Predicting the evolution of sexual size dimorphism

11 , Evolution 34 : 292–305) equations for predicting the evolution of sexual size dimorphism (SSD) through frequency‐dependent sexual selection, and frequency‐independent natural selection, were tested against results obtained from a stochastic genetic simulation model. The SSD evolved faster than predicted, due to temporary increases in the genetic variance brought about by directional selection. Predictions for the magnitude of SSD at equilibrium were very accurate for weak sexual selection. With stronger sexual selection the total response was greater than predicted. Large changes in SSD can occur without significant long‐term change in the genetic correlation between the sexes. Our results suggest that genetic correlations constrain both the short‐term and long‐term evolution of SSD less than predicted by the Lande model.     

The evolution of condition-dependent sexual dimorphism

Theory suggests that the net benefit of allocating resources to a sexual trait depends both on the strength of sexual selection on that trait and on individual condition. This predicts a tight coevolution between sexual dimorphism and condition dependence and suggests that these patterns of within-sex and between-sex variation may share a common genetic and developmental basis. Although condition-dependent expression of sexual traits is widely documented, the extent of covariation between condition dependence and sexual dimorphism remains poorly known. I investigated the effects of condition (larval diet quality) on multivariate sexual dimorphism in the fly Telostylinus angusticollis (Neriidae). Condition determined the direction of sexual size dimorphism and modulated sexual shape dimorphism by affecting allometric slopes and/or intercepts of sexually homologous traits in both sexes. Although the greatest responses to condition manipulation were observed in male sexual traits, both sexual and nonsexual traits exhibited substantial variation in the nature and magnitude of condition effects. Nonetheless, condition dependence and sexual dimorphism were remarkably congruent: variation in the strength of condition effects on male traits explained more than 90% of the variation in the magnitude of sexual dimorphism, whether quantified in terms of trait size or allometric slope. The genetic mechanisms that give rise to multivariate sexual dimorphism in body shape thus function in a strongly condition-dependent manner in this species, suggesting a common genetic basis for body shape variation within and between sexes.     

Chromosome surface area : Further evidence for autosomal sexual dimorphism

The karyotypes of 76 males and 84 females, each assembled by the trypsin banding method, are examined in a study designed to investigate sex differences among autosomes. It is shown that female autosomes have consistently larger surface areas than the males, with respect to both the short and long arm measurements. In addition, discriminant function analysis is used to distinguish between the male and female karyotypes. We find that, using autosomal measurements alone, this can be done with a high probability of success.     

Sex-limited mutations and the evolution of sexual dimorphism

Abstract.— Although the developmental and genetic mechanisms underlying sex differences are being elucidated in great detail in a number of species, there remains a breach between proximate and evolutionary studies of sexual dimorphism. More precisely, the evolution of sex-limited gene expression at autosomal loci has not been well reasoned using either theoretical or empirical methods. Here, I show that a Mendelian genetic model including elementary details of sexual differentiation provides novel insight into the evolution of sex differences via sex limitation. This model indicates that the nature of allelic effects and the pattern of selection must be known in both sexes to predict the evolution of sex differences. That is, selection interacts with genetic variation for sexual dimorphism to produce unanticipated patterns of trait divergence or convergence between the sexes. Ultimately, this model may explain why previous models for the evolution of sexual dimorphism do not predict the erratic behavior of the sex difference during artificial selection experiments.     

Gene duplication in the evolution of sexual dimorphism

Males and females share most of the same genes, so selection in one sex will typically produce a correlated response in the other sex. Yet, the sexes have evolved to differ in a multitude of behavioral, morphological, and physiological traits. How did this sexual dimorphism evolve despite the presence of a common underlying genome? We investigated the potential role of gene duplication in the evolution of sexual dimorphism. Because duplication events provide extra genetic material, the sexes each might use this redundancy to facilitate sex‐specific gene expression, permitting the evolution of dimorphism. We investigated this hypothesis at the genome‐wide level in Drosophila melanogaster, using the presence of sex‐biased expression as a proxy for the sex‐specific specialization of gene function. We expected that if sexually antagonistic selection is a potent force acting upon individual genes, duplication will result in paralog families whose members differ in sex‐biased expression. Gene members of the same duplicate family can have different expression patterns in males versus females. In particular, duplicate pairs containing a male‐biased gene are found more frequently than expected, in agreement with previous studies. Furthermore, when the singleton ortholog is unbiased, duplication appears to allow one of the paralog copies to acquire male‐biased expression. Conversely, female‐biased expression is not common among duplicates; fewer duplicate genes are expressed in the female‐soma and ovaries than in the male‐soma and testes. Expression divergence exists more in older than in younger duplicates pairs, but expression divergence does not correlate with protein sequence divergence. Finally, genomic proximity may have an effect on whether paralogs differ in sex‐biased expression. We conclude that the data are consistent with a role of gene duplication in fostering male‐biased, but not female‐biased, gene expression, thereby aiding the evolution of sexual dimorphism.     

