Evolution takes tiny steps, but over time those steps add up to create new species. Although researchers usually have trouble tracking these miniscule increments, a team has succeeded in explaining one of them--how a spot on a butterfly's wing changes in size. All it takes are a few tweaks in the gene sequence that helps guide the development of the eyespot.
Patricia Beldade, a developmental evolutionary biologist at the University of Leiden, the Netherlands, and her colleagues chose to study the tropical butterfly Bicyclus anynana because others had already determined the genetic pathways that form its eyespot. But no one had pinned down the source of small variations in these wing patterns, which are important to the insect's survival. These researchers focused on a gene called Distal-less, which helps set up the center of the eyespot.
First they bred the butterflies for nine generations to develop one strain with large spots and another with small spots. Because each butterfly carries two of the same copy of Distal-less, the researchers could breed butterflies with all combinations of the genes. When they examined the final strains, the group found that just a few differences in the sequence of "letters" or bases of the Distal-less gene caused spots to grow or shrink. The butterflies with large spots had far more Distal-less activity than the one with small spots, Beldade reports in the 17 January issue of Nature.
The work is significant on two counts, says Fred Nijhout, a developmental evolutionary biologist at Duke University in Durham, North Carolina. For one, "this is probably the first experimental study of the process of evolution, that is, of small genetic variation," he says. Furthermore, finding that sequence variation in genes--and not in genetic regulators of those genes as many have thought--affects key traits suggests new avenues for research.