Assortative mating may have facilitated rapid evolution of locally adaptive phenologies during a biological invasion

Assortative mating for a trait under selection can increase the rate of adaptive evolution and limit introgression of detrimental alleles. We therefore hypothesized that assortative mating facilitated the rapid evolution of flowering phenology observed in the self-incompatible perennial Lythrum salicaria (purple loosestrife) as it invaded eastern North America. We characterized flowering schedules of 600 individuals representing 20 populations sampled along a latitudinal gradient of 10 degrees and grown in a common garden at mid-latitude. Consistent with our hypothesis, we found that flowering schedules were less synchronous in the mixed population of the common garden (s = 0.50) relative to the 20 locally adapted populations comprising it (s = 0.56 to 0.86), and flowering synchrony decreased towards higher latitudes of origin. Despite high synchrony, assortment potential ({rho}) by growth and phenology varied by population ({rho} = 0.03 to 0.61), but the predicted negative correlation between s and {rho} was masked by among-individual fitness variation in the common garden. We show how fitness variation and asynchrony associated with locally adapted phenology can reduce the fitness impacts of maladaptive gene flow and accelerate the rate of adaptive evolution during invasion.