The ecological genetics of homoploid hybrid speciation

Abstract

Our understanding of homoploid hybrid speciation has advanced substantially since this mechanism of species formation was codified 50 years ago. Early theory and research focused almost exclusively on the importance of chromosomal rearrangements, but it later became evident that natural selection, specifically ecological selection, might play a major role as well. In light of this recent shift, we present an evaluation of ecology's role in homoploid hybrid speciation, with an emphasis on the genetics underlying ecological components of the speciation process. We briefly review new theoretical developments related to the ecology of homoploid hybrid speciation; propose a set of explicit, testable questions that must be answered to verify the role of ecological selection in homoploid hybrid speciation; discuss published work with reference to these questions; and also report new data supporting the importance of ecological selection in the origin of the homoploid hybrid sunflower species Helianthus deserticola. Overall, theory and empirical evidence gathered to date suggest that ecological selection is a major factor promoting homoploid hybrid speciation, with the strongest evidence coming from genetic studies.

Figure 1

Example of a phylogenetic tree showing reticulation due to homoploid hybrid speciation. Phylogenetic tree for Helianthus section, Helianthus based on combined chloroplast DNA and nuclear ribosomal DNA data (Rieseberg 1991). The number of mutations are given above and bootstrap percentages below each branch. Dashed lines indicate parentage of homoploid hybrid species.