Abstract
Under different environmental conditions, hybridization between the same species might result in different patterns of genetic admixture. Particularly, species pairs with large distribution ranges and long evolutionary history may have experienced several independent hybridization events over time in different zones of overlap. In birds, the diverse hybrid populations of the house sparrow (Passer domesticus) and the Spanish sparrow (Passer hispaniolensis) provide a striking example. Throughout their range of sympatry, these two species do not regularly interbreed; however, a stabilized hybrid form (Passer italiae) exists on the Italian Peninsula and on several Mediterranean islands. The spatial distribution pattern on the Eurasian continent strongly contrasts the situation in North Africa, where house sparrows and Spanish sparrows occur in close vicinity of phenotypically intermediate populations across a broad mosaic hybrid zone. In this study, we investigate patterns of divergence and admixture among the two parental species, stabilized and nonstabilized hybrid populations in Italy and Algeria based on a mitochondrial marker, a sex chromosomal marker, and 12 microsatellite loci. In Algeria, despite strong spatial and temporal separation of urban early‐breeding house sparrows and hybrids and rural late‐breeding Spanish sparrows, we found strong genetic admixture of mitochondrial and nuclear markers across all study populations and phenotypes. That pattern of admixture in the North African hybrid zone is strikingly different from i) the Iberian area of sympatry where we observed only weak asymmetrical introgression of Spanish sparrow nuclear alleles into local house sparrow populations and ii) the very homogenous Italian sparrow population where the mitogenome of one parent (P. domesticus) and the Z‐chromosomal marker of the other parent (P. hispaniolensis) are fixed. The North African sparrow hybrids provide a further example of enhanced hybridization along with recent urbanization and anthropogenic land‐use changes in a mosaic landscape.
Highlights
Hybridization has recently become widely accepted as a driving force of speciation in animals and plants
The outcome of hybridization may not solely result from the de‐ gree of geneticcompatibility between genomes of parental species, that is, postzygotic reproductive barriers that strengthen over divergence time
The sparrow hybrid system provides another striking case where differentially structured hy‐ brid zones between the same species exist in different regions of their range of overlap
Summary
Hybridization has recently become widely accepted as a driving force of speciation in animals and plants. In Europe, characteristic zoogeographic patterns of parapatry and secondary overlap have arisen from lineage sepa‐ ration in glacial refuges and postglacial range expansion (Hewitt, 2000, 2004; Schmitt, 2007) In these hybrid zones, interspecific gene flow is mediated by a complex interplay of different factors such as the strength of reproductive barriers or the timescale of contact. The North African landscape is characterized by a dense mosaic of agricultural landscapes and human settlements separated by diverse types of steppe and desert (Hirche, Salamani, Abdellaoui, Benhouhou, & Martínez‐Valderrama, 2010) In this mo‐ saic hybrid zone, the two sparrow species and their hybrids occupy suitable habitats in patchy anthropogenic landscape (such as crop fields, palm oases, villages, and cities) but avoid the interspersed in‐ hospitable arid regions such as steppe and desert (Belkacem et al, 2016; Johnston, 1969). The signal of admixture should be weaker in rural popu‐ lations due to the absence of hybrids (Belkacem et al, 2016)
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