Pollinator visitation and foraging behavior can affect rates of outcrossing and patterns of gene dispersal in flowering plant populations. The timing of flowering can affect the availability and flight patterns of pollinators, and foraging behavior may differ among animals with different energy requirements and ecological constraints. The rate of outcrossing and the pattern of gene dispersal determine neighborhood size, in the sense of Wright's isolation-by-distance model, and thus affect evolutionary potential for random genetic differentiation. In this dissertation some of the ecological constraints governing pollinator visitation, flowering time, and pollen dispersal patterns, as well as the potential evolutionary consequences for flowering plant populations are discussed. Chapters 2 and 3 report work with two species of Linanthus, autogamous, beefly-pollinated California grassland annuals. In Chapter 2 it is shown that differences between Linanthus populations in density and flowering phenology can produce differences in genetic structure, and that serpentine populations--which bloom earlier relative to pollinator emergence--may have greater potential for local genetic differentiation. In Chapter 3, evidence is presented that the earlier flowering date on serpentine soil may result from selection for an optimal date of first flowering prior to pollinator emergence. Chapter 4 reports work with three herbaceous perennial Senecio species in the Colorado Rocky Mountains. A comparison of the foraging behavior of butterflies and bumblebees, pollinators with very different energetic requirements and ecological constraints, foraging on the same plant populations is reported. The observed differences lead to the prediction that bumblebee-pollinated populations will have the potential for extensive local genetic differentiation, while the addition of a small amount of butterfly pollination can increase gene dispersal in a population sufficiently to greatly reduce genetic drift and microgeographic adaptive differentiation.