Genetic Variation Segregating in Natural Populations of Tribolium castuneum Affecting Traits Observed in Hybrids

We investigated patterns of within-species genetic variation for traits observed in hybrids (hybrid numbers, hybrid sex ratios, and hybrid male deformities) between two species of flour beetles, Tribolium castaneum and T. Peemani. We found genetic variation segregating among four natural populations of T. castaneum as well as within these populations. For some hybrid traits, we observed as much variation among populations 750 km apart as between populations on different continents, suggesting genetic differentiation at a local scale. Within natural populations, the variation segregating among sires is greater than that found in an earlier study for an outbred laboratory population and comparable to that observed between inbred lines derived from the outbred stock by eight generations of brothersister mating. When sires from T. castaneum are mated to conspecific and heterospecific females, we do not observe a significant correlation at the level of the family mean between the intraspecific and interspecific phenotypes, suggesting the independence of the hybrid traits from comparable traits within species. We discuss our findings in relation to the evolutionary genetics of speciation and the expression of epistatic genetic variance in interspecific crosses I N speciation genetic research, it is common practice to introgress specific regions of chromosomes from one species into another to identify and characterize genetic factors affecting interspecific hybrids, especially hybrid sterility and inviability (c$ COYNE 1992; WU and PALOPOLI 1994; FORJET 1996; TRUE et al. 1996). These studies emphasize the fixed genetic differences between species both in method and in interpretation. In contrast, intraspecific studies in evolutionary genetics and evolutionary ecology attempt to characterize genetic variation segregating within and among natural populations. Once segregating variation is identified, subsequent research focuses on the role of evolutionary forces, like mutation, natural selection, and random genetic drift, in the origin and maintenance of this variation. Ultimately, we wish to understand how the standard evolutionary forces operating within populations give rise to genetic differences at the specific level (LEWONTIN 1974). Thus, the two approaches are com-