→Gossypium davidsonii, a long lived cotton endemic species to the Sonoran Desert, Baja California, Mexico, has been documented to express large (nearly 10-fold) variation in plant defenses (gossypol, a defense compound, gossypol-containing cavity density, and trichome density) across a narrow species range (100s of kilometers). Potential sources of variation explored were from phenotypic plasticity from variable environments, the stochastic events of migration and genetic drift, or a result from natural selection.

→From greenhouse studies testing extreme herbivory, nutrient and water conditions on defense trait expression, we found that most variation in defense traits were attributable to genetic differentiation among sampled populations. We also detected divergent selection on all defense traits, by comparing phenotypic differentiation (P_{ST) to genetic differentiation (FST) among populations, thought the cause of selection is still poorly understood.}

→Variation in defense traits may influence the interaction with associated insect herbivores. We extended the analysis of defense trait phenotypes, genetic relatedness, and spatial structuring of populations of G. davidsonii on fitness responses and seed and leaf damage. Using Multiple Regressions on Distance Matrices (MRDM), we found no common explanatory variables on fitness response variables across four sampled populations, which could be explained by the large environmental variability across site locations.

→We also determined whether ancestral and contemporary gene flow within the boll weevil, A. grandis, and associated populations of G. davidsonii were symmetrical. We found a two-fold difference between plant and herbivore in the degree of population differentiation and that methods detecting gene flow revealed significant incongruence in the geographical network of ancestral gene flow. This incongruence suggests that gene flow in the two species has been influenced by different geographical barriers to dispersal.

→Lastly, we assessed the population genetic structure of the boll weevil in North America on cultivated and wild cotton hosts. Genetic data favor a two-variant classification that takes into account the Sierra Madre Occidental mountain range which likely represents a large barrier to gene flow. We believe that classification of boll weevil would benefit from a phylogeographic perspective, consisting of two geographically distinct forms, each of which utilizes multiple host species.