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Keywords:

  • contact zone;
  • evolutionary mechanisms;
  • introgressive hybridization;
  • literature review;
  • sexual selection

Abstract

  1. A hybrid zone is a region where interbreeding occurs between two or more genetically distinct populations. Recent studies have shown that mammalian hybrid zones occur in nature more frequently than previously thought.
  2. I summarize the history of research on mammalian hybrid zones, highlighting contributions that demonstrate genetic, behavioural and environmental mechanisms responsible for spatial distribution of genotypes, maintenance or elimination of reproductive barriers and evolutionary outcomes of hybridization.
  3. The role of hybridization in mammalian evolution, adaptation and diversification is demonstrated by the existence of hybrid swarms (e.g. caribou) and species genesis via homoploid hybridization (e.g. primates, bats).
  4. Mammalian hybrid zones are ideal settings in which to investigate genetic mechanisms that influence mate preferences as well as other questions regarding sexual selection. Many evolutionary processes and mechanisms associated with sexual selection have been well demonstrated in primate, rodent and seal hybrid zones by combining behavioural and genetic, and environmental techniques.
  5. Investigations of interbreeding chromosomal races of mouse Mus musculus and shrew Sorex araneus provide insight into the role of chromosome rearrangements in the speciation process.
  6. The genomes of some mammals that hybridize in nature are fully sequenced, and rapid progress is being made towards understanding genetic underpinnings of phenotypes governing reproductive isolation. Future studies of mammalian hybrid zones will incorporate genomic techniques with increasing frequency and taxonomic representation. Such techniques may include examination of gene expression patterns using microarrays or large-scale analysis of introgression using markers from multiple genetic systems (e.g. mitochondria, sex chromosomes).
  7. As methods improve, researchers will be better positioned to advise managers and policy makers about the potential outcomes of hybridization. This may be increasingly important as previously isolated forms come into contact as a result of global climate change.