How long does it take to evolve into a new species? Since the 1930s, evolutionary theorists have thought many thousands of generations were typically needed for the complex genetic differences found between most closely related species to accumulate. But new research on Florida's rapidly evolving soapberry bugs reveals that such complex genetic changes can crop up in less than 50 years.
Creatures can adapt fairly quickly to new conditions, such as life on a new host plant. All that it takes, biologists reckon, is the right gene mutation. Other kinds of evolutionary change are more complicated. Like slot machine dials, various gene combinations can sometimes add up to jackpots of useful new features. This type of jumbling would seem to require more time to get right.
Evolutionary biologists led by Scott Carroll of the University of California, Davis, suspected that such mixing and matching might have taken place in soapberry bugs. These pests infest the balloon vine, but some populations have started to take advantage of the Asian goldenrain tree ever since it was introduced in 1955. Within an estimated 100 generations and less than 50 years, bugs on the new hosts have already evolved differences in survivorship of the young, flight apparatus, beak length, and other features, in contrast to vine-dwelling populations. Because beneficial gene mutations are thought to be exceptionally rare, the scientists postulated that the bugs' speedy adaptation must rely on something else.
Carroll's team went looking for the genetic basis of these complex traits. Over 6 months, the team bred hybrids of the two types of soapberry bugs and mated them back with purebred bugs. Instead of the expected ratios of features in hybrid offspring that would indicate the action of new mutations, says Carroll, the team found evidence instead that interactions among combinations of genes underlie characteristics such as increased beak length, the team reports online 14 May in Biology Letters.
Previous work has shown that complex genetic changes are only important in differentiation of species over thousands of years, says evolutionary geneticist William Bradshaw of the University of Oregon in Eugene. This study "shows that [it] can occur on a much more immediate time scale," he says.