Credit: Alan John Lander Phillips

Various studies have been done to analyze the genomics of adaptation in response to selective pressures, mostly using asexual organisms with relatively small genomes, such as bacteria. Now, a new study has looked at this process in a sexually reproducing organism—the tiny fruit fly (Drosophila melanogaster).

In asexual, single-celled bacteria, evolution generally occurs when a new, advantageous genetic mutation arises, 'sweeps' through a population and quickly becomes 'fixed' in the organism's genome. Some theorists believe that this same 'hard sweep' process also drives evolution in sexual organisms. But there are other mechanisms: 'soft sweeps', in which selection acts on existing genetic variation within a population, such that mildly favorable variants of multiple genes become more common; and 'incomplete sweeps', in which an advantageous mutation arises but does not become fixed. The new study suggests that these alternative mechanisms were responsible for evolution in fruit flies exposed to laboratory selection.

Anthony D. Long (University of California Irvine) led the study, in which populations of fruit flies were subjected to laboratory selection for accelerated development over 600 generations. Genomic analysis was used to compare these populations with ancestral control populations that experienced no direct selection on development. Long's group identified several dozen genomic regions in which the frequency of certain genetic variants was significantly different between the two populations (Nature 467, 587–590; 201010.1038/nature09352). These results are not consistent with the type of genomic changes that would be expected to result from a hard sweep, as selection did not lead to the fixation of a new, advantageous genetic variant. Instead, the frequencies of many genetic variants had changed. “This research really upends the dominant paradigm about how species evolve,” said Long in a press release.

The results indicate that in fruit flies, adaptation to selection for accelerated development involved changes in many genes, rather than just one. This observation has repercussions for the process of developing new treatments for certain diseases in humans. Disease susceptibility is a complex trait. For complex traits that are influenced by only a few genes, it should be relatively easy to develop treatments by targeting the products of these genes. For complex traits that involve many genes, however, developing effective treatments may be much more complicated. Michael Rose, an author of the Nature paper, stated in a story published in The New York Times (20 September 2010) that the study results are “very bad news for the pharmaceutical industry in general.”