Rapid evolution of flowering time by an annual plant in response to a climate fluctuation.

Ongoing climate change has affected the ecological dynamics of many species and is expected to impose natural selection on ecologically important traits. Droughts and other anticipated changes in precipitation may be particularly potent selective factors, especially in arid regions. Here we demonstrate the evolutionary response of an annual plant, Brassica rapa, to a recent climate fluctuation resulting in a multiyear drought. Ancestral (predrought) genotypes were recovered from stored seed and raised under a set of common environments with descendant (postdrought) genotypes and with ancestorxdescendant hybrids. As predicted, the abbreviated growing seasons caused by drought led to the evolution of earlier onset of flowering. Descendants bloomed earlier than ancestors, advancing first flowering by 1.9 days in one study population and 8.6 days in another. The intermediate flowering time of ancestorxdescendant hybrids supports an additive genetic basis for divergence. Experiments confirmed that summer drought selected for early flowering, that flowering time was heritable, and that selection intensities in the field were more than sufficient to account for the observed evolutionary change. Natural selection for drought escape thus appears to have caused adaptive evolution in just a few generations. A systematic effort to collect and store propagules from suitable species would provide biologists with materials to detect and elucidate the genetic basis of further evolutionary shifts driven by climate change.