species. These models propose that it is relatively easy for a species to split into new species, but the vast majority of new do not persist over geological timescales.

Explore further

Reproductive isolation driving evolution of species

More information: Macroevolutionary speciation rates are decoupled from the evolution of intrinsic reproductive isolation in Drosophila and birds, www.pnas.org/cgi/doi/10.1073/pnas.1305529110
Citation: Long-held assumption about emergence of new species questioned (2013, September 2) retrieved 20 October 2019 from https://phys.org/news/2013-09-long-held-assumption-emergence-species.html
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Sep 02, 2013
"Most research on the formation of species has assumed that these types of reproductive barriers are a major cause of speciation."

Huh. I thought that these barriers were the result of speciation, not the cause. I thought the cause was mutation and differential environmental selection. I thought that speciation was the result of divergence, and that these barriers evolved as a way of maintaining all the advantages gained from adapting to new environments.

Life works hard to adapt. Groups have much to lose from incursion by unadapted individuals, and those groups which could resist incursion would prevail over those which tended to lose their adaptedness.

Sep 02, 2013
Maybe I am missing something here, but the "one gene - one trait" model has been abandoned elsewhere long since.

And why would "speciation genes" fixate any faster than other genes? I would have been surprised if they would have seen a correlation between fixation and speciation. As TGO notes, speciation ought to drive fixation and the latter may have to play catch up depending on population sizes, preexisting variation et cetera.

FWIW, a newly proposed driver of speciation has been bacterial communities, especially digestive communities. E.g. differences in food intake, differences in bacterial co-communities - and bacterial communities evolve faster than the eukaryote population.

Sep 03, 2013
Speciation doesn't work like that.
It's true that reproductive barriers are still important but only in conjunction with other causes.
They should've used two different environments so the drive for genetic selection work Availability of food and food type, temperature, magnetic fields. Change something in order for the organism to have something to adapt to.
Genetic mutation is a fact but if you have an organism that is fit for a certain environment there will be no need for selection of new mutations as "fitter"

Sep 03, 2013
Reproductive barriers only provide the *opportunity* for speciation.
Actual speciation requires genetic differentiation between to related populations to the degree that reproduction no longer results in fertile offspring.
The forces that drive the differentiation may be stochastic or environmental, or a mixture thereof. If the differentiation does not occur to the required degree, then removal of the barrier will permit breeding between the races that can result in fertile offspring.
The sticking point is just *how much* genetic dfferentiation is required. A single gene polymorphism, in a critical gene, may be sufficient.

Sep 03, 2013
And from this little step in the right direction, a very short, sober and reasonable paradigmatic hop to macroevolution.net?

Sep 04, 2013
And from this little step in the right direction, a very short, sober and reasonable paradigmatic hop to macroevolution.net?

You are correct in that plunging over a precipice of ignorance is but a single hop.

Sep 04, 2013
Gmr, you wouldn't care to explain yourself, would you? Rather than hurling insults, I mean. You know, in the interests of dispelling ignorance and all.

Sep 04, 2013
We already had this dance over this "belief" of the author.

I will not have the noble lineage of the platypus dragged through the mud again, especially when the author has admitted it is a belief rather than a true hypothesis, and therefore not subject to analysis or experimentation.

Sep 07, 2013
I would like to see how the equation changes when one considers the impact that contributions from worlds other than our own are thrown into the genetic soup. That we have been visited in fairly recent historical context can't be denied. The dolicochephaloid races of Peru are definitely alien. Those skulls aren't the result of artificial binding, because that procedure does not increase cranial capacity and those skulls have twice the cranial capacity of the average human. They gave us red, green, and yellow bell peppers, all manner of beans and legumes, the tomato, and much more.

According to the ancient Sumerian accounts of "the sons of the gods", we would not otherwise have sheep.

The list is probably endless, and makes for interesting genetic diversity on planet Earth.

Sep 07, 2013
baudrunner, they are normal people who had their heads sandwiched in boards as infants. It's not anything like what you've seen on late re-runs of "In Search Of." And there is no increase in cranial capacity - actually take and superimpose one of those skulls over a "normal" skull and scale to match markers such as eye sockets and jawline - it's readily apparent they had thinner heads than most, and the volume was the same. Or better yet, get the actual measurement of the interior volume of the head.

And if you actually look, there isn't that much genetic diversity among humans - certainly nothing which requires the intervention of aliens.

I suppose I shouldn't interrupt you from your research of such documentaries as "Puma Man," but I really have to say those skulls are not proof of anything other than cultural relativism in aesthetics.

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