Hybrid Speciation

Abstract

Abstract Hybridisation between genetically divergent populations may lead to the formation of new evolutionary lineages. This may occur via duplication of a hybrid's chromosome complement (allopolyploid speciation) or by stabilisation of a fertile hybrid segregant (homoploid hybrid speciation). Although more common in plants, both modes of hybrid speciation also occur in fungi and in animals, including fish, amphibians and insects. The successful origin and establishment of hybrid species is highly dependent on an ability to occupy novel and/or spatially isolated habitats, in order to escape the effects of competition and gene flow from parental species. This ability is favoured by the generation of genetic novelty resulting from the dramatic effects that hybridisation (and genome duplication in allopolyploids) have on modifying genome structure and gene expression. Examples of recently originated allopolyploid and homoploid hybrid species are important models for investigating processes involved in hybrid speciation and establishment. Key Concepts Hybridisation between genetically divergent populations is promoted by habitat disturbance and may lead to the formation of new evolutionary lineages. Hybrid speciation may occur via the duplication of a hybrid's chromosome complement (allopolyploid speciation) or by the stabilisation of a reproductively isolated, fertile hybrid segregant (homoploid hybrid speciation). Allopolyploidy leads to instantaneous reproductive isolation between the allopolyploid neospecies and its parents and is therefore an example of very rapid speciation. Allopolyploidy is very common in higher plants but less common in animals, although examples are known in insects, crustaceans, molluscs, fish, amphibians, reptiles and mammals. Homoploid hybrid speciation is thought to be relatively rare, although an increasing number of homoploid hybrid species of animals, plants and fungi are being recognised using molecular markers. Homoploid hybrid speciation can be completed within a few generations, although several hundred generations may be required before the genome of the hybrid neospecies is fully stabilised. Considerable genetic novelty is generated in the early stages of allopolyploidy and homoploid hybrid speciation due to the marked effects that hybridisation (and genome duplication in the case of allopolyploidy) have on modifying genome structure and gene expression. Such genetic novelty aids the establishment of hybrid neospecies in ecologically divergent habitats, thus reducing the likelihood of competition and crossing with parents. Several examples are known of new allopolyploid and homoploid hybrid species originating within the past 250 years, and these have become important models for investigating processes involved in hybrid speciation and establishment.