Field mustard is an annual plant. This means field mustard plants die off every winter, and new plants grow in the spring from the seeds that the plants produced. In a certain population of field mustard plants, there are some plants that flower sooner after they germinate and others that flower later. This is because of genetic differences between the plants. After several years, the average number of days between germination and flowering has changed, as shown in the graph below.

graph of the original and current distributions of days to flowering; the current population takes fewer days on average
Data adapted from Franks, Sim, and Weis, Proc. Natl Acad. Sci. USA, January 2007

Which of the following best explains what may have caused this change?