The geography of hybrid speciation in plants

Both homoploid and polyploid hybrid speciation are commonly considered straightforward examples of sympatric speciation. Based on 28 studies of homoploid and 34 studies of polyploid hybrid species I conclude that lati-/longitudinal and/or altitudinal ecogeographical displacement of hybrid lineages from parental lineages has been observed in the majority of cases examined, although more commonly in homoploid than in polyploid hybrid species. This ecogeographical displacement is interpreted as the result of evolutionary novelty in hybrid lineages. As the geographical scale of ecogeographical displacement in relation to gene flow distances may not be large enough to prevent gene flow at the time of initial hybrid formation, I hypothesize that climate-induced changes of distribution ranges may have played a large role in hybrid species formation. In particular, I hypothesize that hybrid lineages initially remain in the secondary contact area of their parental lineages, whereas the parental lineages, responding to climate change, track their niches and withdraw from this contact area. This results in geographical isolation. I conclude that assessment of hybrid speciation as sympatric may be inappropriate in many cases, and that instead a phase of geographical displacement of hybrid and parental lineages into para-/allopatry may often be required.