THE SPATIAL SCALE OF GENETIC DIFFERENTIATION IN A HUMMINGBIRD-POLLINATED PLANT - COMPARISON WITH MODELS OF ISOLATION BY DISTANCE

We examined patterns of genetic differentiation at isozyme loci in natural populations of Ipomopsis aggregata, for which distances of gene flow had been previously estimated. Using genetic neighborhood areas based on direct estimates of gene flow to define subpopulations, we found that F(ST) Values (the inbreeding coefficient of a subpopulation relative to the total population) were consistently positive but small. Allele frequencies were spatially autocorrelated at distances of less than 5 m only. Both measures of genetic structure indicate some isolation by distance but less than predicted by current population genetics theory. Subpopulation size derived from realized gene flow estimates from paternity analysis produced a slightly better fit to theory than did using estimates from tracking pollen dispersal to stigmas. Discrepancies between observation and theory may also arise from other sources including occasional long-distance gene flow that is difficult to observe.