Submitted to: Heredity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 15, 2011
Publication Date: N/A
Interpretive Summary: Problem: Homoploid hybrid species are relatively rare, but represent useful study systems for questions about the speciation process. Interestingly, the first direct comparison of molecular and phenotypic divergence between a homoploid hybrid species and its parents shows higher molecular divergence than phenotypic divergence. This outcome runs counter to our current understanding of the rates of evolution of phenotypic traits compared to molecular markers. Accomplishment: This manuscript represents a summary and perspective on the surprising finding that molecular divergence might exceed phenotypic divergence for a homoploid hybrid species compared to its parental species. It emphasizes the importance of genetic bottlenecks and geographic isolation in influencing the evolution of a new species, especially one invading new habitats, which is ultimately applicable to the evolution of invasive plants. In addition, the manuscript provides the first survey of the levels of genetic divergence between homoploid hybrid species and their parental species, and shows that they may not follow the generally expected patterns for molecular divergence.
Technical 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 via 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 & Rieseberg, 2005). Homoploid hybrid species are very useful for speciation studies because the parental (hybridizing) species are often extant, and provide a baseline against which to measure the changes that accompany the speciation process. In this issue, Brennan et al. (2011) quantify the degree of both molecular and phenotypic divergence between a hybrid species and its progenitors. The results show that molecular divergence exceeds quantitative trait divergence when the hybrid species is compared to its parents, which runs contrary to predictions. These patterns are potentially caused by a severe genetic bottleneck during the origin of the hybrid species and the geographical isolation between the hybrid species and its parents. Interestingly, little is known about how genetically divergent homoploid hybrid species are from their parental species; I provide a survey of existing data to evaluate the patterns.