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Toads point to new mechanism for evolution

Toad evolution Cane toads are shining light on a new process by which genetic traits evolve, say Australian researchers.

They say it has "revolutionary" implications for our understanding of evolution.

Professor Rick Shine of University of Sydney and colleagues report on what appears to be a parallel process to natural selection, called "spatial sorting," in this week's Proceedings of the National Academy of Sciences.

"It's a pretty radical concept. It's the first really new mechanism for evolutionary change for 150 years," says Shine.

"It's a very different process to anything that Darwin talked about because you're having traits evolve not because they help you survive or reproduce, but because they help you disperse faster."

According to Darwin, traits accumulate within a population if they help individuals survive better or have more offspring.

But cane toads are showing that in a dispersing population, traits can evolve without offering any benefit to survival or reproduction.

Cane toads were introduced into Australian state of Queensland in the 1930s and have now spread as far west as Kununurra and as far south as Sydney, says Shine.

He says the invasion front has accelerated dramatically with each generation evolving longer legs and being able to run faster and further than the one before.

"We know it's evolutionary because the kids of toads from different areas run just as fast as their parents," says Shine.

"So the question is what process has caused that evolutionary change?"

Spatial sorting

Natural selection predicts that the toads at the invasion front survive better or have better reproductive success, but studies have shown otherwise.

"None of the evidence says that's the case," says Shine.

Indeed, research shows cane toads at the front are more vulnerable to predators because they have not had a chance to poison them.

Also toads at the front suffer spinal arthritis and don't reproduce as much as other toads.

"So is there any other process that could have assembled this incredibly fast running toad?"

Drawing on previous mathematical models, Shine and colleagues developed a new model that supports a process called "spatial sorting".

This process results in the accumulation of genes that are good for dispersal, without there having to be an advantage to reproduction or survival.

"Individuals at the front line are going to be a very non-random set. They're going to be very bold and adventurous souls. They're going to be fast-moving, fast dispersing," says Shine.

As fast-moving mums mate with fast-moving dads, the genes for such things as long legs and high activity levels traits that help dispersal will accumulate.

"The process can work even if there is not survival or reproductive advantage," says Shine.

Natural selection?

Spatial sorting doesn't replace natural selection, says Shine, rather it runs in parallel with it, and within its bounds.

"Spatial sorting can only occur if natural selection doesn't oppose it," he says.

"If there was too strong a survival disadvantage to be at the invasion front, it simply wouldn't happen. Natural selection would be knocking off the toads at the front."

While Shine expects "squeals of outrage" from people trained in evolutionary biology in response to the idea, some don't think it's so radical.

"I think it's a really significant advance in evolutionary biology," says Professor John Endler, author of Natural Selection in the Wild, who reviewed Shine's article for publication.

Endler, based at Deakin University in Melbourne, is sure spatial sorting plays an important role in any population that disperses into new areas as a result of climate change.