Quantitative genetic models of sexual selection

Estimating selection on the act of inbreeding in a population with strong inbreeding depression

Inbreeding depression is widely regarded as a driving force in the evolution of dispersal, mate choice and sperm selection. However, due to likely costs of inbreeding avoidance, which are poorly understood, it is unclear to what extent selection to avoid inbreeding is expected in nature. Moreover, there are currently very few empirical estimates of the strength of selection against the act of inbreeding (mating with a relative), as opposed to the fitness costs of being inbred. Here, we use data from the individual-based study of red deer on the Scottish island of Rum, a strongly polygynous system which harbours a large inbreeding load, to estimate selection against the act of inbreeding for each sex. We use pedigree and genomic estimates of relatedness between individuals and measure fitness using both lifetime breeding success (number of calves born) and lifetime reproductive success (number of calves surviving to independence), with the latter incorporating inbreeding depression in calf survival. We find for both sexes that the repeatability of the act of inbreeding was low (< 0.1), suggesting little among-individual variation for this trait on which selection can act. Using the genomic measures, there was significant selection against the act of inbreeding in males, but not in females, and there was considerable uncertainty in the estimate in both sexes. We discuss possible explanations for these patterns and their implications for understanding the evolution of inbreeding avoidance in natural populations.

THE EVOLUTION OF SEXUAL DIMORPHISM BY SEXUAL SELECTION: THE SEPARATE EFFECTS OF INTRASEXUAL SELECTION AND INTERSEXUAL SELECTION

Libellula luctuosa, a pond dragonfly found in eastern North America, is apparently sexually dimorphic. Previous studies of the mating behavior in this species suggested that both male-male competition and female mate choice are important influences. Males compete for territories, where they attract females and where mating occurs. Female behavior influences both the copulation success and the fertilization success of males. Because of temporal and spatial separation of these episodes of sexual selection, multivariate and nonparametric statistical techniques could be used to investigate the influence of components of sexual selection on various sexually dimorphic traits. Sexual dimorphism in L. luctuosa was first quantified; then the direct effects and the form of selection were estimated. Sexually dimorphic wing size, body size, wing coloration, and body coloration are distributed either continuously or discontinuously between the sexes in L. luctuosa. These traits have apparently diverged between the sexes as a result of directional sexual selection. Body size is further influenced by stabilizing selection. Intrasexual selection (success in gaining access to a territory) and intersexual selection (success in copulation and fertilization) can influence the same or different sexually dimorphic characters. Body size is influenced by directional selection during the intrasexual phase of sexual selection and is also influenced by stabilizing selection during intersexual selection. The size of the brown wing patch is influenced by directional selection, primarily during the intersexual phase of sexual selection. There is directional selection on the white wing patch during both phases. Thus, the different proximate mechanisms of sexual selection may jointly or separately affect the evolution of sexually dimorphic characters. Further empirical and theoretical investigations into the differences in the effects of intrasexual selection and intersexual selection are needed to clarify the circumstances leading to separate consequences of these two mechanisms of sexual selection.

Selection on parental performance opposes selection for larger body mass in a wild population of blue tits

There is abundant evidence in many taxa for positive directional selection on body size, and yet little evidence for microevolutionary change. In many species, variation in body size is partly determined by the actions of parents, so a proposed explanation for stasis is the presence of a negative genetic correlation between direct and parental effects. Consequently, selecting genes for increased body size would result in a correlated decline in parental effects, reducing body size in the following generation. We show that these arguments implicitly assume that parental care is cost free, and that including a cost alters the predicted genetic architectures needed to explain stasis. Using a large cross-fostered population of blue tits, we estimate direct selection on parental effects for body mass, and show it is negative. Negative selection is consistent with a cost to parental care, mainly acting through a reduction in current fecundity rather than survival. Under these conditions, evolutionary stasis is possible for moderately negative genetic correlations between direct and parental effects. This is in contrast to the implausibly extreme correlations needed when care is assumed to be cost-free. Thus, we highlight the importance of accounting correctly for complete selection acting on traits across generations.

Male-male competition, female mate choice and their interaction: determining total sexual selection

Empirical studies of sexual selection typically focus on one of the two mechanisms of sexual selection without integrating these into a description of total sexual selection, or study total sexual selection without quantifying the contributions of all of the mechanisms of sexual selection. However, this can provide an incomplete or misleading view of how sexually selected traits evolve if the mechanisms of sexual selection are opposing or differ in form. Here, we take a two-fold approach to advocate a direction for future studies of sexual selection. We first show how a quantitative partitioning and examination of sexual selection mechanisms can inform by identifying illustrative studies that describe both male-male competition and female mate choice acting on the same trait. In our sample, the most common trait where this occurred was body size, and selection was typically linear. We found that male-male competition and female mate choice can be reinforcing or opposing, although the former is most common in the literature. The mechanisms of sexual selection can occur simultaneously or sequentially, and we found they were more likely to be opposing when the mechanisms operated sequentially. The degree and timing that these mechanisms interact have important implications for the operation of sexual selection and needs to be considered in designing studies. Our examples highlight where empirical data are needed. We especially lack standardized measures of the form and strength of selection imposed by each mechanism of sexual selection and how they combine to determine total sexual selection. Secondly, using quantitative genetic principles, we outline how the selection imposed by individual mechanisms can be measured and combined to estimate the total strength and form of sexual selection. We discuss the evolutionary consequences of combining the mechanisms of sexual selection and interpreting total sexual selection. We suggest how this approach may result in empirical progress in the field of sexual selection.

