On the origin of an avian species by means of natural hybridization

Abstract

Hybrid speciation has until recently been largely neglected by zoologists. This attitude is now changing. However, data is still scarce, and published examples include few taxa. In this thesis I present the first genetic evidence for hybrid speciation in birds, the long held example of the Italian Sparrow Passer italiae. This bird is intermediate in phenotype between its putative parental taxa, House Sparrow Passer domesticus and Spanish Sparrow Passer hispaniolensis, and its distribution is limited to the Italian peninsula, falling between the much larger distributions of its putative parentals. I analyzed variation in both nuclear (751 individuals genotyped for 14 microsatellite loci) and mitochondrial (21 individuals analyzed for 967 bp of the ND2 gene) markers. Individuals analyzed were sampled over the whole distribution of the Italian Sparrow, including the contact zones with both putative parental taxa as well as areas where these live in allopatry. FST and RhoST-estimates show low but consistent values, and indicate that the Italian Sparrow is intermediate in divergence between the House Sparrow and Spanish Sparrow. Further, Bayesian cluster analysis conducted in STRUCTURE reveals two genetic clusters, clearly corresponding to the House Sparrow and Spanish Sparrow, and assigns Italian Sparrows partially to both clusters. I found evidence of a cline in assignment probabilities in the area of phenotypically pure Italian Sparrows, i.e. assignment to the House Sparrow decreases towards the south, which I interpret as introgression from the House Sparrow. Phylogenetic analyses of the mitochondrial ND2 gene show that the Italian Sparrow is indistinguishable from House Sparrow at this marker whereas Spanish Sparrows form a distinct clade, well differentiated from the House/Italian Sparrow-clade. I argue that, taken together, these results reveal a mixed nuclear genome of recent origin, and hence that the Italian Sparrow is a recently formed homoploid hybrid species. I propose a novel scenario of its origin rooted in the agricultural revolution, and discuss its taxonomic status as well as its future.