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.
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Aaron O. Richardson, Benjamin K. Blackman, and Sarah R. Schaack all provided valuable time and input for manuscript preparation. The authors thank Jennifer L. Durphy, Amanda L. Posto, Sophie Karrenberg, and Nolan C. Kane for their assistance with data collection and analysis in the H. deserticola project. Funding for this project was provided by the National Science Foundation Integrative Graduate Education and Research Traineeship and Predoctoral Fellowship to B.L.G. and National Institutes of Health grant ROI-G59065 to L.H.R. This paper is based on a presentation given at the symposium entitled ‘‘Genomes and Evolution 2004,’’ cosponsored by the American Genetic Association and the International Society of Molecular Biology and Evolution, at The Pennsylvania State University, and Evolution 2004, State College, PA, June 17–20, 2004.