Sparrows are a small family of passerine birds. Hybridization is restricted to the genus Passer. Two cases of hybridization have been studied extensively. First, interbreeding between House Sparrow (P. domesticus) and Eurasian Tree Sparrow (P. montanus). And second, hybridization between House Sparrow and Spanish Sparrow (P. hispaniolensis), that probably resulted in the formation of a hybrid species, the Italian Sparrow (P. italiae).
Hybrids between House Sparrow and Eurasian Tree Sparrow have been documented numerous times (Albrecht, 1983; Cheke, 1969; Cordero, 1990a; Cordero, 1990b; Cordero & Summers-Smith, 1993; Järvinen, 1987; Monsees, 1962; Ruthke, 1930; Solberg & Ringsby, 1996). These hybrids may be fertile (Solberg et al., 2000), but DNA analyses showed that extra-pair fertilization may have confounded this conclusion (Solberg et al., 2006).
Hybrids between House Sparrow and Spanish Sparrow have been bred in captivity (Alonso, 1984; Macke, 1965), but these hybrids have also been found in nature (Alonso López, 1985). Experimental work showed that the post-copulatory prezygotic barrier between these species is insufficient to establish reproductive isolation (Cramer et al., 2014). The resemblance of these hybrids to the Italian Sparrow led to the hypothesis that this sparrow might be a hybrid taxon. Although this hypothesis was first rejected (Topfer, 2006), new morphological and genetic evidence suggests that the Italian Sparrow is indeed of hybrid origin (Hermansen et al. 2011, Elgvin et al. 2017). There are some constraints on the formation of hybrid genomes: not all combinations are possible. Some genes come exclusively from one parental species. The genes from Spanish Sparrow affect external phenotypes, whereas the House Sparrow genes are involved in DNA-repair and mitonuclear functions (Runemark et al., 2018).
A detailed study proposes a particular role for sex-chromosomes in this case of hybrid speciation (Elgvin et al., 2011). But for the Italian Sparrow to be a true hybrid species, it has to be reproductively isolated from his parental species, but a hybrid zone between Italian Sparrow and one of its parental species, House Sparrow, has been described in Italy (Lockley, 1992; Lockley, 1996). The Italian Sparrow also lives sympatrically with its other parent, the Spanish Sparrow. Genomic analyses of several hybrid zones suggested that conflict between nuclear and sex-linked genes with mitochondrial genes might lead to reproductive isolation between Italian Sparrow and its parental species (Trier et al., 2014). A subsequent genomic study suggests that intrinsic barriers isolating the Italian Sparrow from its parent species have mainly developed through the sorting of pre-existing sex-linked parental incompatibilities and that isolation is driven in part by mitonuclear conflict involving the Z chromosome (Hermansen et al., 2014). Cline analysis of the hybrid zone in the Alps suggests that differences in crown colour act as an important isolation mechanism between the hybrid Italian species and its Spanish parent species (Bailey et al., 2015). Competition between these two species did not result in morphological changes (e.g., beak morphology), but was noticeable on a genetic level (Saetre et al. 2017).
The evolution of beak morphology of the Italian Sparrow might not be influenced by its mixed genetic background, in contrast to for example Darwin’s Finches (Eroukhmanoff et al., 2013; Eroukhmanoff et al., 2014). However, a more recent study showed that adaptation to local conditions is influenced by genomic composition in island populations of the Italian Sparrow (Runemark et al. 2018).
In Northern Africa, House Sparrow and Spanish Sparrow also interbreed (Summers‐Smith & Vernon, 1972). In Algeria, House Sparrows and hybrids mainly reside in urban areas, while Spanish Sparrows live in cultivated areas. The latter also breed later and raise only two clutches (House Sparrows and hybrids raise three clutches). The mitogenome of the House Sparrow has almost completed introgressed into the Italian Sparrow (see above). In Algeria, however, a small percentage of the hybrids has a Spanish haplotype (Belkacem et al., 2016).
Albrecht, J. (1983). Courtship behaviour between Tree Sparrow and House Sparrow in the wild—a possible case of hybridization. Sandgrouse 5, 97-99.
Alonso, J. C. (1984). Kreuzung spanischer haus-(Passer domesticus) und weidensperlinge (Passer hispaniolensis) in gefangenschaft. Journal für Ornithologie 125, 339-340.
Alonso López, J. C. (1985). Description of intermediate phenotypes between Passer hispaniolensis and Passer domesticus. Ardeola 32, 31-38.
Bailey, R. I., Tesaker, M. R., Trier, C. N. & Saetre, G. P. (2015). Strong selection on male plumage in a hybrid zone between a hybrid bird species and one of its parents. Journal of Evolutionary Biology 28, 1257-1269.
Belkacem, A. A., Gast, O., Stuckas, H., Canal, D., LoValvo, M., Giacalone, G. & Packert, M. (2016). North African hybrid sparrows (Passer domesticus, P. hispaniolensis) back from oblivion – ecological segregation and asymmetric mitochondrial introgression between parental species. Ecology and Evolution 6, 5190-5206.
Cheke, A. S. (1969). Mechanism and Consequences of Hybridization in Sparrows Passer. Nature 222, 179-&.
Cordero, P. (1990a). Phenotypes of juvenile offspring of a mixed pair consisting of a male House Sparrow and a female Tree Sparrow Passer spp. Ornis Fennica 67, 52-56.
Cordero, P. J. (1990b). Breeding success and behaviour of a pair of House and Tree Sparrow (Passer domesticus, Passer montanus) in the wild. Journal of Ornithology 131, 165-167.
