Fig 1.
Potential evolutionary outcomes of hybridization.
While most hybridization events are evolutionary dead ends, hybridization may also lead to speciation reversal where two taxa merge into one or form a hybrid zone between parapatric taxa. Alternatively, only one species may disappear through genetic swamping if introgression is highly asymmetrical. When one or few heterospecific alleles are advantageous these can also introgress into one of the parent species’ genomes through repeated backcrossing. Hybrids may also form novel lineages that are reproductively isolated from both parent taxa. The coloured fractions of the bars in the bar plots below show the relative proportion of the genome belonging to the blue and green parental lineages respectively. The grey bars represent a speciation reversal where differences are selected against. Finally, if hybridization leads to unfit offspring, it may reduce the fitness of the involved parental taxa due to wasted reproductive effort and may increase extinction risks for these.
Fig 2.
Schematic representation of homoploid and allopolyploid hybrid speciation.
As an example of a homoploid hybrid genome we present a schematic figure of the mosaic genome of the Italian sparrow which is a hybrid resulting from the anthropogenic house sparrow P. domesticus which spread across the Mediterranean with agriculture and encountered and hybridized with local populations of Spanish sparrow P. hispaniolensis [44,47,85]. As allopolyploid example we use the monkeyflower Mimulus peregrinus, an allohexaploid species that has evolved independently at least twice and which involves an intermediate, sexually-sterile but clonally vigorous F1 hybrid [115]. Sterile F1 hybrids have given rise to allopolyploids in other taxa (e.g. Spartina and Senecio), but allopolyploids can also form via fertile intermediate hybrids (e.g. Tragopogon).
Fig 3.
The process of genome stabilization during hybrid speciation and introgression.
Both ecological selection pressures and selection to avoid intrinsic incompatibilities mould hybrid genomes. Depending on the balance between beneficial alleles and incompatibilities hybridisation can result either in an admixed taxon that is reproductively isolated from both parent taxa, or local introgression into a taxon that remains distinct in spite of occasional gene flow. RI abbreviates reproductive isolation. Fd is a measure of introgression and is estimated between a hybrid population and the red parent species, and the haplotypes illustrate example individuals in these populations.