In outcrossing plants differences in pollinator selective pressures among sites can result in inter-population floral trait variation. However, these differences in site-specific pollinator preference cannot explain flower size and shape differences among self-pollinating plant populations. Three factors that could account for morphological differences among sites in selfing species are 1) genetic differences in response to direct selection toward a floral optimum that maximizes anther-stigma contact while minimizing resource expenditure on floral displays, 2) a correlational response to spatially heterogeneous directional selection for locally beneficial vegetative traits, or 3) neutral evolutionary processes. This study analyzes the fitness landscapes, response to selection, and genetic variation of five populations of the outcrossing annual, L. nanus, and its selfing relative, L. bicolor, to determine whether population differences in floral traits are due to direct selection, indirect selection, or neutral evolutionary processes. Fitness landscapes are constructed using Aster regression analyses of vegetative and floral canonical axes, and genetic diversity is calculated using microsatellite data. To measure response to selection, we compare morphological measurements from 2009 and 2010.
Results/Conclusions
Our results show that neither L. nanus nor L. bicolor is experiencing collinearity between floral and vegetative traits. We also find that while L. nanus floral traits responded to direct selection pressures from one year to another, L. bicolor floral traits do not respond to selection. The inability of L. bicolor to respond to selection pressures is likely due to the extremely low levels of within-population genetic diversity we found for this species. This study supports the theory that, while population differences in floral morphology are due to direct selection on floral traits in outcrossing species, neutral evoloutionary processes dominate trait evolution in selfing populations.