Homoploid hybrid speciation in animals is poorly understood compared to the classic bifurcating mode of species diversification, mainly because of the scarcity of well-documented cases. Here, I presents the results of a multilocus sequence analysis on the house sparrow (Passer domesticus), Spanish sparrow (P. hispaniolensis) and their proposed hybrid descendant, the Italian sparrow (P. italiae). The Italian sparrow is shown to be genetically intermediate between the house sparrow and Spanish sparrow, exhibiting genealogical discordance and a mosaic pattern of alleles derived from either of the putative parental species. Further, as the Z chromosome previously has been shown to have a predominant role in bifurcation speciation in birds, I analyzed variation and divergence at autosomal and Z-linked genes, as well as one mitochondrial gene in the three sparrow taxa. Average variation on the Z was significantly reduced compared to autosomal variation in the putative parental taxa. Additionally, interspecific divergence between the two latter taxa was elevated on the Z relative to the autosomes. This pattern of variation and divergence is consistent with a faster-Z effect
(increased rate of adaptive divergence on the Z) and/or reduced introgression of Z-linked genes e.g. due to increased selection against foreign alleles in hybrids and backcrosses (Haldane’s rule).
FST-outlier tests were consistent with the faster-Z hypothesis. Two out of five Z-linked loci were identified as candidates for positive, divergent selection. In addition, isolation with migration analysis
was also consistent with reduced introgression of Z-linked genes, since the estimated rate of gene flow after the initial split of the house sparrow and Spanish sparrow was higher for the autosomal than the Z data set. Interestingly, the two Z-linked candidate loci for positive selection (CHD1Z and PLAA) showed a clearer mosaic pattern in the putative hybrid species than any of the autosomal markers. The Italian sparrow was fixed for Spanish sparrow alleles at CHD1Z and possessed mostly house sparrow alleles at PLAA. I argue that the present results, together with results from another recent study on the same species complex, provide one of the strongest cases of hybrid
speciation in animals.
