Species evolve slowly in arid Australia

By Emma Young 22 April 2010
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Warmer, wetter areas promote biodiversity, a new study suggests.

SPECIES THAT LIVE IN wet areas evolve faster than those in dry areas, suggests a new study of Australian trees. The finding could help to solve the mystery of why there is such an explosion of biodiversity in the planet’s tropics.

Earlier research has found that, in both plants and animals, warmer temperatures seem to increase the rate at which organisms grow and reproduce. With each cell division, there’s a chance of a mutation happening. Natural selection acts for or against mutations, ultimately giving rise to new species. So it’s possible that warmer temperatures help to drive evolution and diversity – provided that there’s enough water available.

Wet and dry habitats

A team of biologists led by Xavier Goldie at the Australian National University in Canberra wanted to investigate just how important water might be to biodiversity. Australia is the perfect place to test it, he says, because closely-related plants grow in places that have similar temperature ranges, but are in either dry outback regions or wetter coastal areas.

The experts looked at a range of plants, including acacias, conifers and melaleucas (paperbark trees) from New South Wales, South Australia, parts of the Northern Territory and Western Australia. They found that the wetter species had faster rates of genetic change than the drier species. “This is the first time that the role of water in evolutionary speed has ever been tested,” Xavier says. “Now, if you’re going to analyse the role of temperature and other variables in driving diversity from a molecular level, this shows we really need to take water into account.”

The study is published this week in the journal The Proceedings of the Royal Society B.

The results are “very exciting”, agrees Stuart Pimm, an expert on biodiversity at Duke University in Durham, North Carolina, US. “This is something quite different from what I’ve seen before, and it does suggest that wet and dry habitats are different.” But, he adds, there are other significant factors to consider when investigating diversity, aside from temperature and water availability.

Species richness

In Australia, the hottest, wettest places do not have the greatest diversity, Stuart points out. If you travel north from Tasmania, and then along the east coast, the steamiest spot is Cape York, at the top. “But the plant species richness peaks at about Brisbane. It drops off as one goes further north, and Cape York has fewer species than Tasmania,” he says.

One explanation for this is a so-called “mid-domain” effect, where more species ranges overlap in the middle of a region (like the east coast of Australia) than at either end – so long as there is enough water. However, Cape York also has a very short monsoonal season, interspersed with periods of drought, and, according to Xavier, this may act to slow evolution.

Xavier agrees, that other factors are probably involved in shaping diversity. The semi-arid regions of Western Australia have much more species diversity than you’d expect from the water availability alone, he says. “So there are other, additional factors at play, which are influencing more local and regional diversity patterns – and that’s something we’d like to get our teeth into.”

Warming world threatens plant diversity