Random and non-random variation in flower color along an urban-rural gradient in the introduced mustard Hesperis matronalis

Premise: Urbanization can alter the interplay of stochastic genetic drift and natural selection but these effects will depend on the biology and history of the species. We investigated the influences of drift and selection on the pattern of flower color variation among populations of the introduced ornamental mustard, Hesperis matronalis, along an urban-rural gradient in eastern Ontario Canada. Methods: We surveyed 136 naturalized stands of H. matronalis over three generations, and for each stand estimated the frequency of the three color morphs (white, pink, purple), the number of reproductive plants and the degree of urbanization based on night sky brightness. Key Results: Flower color morph diversity increased with both stand size and urbanization which is consistent with effects of genetic drift during colonization combined with multiple introductions of this horticultural plant in urban areas. However, the frequency and fixation of the purple morph systematically increased towards the rural end of the gradient. Although lifetime seed production did not vary among morphs, pre-dispersal seed predation by a recently introduced weevil was higher in the purple morph, particularly in rural areas. Estimated seed production in the absence of predation suggests a fitness advantage to the purple and pink morphs in rural areas and to the white morph in urban areas. Conclusions: Random variation in flower color diversity may be influenced by stochastic processes and colonization history, while systematic variation in color morph frequencies may reflect past fitness differences among morphs that have been recently erased by seed predation.