A note on the evolution of sexual dimorphism

Sexual dimorphism is discussed as a diffusion problem. The establishment of different gametic size is favoured through Brownian motion.     

A multivariate view of the evolution of sexual dimorphism

Sexual differences are often dramatic and widespread across taxa. Their extravagance and ubiquity can be puzzling because the common underlying genome of males and females is expected to impede rather than foster phenotypic divergence. Widespread dimorphism, despite a shared genome, may be more readily explained by considering the multivariate, rather than univariate, framework governing the evolution of sexual dimorphism. In the univariate formulation, differences in genetic variances and a low intersexual genetic correlation () can facilitate the evolution of sexual dimorphism. However, studies that have analysed sex‐specific differences in heritabilities or genetic variances do not always find significant differences. Furthermore, many of the reported estimates of are very high and positive. When monomorphic heritabilities and a high are present together, the evolution of sexual dimorphism on a trait‐by‐trait basis is severely constrained. By contrast, the multivariate formulation has greater generality and more flexibility. Although the number of multivariate sexual dimorphism studies is low, almost all support sex‐specific differences in the G (variance‐covariance) matrix; G matrices can differ with respect to size and/or orientation, affecting the response to selection differently between the sexes. Second, whereas positive values of the univariate quantity only hinder positive changes in sexual dimorphism, positive covariances in the intersexual covariance B matrix can either help or hinder. Similarly, the handful of studies reporting B matrices indicate that it is often asymmetric, so that B can affect the evolution of single traits differently between the sexes. Multivariate approaches typically demonstrate that genetic covariances among traits can strongly constrain trait evolution when compared with univariate approaches. By contrast, in the evolution of sexual dimorphism, a multivariate view potentially reveals more opportunities for sexual dimorphism to evolve by considering the effect sex‐specific selection has on sex‐specific G matrices and an asymmetric B matrix.     

Convergent evolution of sexual shape dimorphism in Diptera

Several patterns of sexual shape dimorphism, such as male body elongation, eye stalks, or extensions of the exoskeleton, have evolved repeatedly in the true flies (Diptera). Although these dimorphisms may have evolved in response to sexual selection on male body shape, conserved genetic factors may have contributed to this convergent evolution, resulting in stronger phenotypic convergence than might be expected from functional requirements alone. I compared phenotypic variation in body shape in two distantly related species exhibiting sexually dimorphic body elongation: Prochyliza xanthostoma (Piophilidae) and Telostylinus angusticollis (Neriidae). Although sexual selection appears to act differently on male body shape in these species, they exhibited strikingly similar patterns of sexual dimorphism. Likewise, patterns of within-sex shape variation were similar in the two species, particularly in males: relative elongation of the male head capsule, antenna, and legs was associated with reduced head capsule width and wing length, but was nearly independent of variation in thorax length. However, the two species presented contrasting patterns of static allometry: male sexual traits exhibited elevated allometric slopes in T. angusticollis, but not in P. xanthostoma. These results suggest that a shared pattern of covariation among traits may have channeled the evolution of sexually dimorphic body elongation in these species. Nonetheless, static allometries may have been shaped by species-specific selection pressures or genetic architectures.     

Correlates of sexual dimorphism in primates: Ecological and size variables

The effects of a series of ecological and size factors on the degree of sexual dimorphism in body weight and canine size were studied among subsets of 70 primate species. Variation in body-weight dimorphism can be almost entirely attributed to body weight (83% of variance R² of weight dimorphism). Much smaller amounts of the variation can be attributed to mating system (R² =6.8%,polygynous species being more dimorphic than monogamous ones) and diet (R² = 2.5%,frugivorous species being more dimorphic than folivorous ones). Habitat (arboreal vs. terrestrial) and activity rhythm (nocturnal vs. diurnal) have only an indirect effect on weight dimorphism. Variation in canine-size dimorphism can be explained in terms of canine size (R² =49%),activity rhythm (R² = 20%,diurnal species being more dimorphic than nocturnal ones), and mating system (R² = 10%).Habitat and diet do not play a significant role in canine-size dimorphism. The unexpectedly high contribution of size to sexual dimorphism coupled with the observation of increased sexual dimorphism with increased size leads us to formulate a new selection model for the evolution of sexual dimorphism. We suggest that if there is selection for size increase, whatever its cause, directional selection in both males and females will lead to an increase in sexual dimorphism based on differences in genetic variance between the sexes. Sexual selection, resource division between the sexes, or lopsided reproductive selection need not play a role in such a model.     

