Genetic and phenotypic divergence of homoploid hybrid species from parental species

Abstract

Homoploid hybrid speciation occurs when a stable, fertile and reproductively isolated lineage results from hybridization between two distinct species, without a change in ploidy level. Reproductive isolation between a homoploid hybrid species and its parents is generally attained by chromosomal rearrangements, ecological divergence and/or spatial isolation from the parental species; these factors prevent the incipient hybrid species from being genetically swamped through mating with the parental species, and allow it to evolve as an independent lineage (Gross and Rieseberg, 2005). Homoploid hybrid species are useful for speciation studies because the parental (hybridizing) species are often extant, and provide a baseline against which the changes that accompany the speciation process are measured. In this issue, Brennan et al. (2012) quantify the degree of molecular and phenotypic divergence between a hybrid species and its progenitors. The results suggest that molecular and phenotypic divergence can each follow very different trajectories during the development of a new species, and raises the question of whether quantitative trait divergence is always higher than neutral marker divergence between a homoploid hybrid species and its parents.