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Since the days of Charles Darwin, evolutionary biologists have widely believed that most new species form because they have adapted to different environments – but a new study from the University of Toronto suggests otherwise.
The study, published in the journal Sciencehighlights what researchers have called a “blind spot” in our understanding of why new species form.
“We found that species constantly adapt to similar environmental pressures,” says Sean Anderson, who co-authored the paper with Professor Jason Weir while earning his Ph.D. at the University of Toronto Scarborough. “They undergo classic Darwinian adaptation, but they don’t do it in very different environments.”
Although it is generally accepted that populations must be physically separated to begin evolving into new species, researchers say what happens during this isolation remains obscure. For decades, the dominant theory has been ecological speciation, which holds that groups evolve because they migrate to different environments and experience pressures that the rest of their species does not face, whether new sources of food or predators. Called divergent adaptation, environmental characteristics then determine the natural selection that causes the formation of a new species. An example of this is Darwin’s finches, which have evolved beaks better suited to seeds than insects.
But it’s also common to see species that have evolved to the point where they can no longer reproduce with their closest relatives, while still sharing many of the same characteristics as their counterparts. This gave researchers the impression that the environments in which evolution took place, although geographically distant, may not have been so distinct. This is an established but less adopted explanation, known as parallel adaptation.
“Ideas of divergent adaptation have been dominated to a large extent by the study of model organisms – species that exhibit large ecological differences,” says Anderson. “We wanted to see what patterns we could find by studying as many species as possible.”
The researchers used the largest and broadest dataset of divergent traits found in species and their closest relatives – called sister pairs – ever assembled. They also created a statistical model capable, for the first time, of estimating whether a species evolved through parallel or divergent adaptation. Among nearly 3,000 sister pairs of birds, mammals, and amphibians, species mostly evolved under similar large-scale environmental pressures.
“We found this really consistent signature where parallel adaptation seems to be what dominates – and no matter what traits you look at, it’s the same in almost every species pair group you have,” explains Anderson, who is currently completing postdoctoral research at the University of North Carolina at Chapel Hill. “We were surprised at how consistent this signing was. »
Anderson says that, in some cases, species can develop similar traits while undergoing changes at the genetic level. This can cause them to become different species.
“It’s often not just one pressure: species face a range of similar pressures,” says Anderson. “And the external environment isn’t the only thing that can pose challenges to a species. Its own genome can do this by producing things like selfish genetic elements.
The findings could have far-reaching implications as theories about the causes of species evolution help biologists draw conclusions about biodiversity. While most species evolve through divergent adaptation, building biodiversity requires diverse habitats with different resources and challenges. But if it is a parallel adaptation, biodiversity depends on the geographical distance and the time that separates us.
“I hope this will have the impact that people won’t necessarily assume that divergent adaptations are causing speciation,” Anderson says. “These results could also change the way we think about the evolution of biodiversity and which factors we consider most important. »
Thanks to the University of Toronto for this news.