With crushing heat and humidity, you'd think life would move sluggishly in tropical rainforests. But according to a new study, at least one thing proceeds more like the hare than the tortoise: molecular evolution. Faster evolution in the tropics than more temperate zones could help explain why rainforests are such hotbeds of diversity and have implications for how scientists calculate when one species diverged from another.
Scientists have uncovered hints that evolution progresses faster in regions closer to the equator than in those closer to the poles. But this consensus was never backed up with a solid explanation. "Nobody's tested it properly," says evolutionary ecologist Len Gillman of the Auckland University of Technology in New Zealand. One theory is that the tropics' smaller population sizes make it easier for random genetic changes to accumulate and increase the genetic differences between populations. Another is that the faster metabolism of tropical species, spurred by hotter temperatures and more sunlight, offers more opportunities for cell division to go awry. This could lead to potentially useful DNA mutations.
Gillman and colleagues planned a careful experiment to see what separated the tropical from the temperate. First they turned to 45 pairs of relatively widespread and abundant plants. A pair consisted of two related plants of the same genus, one species coming from tropical rainforests and the other from temperate rainforests. The scientists sequenced a region of DNA estimated to mutate one base pair every million years. By comparing these paired sequences to another sequence of a third, more distantly related plant, they determined how long ago the paired species diverged. That allowed the researchers to compare how many mutations each plant species accumulated in its genetic code since that reference point. On average, tropical plant species had more than twice as many mutations as temperate species in the same genus, the researchers report online this week in Proceedings of the National Academy of Sciences. Gillman says the results point towards faster metabolic rates underlying greater evolutionary rates in the tropics, because his team looked at species with large populations that would be unlikely to pile up random genetic changes as smaller populations might. "This provides an explanation for the pattern [of diversity] that you see across the globe," says Gillman.
"This paper is of extreme importance," says evolutionary ecologist Klaus Rohde of the University of New England in Australia. He says the findings underscore the need for scientists to study warm locales, from rainforests to undersea vents, to get a better understanding of the planet's diversity. Evolutionary ecologist James Brown of the University of New Mexico in Albuquerque says the findings could mean that global warming will speed up rates of evolution. "But it doesn't help to predict which species will benefit and which will suffer."