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Inbreeding depression is expected to play an important but complicated role in evolution. If we are to understand the evolution of inbreeding depression (i.e., purging), we need quantitative genetic interpretations of its variation. We introduce an experimental design in which sires are mated to multiple dams, some of which are unrelated to the sire but others are genetically related owing to an arbitrary number of prior generations of selfing or sib-mating. In this way we introduce the concept of 'inbreeding depression effect variance,' a parameter more relevant to selection and the purging of inbreeding depression than previous measures. We develop an approach for interpreting the genetic basis of the variation in inbreeding depression by: (1) predicting the variation in inbreeding depression given arbitrary initial genetic variance and (2) estimating genetic variance components given half-sib covariances estimated by our experimental design. As quantitative predictions of selection depend upon understanding genetic variation, our approach reveals the important difference between how inbreeding depression is measured experimentally and how it is viewed by selection.

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. freemani. 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 brother-sister 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.

Population genetic methods help propose the anticipated population dynamics of the selfish-gene chromosomal factor Medea in flour beetles. Medea factors extend to fixation for all positive degrees of maternal-effect lethality when there are no harmful effects on gross fertility at a rate that varies with the nature of the effect and with the density. A stable interior polymorphism occurs when medea adversely affects fertility, affecting all offspring genotypes.

Use of quantitative genetic techniques on a laboratory strain of Tribolium castaneum in a study of the genetic basis of variation for the expression of viability reveals genetic variation for two traits of T. castaneum male and T. freemani female hybrids. Variation in hybrid sex ratio and surviving hybrid male fraction with antennal morphological defects increase when T. castaneum animals are inbred. The two traits exhibit a minimal but significant correlation with each other in both the hybrid and inbred lines, indicating that the variation in their expression could arise from the pleiotropic effects of the same set of segregating genes.