Historical divergence and gene flow in the genus Zea.

Gene flow plays a fundamental role in plant evolutionary history, yet its role in population divergence--and ultimately speciation--remains poorly understood. We investigated gene flow and the modalities of divergence in the domesticated Zea mays ssp. mays and three wild Zea taxa using sequence polymorphism data from 26 nuclear loci. We described diversity across loci and assessed evidence for adaptive and purifying selection at nonsynonymous sites. For each of three divergence events in the history of these taxa, we used approximate Bayesian simulation to estimate population sizes and divergence times and explicitly compare among alternative models of divergence. Our estimates of divergence times are surprisingly consistent with previous data from other markers and suggest rapid diversification of lineages within Zea in the last approximately 150,000 years. We found widespread evidence of historical gene flow, including evidence for divergence in the face of gene flow. We speculate that cultivated maize may serve as a bridge for gene flow among otherwise allopatric wild taxa.