A change in climate causes rapid evolution of multiple life-history traits in an annual plant

We studied the effects of a natural drought on traits related to the timing of flowering in two populations of Brassica rapa. Seeds were collected before and after a five year drought in southern California and raised in the greenhouse. We found that the drought caused changes in several flowering parameters, including a shift to earlier flowering, longer duration of flowering, reduced peak flowering and greater skew of the flowering schedule. Time to first flowering explained little of the variation in other flowering schedule parameters. Descendants had thinner stems and fewer leaf nodes at time of flowering than ancestors, which indicates that the drought selected for plants that flowered at a smaller size and earlier developmental stage. Drought also caused evolutionary changes in correlations among traits, as determined by common principal component analysis. There was selection on several flowering schedule parameters, with selection on some parameters differing depending on the watering (season length) treatment. As expected, there was selection for later flowering under long season treatments and selection for earlier flowering under shorter season treatments. The results show that changes in climate are likely to cause a wide array of evolutionary shifts in phenology and patterns of reproductive availability, with important consequences for evolutionary responses to global change.