Tropical mammals are evolving faster than those found at high latitudes or elevations, according to a study published online today (June 23) in Proceedings of the Royal Society B. This pattern had previously been found in plants and marine protists but until now was assumed to apply only to cold-blooded organisms.

Structure of DNA helix Image: Richard Wheeler, Wikimedia Commons

"There's lots of reasons to believe that temperature plays a substantial role in generating [differences in the rate of evolution]," said evolutionary ecologist James Brown of the University of New Mexico, who did not participate in the study. "What's particularly interesting here is that [this same pattern] occurs in mammals, which take their body temperatures with them wherever they go around the world." That means, he said, that "this [difference] can't be a direct effect of temperature per se."

In 1799, German naturalist and explorer Alexander von Humboldt noticed that there was greater species richness near the equator. Nearly two centuries later, evolutionary ecologist Klaus Rohde of University of New England in Australia suggested a mechanism. "At higher temperatures, mutations are more frequent and generation times are shorter," resulting in faster evolution, said Rohde, who was not involved in the research. This could in turn lead to higher rates of speciation, which could result in the greater species diversity found in the tropics.

However, because they are able to regulate their body temperature independent of habitat, "the theory wasn't thought to apply to warm blooded animals," said ecologist Gary Mittelbach of the Kellogg Biological Station in Michigan.

Evolutionary biologist Len Gillman of Auckland University of Technology in New Zealand and colleagues compiled a global dataset of 130 pairs of closely related species whose ranges differed in either latitude or elevation. Comparing sequences of cytochrome b -- the most consistently available mammalian gene in the GenBank database -- for each of these pairs as well as two closely related species that served as reference points for the comparison, Gillman discovered that DNA substitution rates were substantially faster for those species that live at lower latitudes and elevations.

"[It's] an empirical pattern that is begging for an explanation," Brown said. "The most likely mechanism, which would have to do with the effect of temperature, can't hold for bird and mammals" -- because they are both warm-blooded -- "so we've got to look else where for the explanation."

The authors propose two reasons why even warm-blooded animals may evolve faster in warmer climates. The first, known as the Red Queen hypothesis, suggests that because the other organisms that interact with mammals, such as parasites and plants, are undergoing faster evolution, there is a coevolutionary pressure for mammals to keep up. "The biotic environment is changing more rapidly, and therefore the mammals are evolving more rapidly in response," Gillman said.

Alternatively, behavioral adaptations to temperate climates, such as hibernation and torpor, may reduce the annual average metabolic rate of mammals at high latitudes and elevations. This in turn could decrease the chance of a germline mutation that may spread through the population. The paper can't distinguish between the two, Rohde said, but "I suspect the combination of both will be found for mammals."

Gillman is now looking at the effects of water availability on the rate of evolution. "For most species," he said, "it's not just temperature that's important for species richness, it's productivity."

Another area in need of research is the link between the rates of molecular evolution and the patterns of species richness. "No one knows whether these intriguing patterns of microevolutionary change are in fact going to be drivers of speciation," said Mittelbach. "So there's still a fundamental step here that's missing."


Related stories:

Genetic evidence for punctuated equilibrium
[6th October 2006]

Mutations go tick, tock
[20th July 2004]

Evolution at warp speed
[19th August 2002]