Cordero, P. J. & Summers-Smith, J. D. (1993). Hybridization between House and Tree Sparrow (Passer domesticus, P. montanus). Journal für Ornithologie 134, 69-77.
Cramer, E. R. A., Laskemoen, T., Eroukhmanoff, F., Haas, F., Hermansen, J. S., Lifjeld, J. T., Rowe, M., Saetre, G. P. & Johnsen, A. (2014). Testing a post-copulatory pre-zygotic reproductive barrier in a passerine species pair. Behavioral Ecology and Sociobiology 68, 1133-1144.
Elgvin, T. O., Hermansen, J. S., Fijarczyk, A., Bonnet, T., Borge, T., Saether, S. A., Voje, K. L. & Saetre, G. P. (2011). Hybrid speciation in sparrows II: a role for sex chromosomes? Molecular Ecology 20, 3823-3837.
Elgvin, T. O., C. N. Trier, O. K. Tørresen, I. J. Hagen, S. Lien, A. J. Nederbragt, M. Ravinet, H. Jensen and G.-P. Sætre (2017). The genomic mosaicism of hybrid speciation. Science Advances 3(6): e1602996.
Eroukhmanoff, F., Elgvin, T. O., Rojas, M. F. G., Haas, F., Hermansen, J. S. & Saetre, G. P. (2014). Effect of Species Interaction on Beak Integration in an Avian Hybrid Species Complex. Evolutionary Biology 41, 452-458.
Eroukhmanoff, F., Hermansen, J. S., Bailey, R. I., Saether, S. A. & Saetre, G. P. (2013). Local adaptation within a hybrid species. Heredity 111, 286-292.
Hermansen, J. S., Haas, F., Trier, C. N., Bailey, R. I., Nederbragt, A. J., Marzal, A. & Saetre, G. P. (2014). Hybrid speciation through sorting of parental incompatibilities in Italian sparrows. Molecular Ecology 23, 5831-5842.
Hermansen, J. S., Saether, S. A., Elgvin, T. O., Borge, T., Hjelle, E. & Saetre, G. P. (2011). Hybrid speciation in sparrows I: phenotypic intermediacy, genetic admixture and barriers to gene flow. Molecular Ecology 20, 3812-3822.
Järvinen, A. (1987). A successful mixed breeding between Parus cinctus and Parus montanus in Finnish Lapland. Ornis Fennica 64, 158-159.
Lockley, A. K. (1992). The Position of the Hybrid Zone between the House Sparrow Passer-Domesticus-Domesticus and the Italian Sparrow Pd Italiae in the Alpes Martimes. Journal of Ornithology 133, 77-82.
Lockley, A. K. (1996). Changes in the position of the hybrid zone between the House Sparrow Passer domesticus domesticus and the Italian Sparrow P-d-italiae in the Alpes Maritimes. Journal of Ornithology 137, 243-248.
Macke, T. (1965). Gelungene Kreuzung von Passer domesticus und hispaniolensis. J. Orn 106, 461-462.
Monsees, G. (1962). Huismus gepaard met Ringmus. Levende Natuur 65, 274.
Runemark, A., Trier, C.N., Eroukhmanoff, F., Hermansen, J.S., Matschiner, M., Ravinet, M., Elgvin, T.O. & Saetre, G.-P. (2018) Variation and constraints in hybrid genome formation. Nature Ecology & Evolution 2, 549-556.
Runemark, A., Fernandez, L.P., Eroukhmanoff, F. & Saetre, G.-P. (2018) Genomic Contingencies and the Potential for Local Adaptation in a Hybrid Species. The American Naturalist 192(1), 10-22.
Ruthke, P. (1930). Kommen Mischpaare von Passer domesticus und montanus vor. Beiträge zur Fortpflanzungsbiologie der Vögel 6, 29.
Saetre, G. P., A. Cuevas, J. S. Hermansen, T. O. Elgvin, L. P. Fernandez, S. A. Saether, C. L. Cascio Saetre and F. Eroukhmanoff (2017). Rapid polygenic response to secondary contact in a hybrid species. Proc Biol Sci 284(1853).
Solberg, E. J., Jensen, H., Ringsby, T. H. & Saether, B. E. (2006). Fitness consequences of hybridization between house sparrows (Passer domesticus) and tree sparrows (P-montanus). Journal of Ornithology 147, 504-506.
Solberg, E. J. & Ringsby, T. H. (1996). Hybridisation between house sparrow Passer domesticus and tree sparrow Passer montanus. Journal Fur Ornithologie 137, 525-528.
Solberg, E. J., Ringsby, T. H., Altwegg, A. & Saether, B. E. (2000). Fertile House Sparrow x Tree Sparrow (Passer domesticus x Passer montanus) hybrids? Journal Fur Ornithologie 141, 102-104.
Summers‐Smith, D. & Vernon, J. (1972). The distribution of Passer in northwest Africa. Ibis 114, 259-262.
Topfer, T. (2006). The taxonomic status of the Italian Sparrow Passer italiae (Vieillot 1817): Speciation by stabilised hybridisation? A critical analysis. Zootaxa 1325, 67-68.
Trier, C. N., Hermansen, J. S., Saetre, G. P. & Bailey, R. I. (2014). Evidence for Mito-Nuclear and Sex-Linked Reproductive Barriers between the Hybrid Italian Sparrow and Its Parent Species. Plos Genetics 10.