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Hybrid zones and speciation – insights from the European crow hybrid zone

  • Fredrik Haas
Publishing year: 2008
Language: English
Document type: Dissertation
Publisher: Theoretical Ecology, Ecolgy Building, Lund University
Additional info: The information about affiliations in this record was updated in December 2015. The record was previously connected to the following departments: Theoretical ecology (Closed 2011) (011006011)

Abstract english

In hybrid zones genetically differentiated populations meet and interbreed. As they result from ongoing divergence and potential speciation, such zones provide opportunities to study how different factors affect this process. The hybrid zone between the all black carrion crow (Corvus corone corone) and the grey and black hooded crow (Corvus c. cornix) stretches c. 2400 km throughout Europe. I have studied the German-Danish part of this hybrid zone.

Hybrids zones are often maintained by a balance between immigration into the zone and selection against hybrids. The latter requires that some kind of cost is associated with hybridization, for example that hybrids suffer from reduced fertility or viability. In that case selection will favor individuals that choose partners of their own type. I found that hooded and carrion crows mated assortatively with respect to subspecies. However, no evidence of hybrid unfitness has been found in the crow hybrid zone. Another possible explanation for the observed mating pattern could be that the two subspecies are spatially separated because of phenotypic specific habitat preferences. I found no indications of that. Carrion, hooded and hybrids preferred the same habitat and this habitat was the most common in my study area.

Sexual imprinting, i.e. the process whereby young individuals learn species-specific characteristics that enables them to choose appropriate mates as adults, is a mechanism that has the potential to cause assortative mating without demanding selection against hybridization. We mimicked this process by training artificial neural networks to separate their own type from the other two with hybrids as the third phenotype. The network learned pure phenotypes faster and better than the hybrid patterns showing that already at the receptor level there may be signal reception properties that will cause assortative mating. In another simulation we used partner choice preferences extracted from the neural network simulations and showed that they could maintain a hybrid zone resembling the crow hybrid zone.

Under this scenario gene flow between the hybridizing populations is likely to be high because heterotypic pairings will produce as many viable and fertile offspring as homotypic pairings. This is supported by an analysis in which we analyzed 18 microsatellite loci and found no evidence for genetic differences between hooded, hybrids and carrion phenotypes sampled within the zone. We also found that the gene MC1R, known to be associated with melanic patterning in other organisms, could not explain the colour pattern in crows.


Blå Hallen, Ekologihuset, Sölvegatan 37, Lund
  • Glenn-Peter Sætre (Professor)


  • Ecology
  • microsatellites
  • mc1r
  • introgression
  • hybrid zone
  • speciation
  • European crow
  • Corvus corone
  • habitat preferences
  • neural network
  • sexual imprinting
  • assortative mating


  • Anders Brodin
  • ISBN: 978-91-7105-272-8
Fredrik Haas
E-mail: fredrik [dot] haas [at] biol [dot] lu [dot] se



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