Population Structure and Evolutionary Dynamics of Wild–Weedy–Domesticated Complexes of Common Bean in a Mesoamerican Region

The diversity, structure, genetic flow, and evolutive relationships within and among three wild–weedy–domesticated complexes and a wild population isolated from crops of common bean (Phaseolus vulgaris L.) were analyzed under traditional agriculture, within a region of the Mesoamerican center of domestication. Their diversity was compared with the diversity of a local commercial variety and a breeding line. A morphological marker and 37 polymorphic ISSR loci were used. Total genetic diversity within the wild, weedy, and domesticated populations across the complexes was very similar (0.24, 0.22, and 0.26, respectively). Weedy populations within each complex were more closely related to the domesticated than to the wild populations, suggesting they originated from introgression of wild-type alleles into the domesticated populations or predominant gene flow from domesticated to wild populations. The wild population in closest proximity to the crop within its complex was more similar to the domesticated and weedy populations of its complex than to the rest of the wild populations, suggesting displacement of the wild genetic diversity by gene flow from the domesticated population within its complex. The high values of differentiation among wild, weedy, and domesticated populations within each complex suggest high autogamy or genetic drift. However, the values of gene flow among populations within the complexes were close to one, theoretically sufficient to counteract genetic drift and/or autogamy. We therefore assume that human selection is the most important evolutionary mechanism for maintaining the high wild-domesticated differentiation by negative farmer selection of cultivated plants with morphological characters that suggest introgression. Farmers may influence the magnitude and characteristics of gene flow among populations within each complex by the management of the distance between the crops and the wild populations, the diversity within the landraces sown, and the tolerance and harvesting of weedy populations. The high geographic differentiation of the wild populations, together with the local differences in human selection practices and agronomic management, could have generated multiple evolutionary lineages after domestication. Domesticated populations within complexes were between two and four times more diverse than the local commercial variety and four and nine times more diverse than the breeding line. New conservation and breeding strategies are suggested to maintain and use the gene pools from these complexes.