Current Research
E-mail: peterson@biol.wwu.edu

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Evolution of Reproductive Barriers

According to reinforcement theory, reproductive barriers arise as a result of selection imposed on hybridizing species for which hybrid offspring have low relative fitness. One of the frequent criticisms levelled against tests of this theory is that they have been performed with species that are not known to hybridize in nature and for which mating behaviors are difficult to observe in the field. Thus, although compelling results have been obtained in the lab for many species, it is difficult to be confident that the results can be used to infer that reinforcing selection is operating in nature in those species. Furthermore, it is often the case that studies of reinforcement lack the population-genetic perspective to allow one to ask whether reinforcing selection might be offset by other evolutionary forces such as gene flow or genetic drift.

C. auratus F1 Hybrid C. cobaltinus

I have begun a series of studies with hybridizing chrysomelid beetles to address these weaknesses that plague much of the reinforcement research to date. In south-central Washington, Chrysochus auratus and C. cobaltinus form an approximately 75km wide hybrid zone along the Yakima River and one of its tributaries. Within this hybrid zone, hybrids are distinct in coloration (see photos), and antennal morphology, and are heterozygous for a suite of species-specific allozyme loci. These hybrids apparently have very low fitness compared to pure C. auratus or C. cobaltinus, based on direct measures of fecundity and offspring mortality, as well as studies of allozyme and mtDNA variation in hybridizing populations.

Since these beetles mate on average once per day and males engage in extended periods of postcopulatory mate guarding, it is quite easy to study the mating behaviors of both species in the field and the lab. We have found that the degree of assortative mating is stronger inside of the hybrid zone than outside of the hybrid zone, as predicted by reinforcement theory. Currently, we are studying patterns of genetic isolation by distance in both species to determine if gene flow in either species might decline across the hybrid zone. If so, this would suggest that within the hybrid zone, there should be positional effects that result from the combined effect of reinforcing selection and gene flow from allopatric populations in which that selection is not in effect. In addition, we are assessing geographic variation in the strength of reinforcing selection on each sex of each species, and the degree to which conspecific sperm precedence buffers females against mating mistakes. My goal is to integrate these analyses of selection and gene flow to make testable predictions regarding geographic variation in reproductive character displacement in each species.