Circadian and other natural clock-like endogenous rhythms may have evolved to anticipate regular temporal changes in the environment. We report that a mutation in the circadian clock gene timeless in Drosophila melanogaster has arisen and spread by natural selection relatively recently in Europe. We found that, when introduced into different genetic backgrounds, natural and artificial alleles of the timeless gene affect the incidence of diapause in response to changes in light and temperature. The natural mutant allele alters an important life history trait that may enhance the fly's adaptation to seasonal conditions.

favored in a small region of southern Italy but less favored farther north or south. We propose that an origin of the derived ls-tim allele in southern Europe, followed by its subsequent spread by directional selection, providescounterintuitively-a more compelling model for understanding the elevated frequencies of ls-tim in this geographical region.