Around a decade after a new form of evolution was proposed to explain the behavior of cane toads in Australia, a study of beetles has confirmed it in the lab.
Darwin called his most famous work the “theory of evolution by natural selection”, but even the great man himself knew that survival of the fittest was not the only way in which species change. He also described sexual selection, in which those deemed most attractive get to pass on their genes, even if being sexy is not necessarily helpful for survival (we're looking at you, peacocks).
Eleven years ago scientists studying the spread of cane toads across Australia proposed another way in which evolution can operate, albeit a rarer one. They argued that when an invasive species is extending its range "spatial sorting" occurs, with those most suited to expansion inhabiting the frontier. Since these individuals mate with those around them, who have similar traits, these get re-enforced until the leading edge of the invasion can be quite different from those they left behind. Eventually, the two might even become separate species.
The authors of this theory provided observational evidence, as well as modeling to show it makes sense, but confirming through experiments is harder. “We can't replicate the spread of cane toads across Australia, gypsy moths across the Northeast or zebra mussels in the Great Lakes, for example,” said Dr Tom Miller of Rice University in a statement. Miller set out to address this problem using an easier study animal, the bean beetle (
Miller set out to address this problem using an easier study animal, the bean beetle (Callosobruchus Maulatus), for which a generation lasts just one month. He built a network of habitats for the beetles, amply stocked with black-eyed peas, on which the beetles lay eggs. Colonies were started with 100 insects, but in such favorable conditions numbers grew rapidly so that throughout the period almost 300,000 were hatched within 10 generations.
The more adventurous beetles crawled through tunnels from their starting places to uncover untapped beans, while others competed for access to the beans closer to home. In one experiment Miller occasionally shuffled the beetles so that some pioneers were moved back to base, while stay-at-homes got moved to new territory. In another version, the beetles were allowed to stay or go as they pleased.
In the study published in Nature Communications, Miller reports that when the beetles were left to their own devices they expanded 8.9 percent further. This was a product of the adventurous beetles mating with each other, producing offspring even more inclined to expansion.
First author, doctoral student Brad Ochocki, calls this the "Olympic Village Effect". If Olympic athletes conceived children during their time in the village, the results would be a select group of children likely to push records by being faster and stronger two decades later. Meanwhile, the rest of the population, deprived of those athletic genes, would become even less sporty.
The findings are not simply a matter of curiosity. "Farmers and other people who have an interest in maintaining a natural resource are good at detecting initial outbreaks," Miller said. "When they detect a new corn pest in a field, they want to know how far might it get the next season and how far ahead of a wave they should warn growers to spray for this new bug.” Miller and Ochocki's results provide a warning that such expansion may accelerate, forcing regular reassessments of how far species can be expected to spread.
A team at the University of Colorado conducted similar tests in parallel on red flour beetles. Their finding, of a 6 percent faster rate of spread with the Olympic Village Effect than without, was published in the same edition of Nature Communication.