Population structure and local selection in Impatiens pallida (Balsaminaceae), a selfing annual

Population structure in a predominantly self-pollinating annual, Impatiens pallida (Balsaminaceae), was investigated at two localities, Allerton and Brownfield, in central Illinois, USA. A hierarchical ANOVA based on 19 quantitative characters measured for 60 families at each locality indicated significant genetic differentiation between transects (3 transects/locality, 32-50 m apart) and among-families within transects (20 families/transect). Discriminant function analysis conducted on family means indicated complete multivariate discrimination between transects. Among-family variance, calculated by transect, was significant for virtually all characters at both localities and was heterogenous across transects for a number of characters. These results demonstrate that inbreeding populations can maintain significant genetic variation, with local differences in the magnitude of variation. Reciprocal seed-transplant experiments demonstrated local adaptation at Brownfield, but not Allerton. Differential fitness of Brownfield transplants was due to variation in post-germination survivorship to reproduction. Striking local differences in survivorship at Brownfield have resulted in spatial variation in selection pressures, and life-history evolution. Catastrophic mortality caused by a specialist herbivore in the Brownfield forest interior has promoted the evolution of early reproduction in forest plants, as compared to neighboring, herbivore-free sites. A significant negative regression of seed production on flowering time for seedling transplants at a Brownfield forest transect indicated the potential for local selection. These results illustrate that the population structure of inbreeding plant species is a complex function of ecological and genetic factors.