| Two processes may lead to genetic divergence among populations. One is mediated by geography, whereby physical barriers and geographic distance limit gene flow among populations, resulting in divergence due to drift or mutation. Another is ecological speciation, whereby populations adapt to their local environments via natural selection, and gene flow is impeded by selection against dispersers in favor of adapted residents. I used natural populations of guppies (Poecilia reticulata) to determine the relative influence of these two processes in the structuring of populations. If geography is playing a strong role, I predicted that gene flow would be greatly impeded by physical barriers and geographic distance. If ecology is playing a strong role, I predicted that gene flow would decrease with increasing strength of divergent selection among populations. Specifically, I examined the relative roles of physical barriers, geographic distance, predation, and various other habitat features (e.g. canopy cover, water velocity) on the amount of gene flow among populations. I was thus able to determine whether natural selection or decreased dispersal plays a greater role in the reduction of gene flow. I found that physical barriers and geographic distance played a large role in the regulation of gene flow among populations. Predation and physical habitat features did not play a role in the reduction of gene flow. My research clarifies the mechanisms involved in speciation and the production and maintenance of biodiversity, important issues in conservation and evolutionary biology. |
