Abstract
BackgroundCharacterizations of the dynamics of hybrid zones in space and time can give insights about traits and processes important in population divergence and speciation. We characterized a hybrid zone between tanagers in the genus Ramphocelus (Aves, Thraupidae) located in southwestern Colombia. We evaluated whether this hybrid zone originated as a result of secondary contact or of primary differentiation, and described its dynamics across time using spatial analyses of molecular, morphological, and coloration data in combination with paleodistribution modeling.ResultsModels of potential historical distributions based on climatic data and genetic signatures of demographic expansion suggested that the hybrid zone likely originated following secondary contact between populations that expanded their ranges out of isolated areas in the Quaternary. Concordant patterns of variation in phenotypic characters across the hybrid zone and its narrow extent are suggestive of a tension zone, maintained by a balance between dispersal and selection against hybrids. Estimates of phenotypic cline parameters obtained using specimens collected over nearly a century revealed that, in recent decades, the zone appears to have moved to the east and to higher elevations, and may have become narrower. Genetic variation was not clearly structured along the hybrid zone, but comparisons between historical and contemporary specimens suggested that temporal changes in its genetic makeup may also have occurred.ConclusionsOur data suggest that the hybrid zone likey resulted from secondary contact between populations. The observed changes in the hybrid zone may be a result of sexual selection, asymmetric gene flow, or environmental change.
Highlights
Characterizations of the dynamics of hybrid zones in space and time can give insights about traits and processes important in population divergence and speciation
Niche models suggest that the hybrid zone may have originated via secondary contact as a result of climatic change since the Last Glacial Maximum (LGM) causing population expansions, of flammigerus from the Cauca Valley
For each of seven genetic loci sampled across a Manacus hybrid zone in Panama, values of cline center and width estimated using Hill functions are similar to point estimates of these parameters obtained by a previous study [66] using cline-fitting algorithms HZAR and Analyse
Summary
Characterizations of the dynamics of hybrid zones in space and time can give insights about traits and processes important in population divergence and speciation. Tests of hypotheses posed to account for the origin of hybrid zones may be conducted by examining the historical distribution of hybridizing taxa using paleodistributional modeling, an approach employing niche models that characterize the current distribution of species in climatic space to infer historical potential distributions given climatic conditions of the past [8, 9] Such models of historical distributions represent hypotheses one can further test using molecular data to evaluate their population-genetic predictions, such as signatures of population growth for presumably expanding populations and of constant population size and isolation by distance in populations occurring within climatically stable areas [10, 11]. This approach has revealed that several hybrid zones likely originated following range expansions leading to secondary contact [12,13,14,15]
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