Genetic correlations with floral display lead to sexual dimorphism in the cost of reproduction

In dioecious plants, females typically invest more biomass in reproduction than males and consequently experience stronger life-history trade-offs. Sexual dimorphism in life history runs counter to this pattern in Silene latifolia: females acquire less carbon and invest more biomass in reproduction, but males pay a higher cost of reproduction. The species is sexually dimorphic for many traits, especially flower number, with males producing many, small flowers compared to females. We tested whether the cost of reproduction is higher in males because flower number, which we presume to be under sexual selection in males, is genetically correlated with traits that would affect life-history trade-offs. We performed artificial selection to reduce the sexual dimorphism in flower size and looked at correlated responses in ecophysiological traits. We found significant correlated responses in total vegetative mass, leaf mass, leaf thickness, and measures of CO(2) exchange. Individuals in the many-and-small-flowered selection lines did not grow as large or invest as much biomass in leaves, and their leaves exhibited an up-regulated physiology that shortened leaf life span. Our results are consistent with the hypothesis that genetic correlations between floral display and ecophysiological traits lead to a higher cost of reproduction for males.

The quantitative genetics of sexual dimorphism: assessing the importance of sex-linkage

Sexual dimorphism (SD) is a defining feature of gonochorous animals and dioecious plants, but the evolution of SD from an initially monomorphic genome presents a conundrum. Theory predicts that the evolution of SD will be facilitated if genes with sex-specific fitness effects occur on sex chromosomes. We review this theory and show that it generates three testable predictions. For organisms with an XX/XY chromosomal system of sex determination: (1) SD should be associated with X-linked effects; (2) X-linked effects should show strong directional dominance for sexually dimorphic traits favored in males but expressed in both sexes; and (3) SD should be associated with a reduction in the between-sex additive genetic covariance and correlation. A literature review reveals that empirical evaluations of the association between sex-linkage and SD have lagged behind theory. Tests for the presence of sex-linked effects have been plagued by the need to make simplifying assumptions, such as the absence of dominance or maternal effects, that greatly weaken their discriminatory power. Further, most have used comparisons between species or populations, whereas the correct level of analysis is within populations. To overcome these problems, we derive a novel pedigree design that permits separate estimation of X-linked, dominance and maternal effects. We suggest that the data from such a design would be most appropriately analyzed using the animal model. This novel protocol will allow quantitative evaluation of the above predictions, and hence should spur progress in understanding the role of sex-linkage in the evolution of SD.

Genetic variability of sexual size dimorphism in a natural population of Drosophila melanogaster: an isofemale-line approach

Most animal species exhibit sexual size dimorphism (SSD). SSD is a trait difficult to quantify for genetical purposes since it must be simultaneously measured on two kinds of individuals, and it is generally expressed either as a difference or as a ratio between sexes. Here we ask two related questions: What is the best way to describe SSD, and is it possible to conveniently demonstrate its genetic variability in a natural population? We show that a simple experimental design, the isofemale-line technique (full-sib families), may provide an estimate of genetic variability, using the coefficient of intraclass correlation. We consider two SSD indices, the female-male difference and the female/male ratio. For two size-related traits, wing and thorax length, we found that both SSD indices were normally distributed. Within each family, the variability of SSD was estimated by considering individual values in one sex (the female) with respect to the mean value in the other sex (the male). In a homogeneous sample of 30 lines of Drosophila melanogaster, both indices provided similar intraclass correlations, on average 0.21, significantly greater than zero but lower than those for the traits themselves: 0.50 and 0.36 for wing and thorax length respectively. Wing and thorax length were strongly positively correlated within each sex. SSD indices of wing and thorax length were also positively correlated, but to a lesser degree than for the traits themselves. For comparative evolutionary studies, the ratio between sexes seems a better index of SSD since it avoids scaling effects among populations or species, permits comparisons between different traits, and has an unambiguous biological significance. In the case of D. melanogaster grown at 25 degrees C, the average female/male ratios are very similar for the wing (1.16) and the thorax (1.15), and indicate that, on average, these size traits are 15-16% longer in females.

Sexual selection in Nauphoeta cinerea: inherited mating preference?

In the cockroach, Nauphoeta cinerea, female preferences with respect to sexual selection can be easily examined. Mating is nonrandom due to female preferences based on olfactory differences among males. This mate choice results in a mating advantage for socially dominant males. In this paper, results of experiments designed to explore the relationship between female choice in daughters and the preferred male character are described. Using females with male parents of known status and an olfactometer to determine preferences, it is shown that the type of male preferred depends on the status of the female's father. Because social experiences of females were experimentally controlled, these preliminary results suggest that (1) variation in female preferences may have a genetic basis and (2) because of assortative mating, mate choice may be genetically linked to the preferred male character, as predicted by mathematical models of the sexual selection process. N. cinerea appears to be an ideal species for testing predictions and assumptions of models of sexual selection.