The historical pattern of gene flow among migratory and nonmigratory populations of prairie warblers (Aves : parulinae)

Within a group of interbreeding organisms, the balance of gene flow among p opulations and microevolutionary forces acting within populations is expect ed to result in clinal transitions in the phenotypes possessed by members o f differentiated populations. Discontinuous variation between geographicall y adjacent populations suggests the presence of a significant barrier to ge ne flow. Here I present genetic evidence for restricted gene flow between m igratory and nonmigratory populations of prairie warblers. The nonmigratory form of this species is restricted to coastal mangroves in Florida and is morphologically distinguishable from the typical, migratory form that occur s across the remainder of the eastern United States. Pairs of migratory pop ulations exhibited little population subdivision (phi(ST) less than or equa l to 0.09), whereas pairs of migratory and nonmigratory populations are muc h more differentiated (phi(ST) = 0.27-0.42). A phylogenetic analysis of mit ochondrial DNA haplotypes did not offer evidence of long-term isolation of migratory and nonmigratory populations. Together with the population geneti c analysis, the phylogenetic relationship of haplotypes suggests that isola tion between these forms must have arisen relatively recently in their hist ory. Evidence for significant population structure is unexpected, given the geographic proximity of migratory and nonmigratory populations, the capaci ty for long-distance movements (e.g., migration) by prairie warblers, and s everal previous studies of population structure in North American birds. Ho wever, the findings are consistent with the geographic distribution of morp hological and behavioral variation and demonstrate that significant boundar ies between populations of vagile organisms may be relatively cryptic.