Cane toads evolve in space reported in ABC News in Science 22 March 2011 and PNAS doi: 10.1073/pnas.1018989108. Rick Shine of University of Sydney and colleagues claim to have found a new mechanism of evolution after studying the relentless march of cane toads across northern Australia. According to the researchers, the rate of toad invasion has accelerated due to the toads evolving longer legs and being able to run faster than previous generations. Shine explained: “We know it’s evolutionary because the kids of toads from different areas run just as fast as their parents. So the question is what process has caused that evolutionary change?”
According to Darwin’s theory of natural selection, evolutionary changes are preserved in populations because they enhance survival and reproduction. Therefore, the long legged toads at the invasion front should survive better or reproduce more, but according to Shine, “None of the evidence says that’s the case”. In fact, the long legged toads are proving more vulnerable to predators, tend to get spinal arthritis, and reproduce less. Shine and his colleagues have come up with a new explanation: “spatial sorting”. Shine explained that the toads at the front line are a “very non-random set”. They would be “very bold and adventurous souls” that are “going to be fast-moving, fast dispersing”. Therefore, as toads with these traits find themselves in the same place they mate with each other, and the genes for the fast moving traits accumulate in the succeeding generations. Shine said: “The process can work even if there is not survival or reproductive advantage”. Shine claimed: “It’s a pretty radical concept. It’s the first really new mechanism for evolutionary change for 150 years”.
Editorial Comment: In 2006 we described the process that Shine and colleagues claim to have discovered, but congratulations to them for coming up with a name for it: “spatial sorting”. We wrote: “…the toads which are winning the race across Australia, are the ones that already have a gene for longer legs. As a result of leaving the others behind the faster toads are becoming temporarily separated from the shorter legged toads. Therefore, toads at the invasion front can only mate with toads of similar leg length and thus reinforce the long legged tendency”. See our original report Toads Evolve into Eco-Nightmare here. We pointed out then that the long legged toads at the invasion front were not evolving by Darwinian natural selection, and they are still not evolving. It’s not evolution since no new genes are actually appearing at all and the toads are still the same species. (Ref. amphibians, Bufo marinus)
Evidence News 8 August 2012