Concept 24.3: Hybrid zones reveal factors that cause reproductive isolation

Flashcards from Concept 24.3 of Pearson's Campbell Biology, Twelfth Edition.

Hybrid zone

A region in which members of different species mate and produce hybrid offspring, given two species with incomplete reproductive barriers, albeit with reduced survival and reproduction rates

• Some of these form as narrow bands where habitats of two or more closely related species meet; most are isolated patches, however

  • Two species of toad in the genus Bombina interbreed in one that is long and narrow

• Either result in reinforcement, fusion, or stability as an outcome if hybrids do not become reproductively isolated

Environmental change

A potential driver of the relocation of hybrid zones

• Seen with the zone between the black-capped and Carolina chickadees shifting northward in response to climate change

Allele transfer

This can be done between parent species through breeding between parents and hybrids

• This may help parent species cope with changing environments with new alleles

Reinforcement

One possible outcome of a hybrid zone that strengthens reproductive barriers

• If hybrids are less fit than the parent species, than strong selection for prezygotic barriers should reduce hybrid production

• Stronger for sympatric populations

Fusion

One possible outcome of a hybrid zone that weakens reproductive barriers

• If hybrids are as fit as the parent species, reproductive barriers can weaken, and the two parent species may fuse into a single species

  • Pollution in Lake Victoria has reduced the ability of female cichlids to visually distinguish between males of their own and different species

Stability

One possible outcome of a hybrid zone that continues the formation of hybrid individuals

• Extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone

  • Members of both parent species of Bombina routinely migrate into the narrow hybrid zone, resulting in ongoing hybridization