Mammals Evolve Faster in the Tropics, Confounding Scientists

By Eliza Strickland | June 24, 2009 2:37 pm

primatesIn the heat and humidity of the tropics you might expect that mammals take it slow and easy–but on the genetic level, they’re accelerating past their mammalian relations that live in more temperate zones. A new study has discovered that tropical mammals are accumulating mutations more quickly and are therefore evolving faster, in a finding that could help account for the phenomenal biodiversity of the rainforests. But the study’s unexpected results have posed a puzzle for biologists. “[It's] an empirical pattern that is begging for an explanation” [The Scientist], says evolutionary ecologist James Brown, who was not involved in the current study.

Previous research had shown that plants and marine microorganisms evolve more quickly in the tropical zone near the equator, but scientists believed that pattern would hold true only for cold-blooded creatures, whose body temperatures and metabolisms are determined by the temperature of the surrounding environment. Scientists believe that this link between temperature and metabolic rate means that, in warmer climates, the germ cells that eventually develop into sperm and eggs divide more frequently. “An increase in cell division provides more opportunities for mutations in the population over a given time,” explained [lead researcher Len] Gillman. “This increases the probability of advantageous mutations that are selected for within the species” [BBC News]. But this mechanism wouldn’t work in warm-blooded mammals, whose body temperatures remain roughly constant regardless of environmental factors.

In the study, to be published in the Proceedings of the Royal Society B, the researchers selected 130 pairs of closely related species where the two species in each pair lived at different latitudes. Gillman looked for small genetic changes known as microevolution, in which mutations cause differences within a population but haven’t yet given rise to new species. He focused on a single gene in each species pair, comparing them both to a “reference” gene in a common ancestor. By looking for mutations in the DNA code for this gene – each point where one letter in the code was substituted for another – the researchers were able to see which of the two mammals had “microevolved” faster. Animals living in environments where the climate was warmer had about 1.5 times more of these substitutions than the animals living in cooler environments [BBC News].

The first possibility, researchers say, is that animals’ behavioral adaptations in colder climates–like periods of hibernation or torpor during the cold months–have slowed down their metabolisms on average, reducing the frequency of their cell divisions. The second possibility, 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 [in the tropics], 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 [The Scientist]. The hypothesis takes its name from the eponymous queen in Lewis Carroll’s Through the Looking Glass, who tells Alice: “It takes all the running you can do, to keep in the same place.”

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Image: flickr / Ryan Somma

CATEGORIZED UNDER: Environment, Living World
  • tim

    Is there any evidence that humans living in the tropics are evolving faster than humans living in (1) temperate regions and/or (2) the artic? Also, are humans living in temperate regions evolving faster than those living in the artic?

  • Grant H

    @ tim:
    I don’t believe that would be the case. I would’ve thought that a percentage of humans globally are quite mobile and I suspect there would be sufficient gene mixing to prevent that scenario.

    It would be very interesting to find out what the answers to your questions are. But as the article indicates that these evolutionary differences in mammals have only just been established, I imagine you’re questions as yet, have no answers.

  • YouRang

    Grant H’s comment is relevant to the Out of Africa DNA hypothesis. Maybe humans are mobile now, but 50 kyr ago that wouldn’t have been true. So there would have been a drive for greater diversity in the tropical populations. Admittedly, the 2 measures of DNA age (ychromosome and mitochondrial chromosomes) have relatively little drive components to them (if any) by virtue of interacting very little with the Red Queen’s followers.
    Aside from all of which, the Red Queen hypothesis is not a theory yet. It might be that the stress response to cool environments is less consistent with variety than the stress response to hot environments. In the study, it might also be that the ancestral population that moved out of the intermediate land that was the homeland to both populations had to be more diverse to respond to a variety of harsher environments in the tropics. I.E. it seems to me that there are more different kinds of climate regions in the tropics than in the temperate zones. (I’m not sure where my climatology book is to verify that.)
    And as to whether evolution in humans is faster in the tropics: The main driver of human evolution is the human induction of our habitat. IOW there is no particular evolutionary virtue to the ability to run a 9.6 100 m dash. Maybe a few groupies might be seduced (as in Wilt Chamberlain’s pointless screwing with 20k women); but it won’t last in the long run (since running a 9.61 100 m in the next generation would be no big deal).

  • Adrian

    I often hear people wondering about human evolution these days – will people evolve in response to technology for instance – and it seems that a fundamental component of evolution is being left out of these ideas. Evolution works through the death of individuals who don’t have genetics that give them a competitive edge. Individuals do not of themselves evolve. Evolution occurs through the transfer by reproduction of successful genes.

    At the moment Chinese people may be evolving a resistance to breathing air pollution by virtue of an increased death rate in pre- sexually mature (or at least reproductive) individuals. However if medical interventions keep a large portion of people alive who otherwise would have died then their less effective genes are able to be passed on. Basically unless an environmental force is killing people before they reproduce we can’t evolve in response to it. So those people who can’t figure out how to use technology like cell phones or computers would actually have to be killed by those technologies before they reproduce in order for us to evolve into more technologically savvy animals.

    It is evident that our modern society is using technological interventions to prevent human deaths from all manner of causes. The actual effect of this is counter evolutionary. We are allowing individuals with increasingly defective genes to survive and reproduce. At this point in time the human genome is deteriorating not evolving.

    Having said that there is some recent evidence from twin studies that environmental factors can affect the expression of an individual’s genes. In other words that genes are actually changing in individuals in response to environmental demands. This is a radical new possibility for evolution but these findings don’t negate the dominant role that surviving to breeding age plays.

  • Adrian

    PS Sorry, by Chinese people I meant those people living in Chinese cities with severe air pollution.

  • stephen

    adrian i would consider retracting or revising this part of your statement “Evolution works through the death of individuals who don’t have genetics that give them a competitive edge” i understand what your getting at, its survival of the fittest but the fact that the individuals that are week may die out and become extinct has little effect on the strong left behind continuing to evolve its not as simple as that, evolution is better described in terms of adaptation and changes on many biological system which are transferred to subsequent generations and the fact that the weak may die out doesnt mean they couldnt or wouldnt have contributed to that effect on some level by reproducing with what ever or who ever that had a beneficial contribution. the fact they die is irrelevant if they reproduce and if they reproduce they do contribute to the effects of evolution.

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