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Genetic variation among populations in the degree of sexual dimorphism may be a consequence of selection on one or both sexes. We analysed genetic parameters from crosses involving three populations of the dioecious plant Silene latifolia, which exhibits sexual dimorphism in flower size, to determine whether population differentiation was a result of selection on one or both sexes. We took the novel approach of comparing the ratio of population differentiation of a quantitative trait (Q(ST) ) to that of neutral genetic markers (F(ST) ) for males vs. females. We attributed 72.6% of calyx width variation in males to differences among populations vs. only 6.9% in females. The Q(ST) /F(ST) ratio was 4.2 for males vs. 0.4 for females, suggesting that selection on males is responsible for differentiation among populations in calyx width and its degree of sexual dimorphism. This selection may be indirect via genetic correlations with other morphological and physiological traits., (© 2011 The Authors. Journal of Evolutionary Biology © 2011 European Society For Evolutionary Biology.)

Kin and levels-of-selection models are common approaches for modelling social evolution. Indirect genetic effect (IGE) models represent a different approach, specifying social effects on trait values rather than fitness. We investigate the joint effect of relatedness, multilevel selection and IGEs on response to selection. We present a measure for the degree of multilevel selection, which is the natural partner of relatedness in expressions for response. Response depends on both relatedness and the degree of multilevel selection, rather than only one or the other factor. Moreover, response is symmetric in relatedness and the degree of multilevel selection, indicating that both factors have exactly the same effect. Without IGEs, the key parameter is the product of relatedness and the degree of multilevel selection. With IGEs, however, multilevel selection without relatedness can explain evolution of social traits. Thus, next to relatedness and multilevel selection, IGEs are a key element in the genetical theory of social evolution.

A biallelic viability model based on human data for maternal-fetal interactions reported by Hedrick (1997) gives the interesting result of neutral stability at all gene frequencies. I show that there are two levels of selection, within and among families, acting in opposing directions in this model and that the neutral stability occurs when the two levels of selection exactly balance one another, as they do in a randomly mating population. Deviations from random mating disrupt the balance and consequently destroy the neutral stability. However, with inbreeding avoidance, which characterizes the human histocompatibility loci, within-family selection is strengthened and among-family selection is weakened. This favors the invasion of new alleles and contributes to a high equilibrium level of genetic diversity at loci with maternal-fetal interactions affecting offspring viability in the pattern described by Hedrick. This pattern of selection is remarkably similar to that observed for the maternal effect selfish genes, Medea in flour beetles and scat in the mouse, and the Gp-9 gene in the fire ant.

Hybrid inviability per se is not generally considered adaptive; it is believed to evolve as a pleiotropic consequence of other genetic changes, either adaptive or neutral, between diverging populations. However, given the pre-existence of hybrid sterility, there are circumstances under which hybrid inviability may be directly selected. In this report, we model the evolution of hybrid inviability in the context of within-family density-dependent selection (soft selection). This paper shows that under certain conditions, an allele causing hybrid inviability can increase when rare, despite also decreasing viability in conspecifics. These conditions depend upon the strength of density effects within families and upon the frequency of matings with heterospecifics. It is found that, under the most favorable conditions (strong soft selection and frequent hybridization), the inviability allele can invade when its deleterious effect on the hybrid viability exceeds four times its deleterious effects on conspecific heterozygotes. The conditions for fixation of the allele are also investigated and the circumstances under which this process might operate in natural populations are discussed. The conclusion is that the most restrictive condition is that for this process to evolve, there must be little opportunity for postzygotic reproductive isolation to evolve.

We investigated intraspecific and interspecific patterns of paternity in Tribolium castaneum, the flour beetle, by mating females either to pairs of conspecific males or to one conspecific and one heterospecific male of the closely related species, T. freemani. Females of both species store sperm in the spermatheca after copulation and postcopulatory, prezygotic reproductive isolation has been reported between this pair of species. When conspecific males of contrasting genotype are mated simultaneously to T. castaneum females, we observed extremely high levels of variation among females in the pattern of sperm precedence as shown by the offspring genotypes. In contrast, T. castaneum females mated simultaneously to a conspecific and a heterospecific male produced over 99 per cent conspecific progeny. When conspecific males were mated sequentially to T. castaneum females, within 3-7 days, all offspring were sired by the second male. In contrast, when a conspecific male was replaced with a T. freemani male, most females continued to produce only first-male, conspecific offspring for the next 10 days. When a heterospecific male was replaced with a conspecific male, T. castaneum females changed from producing hybrid to conspecific offspring within 3 days. We discuss the evolutionary implications of these findings.

Theoretical models of sexual selection assume that females choose males independently of the actions and choice of other individual females. Variance in male mating success in promiscuous species is thus interpreted as a result of phenotypic differences among males which females perceive and to which they respond. Here we show that, if some females copy the behavior of other females in choosing mates, the variance in male mating success and therefore the opportunity for sexual selection is greatly increased. Copying behavior is most likely in non-resource-based harem and lek mating systems but may occur in polygynous, territorial systems as well. It can be shown that copying behavior by females is an adaptive alternative to random choice whenever there is a cost to mate choice. We develop a statistical means of estimating the degree of female copying in natural populations where it occurs.