The tropics are home to the vast majority of plant and animal species on Earth. This is why scientists have long considered the equatorial regions of our planet to be where evolution is at its most exuberant. But new research by Jason Weir, assistant professor of evolutionary biology at the University of Toronto Scarborough, suggests that if you really want to see evolution in action—along with some of the bird world’s sexiest singers—you might want to skip the tropics altogether.
For some time now, conventional wisdom has held that speciation—the process by which new species are created—occurs in tropical regions at higher rates than anywhere else, and this is why the tropics are such a hotbed of biological diversity. But Weir is the lead author on a new study, just published online in the Proceedings of the Royal Society B, that suggests the opposite—that speciation is actually happening faster in temperate regions than it is at the equator.
Weir and his coauthor, David Wheatcroft of the University of Chicago, measured the differences in the DNA of 116 closely related pairs of bird species, in order to estimate when they last shared a common ancestor. Then, for each pair, they compared differences in the length and complexity of their songs. Together, these two measurements provided the researchers with an estimate of how quickly each set of birdsongs had diverged, or evolved, over time.
When they plotted these results against the latitudes where the species are found, Weir and Wheatcroft discovered something surprising: the farther away from the equator, the faster the evolutionary change.
“Our results show the fastest rates of song evolution in places where diversity is relatively low,” says Weir. “This is surprising, because it suggests that a fast evolutionary pace does not drive high biodiversity. We are going to have to rethink why places like the Amazon have so many species. ”
According to Weir, there could be a number of explanations for his results. Over the last few million years, the climate of the tropics has been relatively stable, while the climate of temperate latitudes has undergone much more extreme, long-term fluctuations such as ice ages. These fluctuations result in many more species going extinct at higher latitudes than they do near the equator. Ongoing extinction events provide a continual flow of vacancies in the ecosystem into which new species can quickly evolve.
Another explanation may lie with the sexual behavior of female birds. In higher latitudes, breeding seasons become shorter, and females must select their mates much faster than they do at the equator. This means there is a great deal of pressure on the males to evolve highly distinct and complex songs in order to attract these highly motivated females.
“Females are always looking for the sexiest singers,” says Weir. “But at higher latitudes, with shorter breeding seasons, females may not have time to evaluate the males’ other qualifications as they do in the tropics. This pressure would likely drive the rapid evolution of complexity in birdsong. The higher the latitude, the sexier the singers have to be.”
Weir’s recent study supports some of his earlier work, published in Science in 2007, which was the first study to indicate that species in higher latitudes may evolve much faster than those near the equator. Taken together, Weir’s research suggests the accepted wisdom about the connection between biological diversity and the pace of speciation requires a significant rethink.
Jason Weir is one of UTSC’s newest faculty members, having arrived in July after completing a post-doc at the University of Chicago. Malcolm Campbell, vice-principal (research) at UTSC, is thrilled to welcome Weir to the Department of Biological Sciences.
"Professor Weir is a fantastic addition to the faculty here at UTSC,” says Campbell. “His latest research provides key insights into the underpinnings of biodiversity, and poses important questions regarding the interplay between regional climate and the tempo of evolution. Such studies will be crucial as we work to understand and safeguard biodiversity and evolutionary processes in a changing world."