Sparrows illustrate the possible outcomes of hybridization: from a hybrid species to a mosaic hybrid zone

“It’s Difficult to Make Predictions, Especially About the Future”

– Niels Bohr (possibly)

Predicting the outcome of hybridization is challenging, partly because there are numerous outcomes. First, when two species hybridize, they could merge into one species. This phenomenon – known as species collapse – might be occurring with tree finches (genus Camarhynchus) on Floreana Island, Galápagos Archipelago. Alternatively, the hybridizing species might form a hybrid zone, which could (1) remain stable for long periods of time, (2) shift due to changes in local conditions or (3) dissolve as the interacting species evolve complete reproductive isolation (through reinforcement). Finally, hybridization can give rise to a new species by means of hybrid speciation. Several hybrid bird species have been proposed and I have recently reviewed the evidence for these cases. To recap, here are some possible outcomes of hybridization:

  • Species collapse
  • Stable hybrid zone
  • Completion of reproductive isolation
  • Hybrid speciation

A study in the journal Ecology and Evolution found evidence for three of these outcomes within one study system. An evolutionary goldmine!


A singing House Sparrow in Brussels © Luc Viatour | Wikimedia Commons


Hybrid Species

The study system I am referring to concerns hybridization between House Sparrow (Passer domesticus) and Spanish Sparrow (P. hispaniolensis). In Italy, they gave rise to one of the most iconic hybrid species: the Italian Sparrow (P. italiae). Here is a quick summary from my review paper:

Captive‐bred hybrids between house sparrow (Passer domesticus) and Spanish sparrow (P. hispaniolensis) so resemble the Italian sparrow (P. italiae) that it was hypothesized to be of hybrid origin. The Italian Sparrow shares mitochondrial haplotypes with both parental species and genetic analyses of both microsatellite data and nuclear sequences already indicated an admixed nuclear genome. These results were later confirmed by genomic data. The hybrid speciation event probably occurred less than 10,000 years ago when house sparrows expanded across Europe and came into contact with the Spanish sparrow. The Italian Sparrow appears to be reproductively isolated from the Spanish sparrow, because no signs of interbreeding were detected in a sympatric area on the Gargano Peninsula in Italy. In contrast, the Italian sparrow does interbreed with the house sparrow in the Alps, but mito‐nuclear and sex‐linked incompatibilities probably result in partial reproductive isolation between these species.

That is one possible outcome of hybridization confirmed: hybrid speciation. You can read more about this case in two other blog posts (see here and here).


An Italian Sparrow in Riva San Vitale, Svizzera. © Omar Bariffi | Flickr


Reproductive Isolation

The researchers also sampled birds from the Iberian Peninsula (Spain and Portugal) where the House Sparrow and Spanish Sparrow live side by side. Genetic analyses uncovered weak signs of gene flow between the species. Despite some gene flow, there were no obvious hybrids in the area. Hence, the authors concluded that “the near absence of phenotypic hybrids in the Eurasian area of sympatry rather suggests that despite limited interspecific gene flow, species integrity of these two is maintained due to genomic parental incompatibilities.” These results indicate that reproductive isolation is nearly complete between Iberian House Sparrows and Spanish Sparrows. Second possible outcome confirmed: (nearly) complete reproductive isolation.


A couple of Spanish Sparrow © Dûrzan | Wikimedia Commons


Hybrid Zone

For the third outcome of hybridization, we need to cross the straight of Gibraltar. Here, in northern Africa, House Sparrows and Spanish Sparrows are also living side by side. In contrast to the Iberian situation, they are hybridizing extensively. The genetic analyses indicated high levels of gene flow between the two species. This situation is likely due to increasing urbanization and intensive agriculture in the late 19th century. These developments created patches of urban areas where House Sparrows and Spanish Sparrows meet and interbreed, resulting in a mosaic hybrid zone. And that is a third outcome confirmed: a stable (?) hybrid zone.



Ottenburghs, J. (2018). Exploring the hybrid speciation continuum in birds. Ecology and Evolution, 8(24), 13027-13034.

Päckert, M., Ait Belkacem, A., Wolfgramm, H., Gast, O., Canal, D., Giacalone, G., Lo Valvo, M., Vamberger, M., Wink, M., Martens, J. & Stuckas, H. (2019). Genetic admixture despite ecological segregation in a North African sparrow hybrid zone (Aves, Passeriformes, Passer domesticus × Passer hispaniolensis). Ecology and Evolution.


This paper has been added to the Passeridae page. And a big thanks to Martin Päckert for sending me this study.

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