Background
The authors write that "one rigorous way to determine if evolution has occurred in a natural population is to collect propagules before and after an environmental change and raise them under common conditions," stating that "this approach was used previously to show that Brassica rapa [a self-incompatible weedy winter annual] evolved drought escape through earlier flowering following a series of recent dry years in Southern California, and that early flowering results in higher fitness under drought conditions (Franks et al., 2007)," while noting that "a related study showed that multiple phenological traits and their interactions evolved in response to the drought (Franks and Weis, 2008)."

What was done
Working with the same pre- and post-drought collection lines from the Franks et al. (2007) experiment, the two researchers estimated the amount of assortative mating within, and the degree of phenological isolation between, two B. rapa populations.

What was learned
Franks and Weise (2009) determined that "climate change can alter plant phenology, which can change patterns of assortative mating within populations," and that "this assortative mating can directly change genotype frequencies and can also increase the rate of evolution by interacting with selection." In addition, they demonstrated that "climatically driven changes in phenology can potentially influence gene flow among populations due to changes in overlap in flowering schedules," and that "these changes in gene flow can also influence both the rate and pattern of evolutionary change."

What it means
The two scientists conclude that "the high degree of interdependence of flowering time, assortative mating, selection and gene flow make predicting evolutionary responses to changes in climate particularly complex and challenging." We further note that this great degree of complexity suggests that among the multiplicity of outcomes, there is a good chance that one or more of them will be just what the plants need to successfully respond to the climate change that elicited the many outcomes.

References
Franks, S.J., Sim, S. and Weis, A.E. 2007. Rapid evolution of flowering time by an annual plant in response to a climate fluctuation. Proceedings of the National Academy of Sciences USA 104: 1278-1282.

Franks, S.J. and Weis, A.E. 2008. A change in climate causes rapid evolution of multiple life-history traits and their interactions in an annual plant. Journal of Evolutionary Biology 21: 1321-1334.