Answer by Diane Eager

The cane toad Bufo marinus was introduced from Hawaii to the east coast of northern Australia in the 1930s, in an attempt to control cane beetles in sugar cane farms. It was a great failure of biological control. Since then the toads have not only largely ignored the cane beetles, they have hit the road and have been migrating across northern Australia.

In 2006 biologists from University of Sydney studied the movement of toads and compared the size of the toads found at the front with toads in older more established populations. They found the toads on the front line had longer legs than those further behind. One researcher explained the results as follows: “During an invasion process the individuals at the front are there because they have moved the furthest. We showed that the toads that are the first to arrive at the front are the ones with the longest legs, and the ones last to arrive have shorter legs. The front toads also have much longer legs than the older populations in Queensland.” (BBC News 15 February 2006) The long legged toads were then claimed to be an example of rapid evolution.

We first wrote about this study in Evidence News, our free e-mail newsletter, and made the following comment: “All that is happening here is the toads that are winning the race across Australia already have genes 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.” (Read our report and comment here)

Since then further research has confirmed our conclusion that toads at the front of migration had become separated from the main population and were only breeding with one another, and therefore passing on the ‘long legged’ genes that enabled them to stay out in front. One group of researchers has also claimed this phenomenon is a new type of evolutionary process, and has given it the name “spatial sorting”. (ABC News in Science, 22 March 2011)

The reason for claiming a new evolutionary process is that evolution is supposed to occur by natural selection of individuals who have developed new genes that give them some survival advantage over the remaining population, and therefore they reproduce more and pass on the advantageous genes, which then spread throughout the population over succeeding generations. However, the long legged cane toads show no survival advantage compared with the shorter legged toads they left behind. Other studies show the long legged toads are not fitter than short legged toads. They have a tendency to spinal arthritis, are more vulnerable to predators, and reproduce less. Therefore, there had to be another reason for the development of a long legged population.

“Spatial sorting” is a good name for what is happening amongst migrating toads, but it is not evolution. If toads with genes for long legs can only breed with other long legged toads because they have been separated in space from short legged toads, the next generation will inherit long legs, and therefore stay at the front of the great toad race across Australia. As long as the invasion front keeps moving the faster, long legged toads will be separated from short legged toads and continue to breed a population of long legged toads. This is not evolution – it is just normal ecology and genetics.

In our 2006 report we also made the following prediction based on the fact that Australia happens to be an island, and eventually the invasion front will reach the western coast and stop moving. We predicted: “when the short legged toads catch up to the front line they will breed with the long legged animals once more, resulting in a return to the average leg length seen in older established populations.” We stand by this prediction because the toads have not evolved into a new kind of animal. They are still cane toads of the same species, Bufo marinus. Therefore, when the short legged toads catch up they will be able to breed with the long legged varieties, and continue to multiply after the kind, just a Genesis says that living creatures were created to do.

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