Extensive introgressive hybridization within the northern oriole group (Genus I cterus ) revealed by three‐species isolation with migration analysis

Until recently, studies of divergence and gene flow among closely‐related taxa were generally limited to pairs of sister taxa. However, organisms frequently exchange genes with other non‐sister taxa. The “northern oriole” group within genus I cterus exemplifies this problem. This group involves the extensively studied hybrid zone between B altimore oriole ( I cterus galbula ) and B ullock's oriole ( I . bullockii ), an alleged hybrid zone between I . bullockii and black‐backed oriole ( I . abeillei ), and likely mt DNA introgression between I . galbula and I . abeillei . Here, we examine the divergence population genetics of the entire northern oriole group using a multipopulation Isolation‐with‐Migration ( IM ) model. In accordance with H aldane's rule, nuclear loci introgress extensively beyond the I . galbula– I . bullockii hybrid zone, while mt DNA does not. We found no evidence of introgression between I . bullockii and I . abeillei or between I . galbula and I . abeillei when all three species were analyzed together in a three‐population model. However, traditional pairwise analysis suggested some nuclear introgression from I . abeillei into I . galbula , probably reflecting genetic contributions from I . bullockii unaccounted for in a two‐population model. Thus, only by including all members of this group in the analysis was it possible to rigorously estimate the level of gene flow among these three closely related species.