Natural Selection Saves Majestic Butterfly

Author:  Metcalfe David

Date:  July 2007

While many students of evolution are familiar with the story of the peppered moth, the most recent chapter of evolutionary theory has been written about a black butterfly. The butterfly species Hypolimnas bolina, H. bolina, has experienced "the fastest evolutionary change that has ever been observed", according to Sylvain Charlat, a post-doctoral researcher, who led a study published in Science earlier this month. Although Charlat holds a joint appointment at the University of California, Berkeley and University College London, she conducted her fieldwork on the South Pacific islands of Upolu and Savaii. This research may change how biologists view evolution and has already led Gregory Hurst, a senior co-author on the paper, to report that "we usually think of natural selection as acting slowly, over hundreds or thousands of years. but the example in this study happened in a blink of the eye in terms of evolutionary time".

H. bolina are a species of butterfly that has been battling, in evolutionary terms, for a long time with the Wolbachia, a genus of maternally inherited bacteria which selectively kill male larvae. Only a year ago, H. bolina accounted for just 1 percent of all butterflies on the Samoan island of Savaii. Recently, however, these selection pressures have resulted in the propagation of a suppressor gene through the butterfly population which protects males from Wolbachia infection. A suppressor gene is one that confers some protective benefit to individuals that inherit it. As a result, some 39 percent of butterflies on Savaii are now identified as H. bolina. In addition to challenging the assumption that natural selection is always a slow and gradual process, the case of H. bolina also "strengthens the view that parasites can be major drivers in evolution".

Although females accounted for 99 percent of the H. bolina population at the beginning of 1996, this number was reduced to 61 percent by the end of the year. It has not yet been established whether the suppressor mutation that rescued this species came about by chance or from another butterfly population.

By David Metcalfe