A multidisciplinary approach to divergence in the Icterus graduacauda and Icterus chrysater group: genetic, morphometric and ecological divergence across the Isthmus of Tehuantepec.

Researchers studying earliest stages of speciation have the opportunity to study the divergence of multiple sets of characters. In this study I compared genetic, morphometric and ecological variation between two sister species of orioles, Icterus graduacauda and Icterus chrysater, across a well-known geographic gap, The Isthmus of Tehuantepec (50 km wide). In addition, I compared populations of Icterus chrysater across a much larger gap (around 600 km). s Morphometric analyses comparing Icterus graduacauda and Icterus chrysater revealed that both species show similar overall size with no diagnosable character differences between them. Furthermore, Ecological Niche Modeling (ENM) showed niche conservatism east and west of the isthmus, suggesting the two species share abiotic requirements, thus could potentially inhabit the same areas. However, in spite of the close proximity and bioclimatic similarity, coalescent analysis with the program IM shows no evidence of gene flow. These two taxa have very distinct plumage patterns, and in orioles plumage likely plays an important role in mate choice. Thus, this lack of gene flow may be the result of behavioral isolation based on plumage differences. However, for the comparison within I. chrysater, despite the large gap between north and south populations, IM shows strong evidence of gene flow. Additionally, ENM revealed evidence of ecological divergence between northern and southern populations, and there is some morphometric divergence. Therefore, within I. chrysater, I found evidence of gene flow in spite of a larger geographic gap, along with some niche and morphometric divergence. However, as these two populations have no known plumage differences, gene flow across the gap could be facilitated by the similarity in plumage patterns. The graduacauda/chrysater group is interesting because despite their similar overall morphometrics, environment and behavior, no hybrids have been found. Importantly, the differences in plumage could be one way that behavioral isolation is maintained. In contrast, within I. chrysater, the lack of plumage differences could be contributing to the gene flow that occurs in spite of the large gap between the two populations. Future studies are needed to test the importance of plumage in preventing and allowing gene flow in this species complex