Processes that constrain and facilitate the evolution of sexual dimorphism

Sexual dimorphism, or differences between the sexes, is pervasive in both plants and animals despite genetic and developmental constraints on its evolution. This special issue of the American Naturalist, which is based on the annual Vice Presidential Symposium, documents how the underlying processes responsible for the presence and extent of sexual dimorphism can be qualified and quantified by a variety of approaches. These include estimates of the G matrix and phenotypic selection, artificial selection, phenotypic manipulation of hormones, estimates of sex-differential sensitivity to maternal effects, among-population and phenotypic plasticity studies, and the mapping of sexual dimorphism onto a phylogeny. The questions addressed in the articles in this issue vary depending on the motivation for the studies and the taxa being investigated, but taken together, they show how the integration of genetic, developmental, physiological, ecological, and phylogenetic approaches can illuminate the processes underlying the evolution of sexual dimorphism.     

Testosterone,growth and the evolution of sexual size dimorphism

The integration of macroevolutionary pattern with developmental mechanism presents an outstanding challenge for studies of phenotypic evolution. Here, we use a combination of experimental and comparative data to test whether evolutionary shifts in the direction of sexual size dimorphism (SSD) correspond to underlying changes in the endocrine regulation of growth. First, we combine captive breeding studies with mark‐recapture data to show that male‐biased SSD develops in the brown anole lizard (Anolis sagrei) because males grow significantly faster than females as juveniles and adults. We then use castration surgeries and testosterone implants to show that castration inhibits, and testosterone stimulates, male growth. We conclude by reviewing published testosterone manipulations in other squamate reptiles in the context of evolutionary patterns in SSD. Collectively, these studies reveal that the evolution of SSD has been accompanied by underlying changes in the effect of testosterone on male growth, potentially facilitating the rapid evolution of SSD.     

A developmental perspective on the evolution of sexual size dimorphism of a moth

Males and females of almost all organisms exhibit sexual differences in body size, a phenomenon called sexual size dimorphism (SSD). How the sexes evolve to be different sizes, despite sharing the same genes that control growth and development, and hence a common genetic architecture, has remained elusive. Here, we show that the genetic architecture (heritabilities and genetic correlations) of the physiological mechanism that regulates size during the last stage of larval development of a moth, differs between the sexes, and thus probably facilitates, rather than hinders, the evolution of SSD. We further show that the endocrine system plays a critical role in generating SSD. Our results demonstrate that knowledge of the genetic architecture underlying the physiological process during development that ultimately produces SSD in adults can elucidate how males and females of organisms evolve to be of different sizes.     

The evolution of sexual dimorphism in animals: Hypotheses and tests

Three major hypotheses, based upon mechanisms of sexual selection, intersexual food competition and reproductive role division, have been advanced to explain the evolution of sexual dimorphism in body size and morphology of animals. Genetic models suggest that all of the hypotheses are plausible, and empirical studies demonstrate that each of the three mechanisms operates in natural populations. However, problems arise in testing hypotheses for the evolution of sexual dimorphism: more than one mechanism may be operating simultaneously, and the demonstrated occurrence of a mechanism does not indicate that it actually results in selection for dimorphism. A recent statistical technique offers a solution to these problems and provides a promising new approach to the study of sexual dimorphism, in which researchers can assess the relative importance of each mechanism in present-day selection for sexual dimorphism within a species.     

Sex ratio represents a unique context for selection on attractive traits: consequences for the evolution of sexual dimorphism

We explored the idea that sex ratio represents a unique context for selection on attractive traits by manipulating sex ratio and pollinator abundance in experimental populations of a gender-dimorphic wild strawberry Fragaria virginiana. We found that increasing the frequency of functional males (the pollen-bearing morph) increased the frequency of pollen-collecting syrphid flies in the pollinator assemblage, decreased pollinator visitation to less preferred morph (females), and decreased the degree of pollen limitation of females. Moreover, sex ratio influenced the strength of selection on petal size through female fitness but did not alter the strength of selection through male fitness components, suggesting that sex ratio can alter the gender bias of selection on an attractive trait. This study of context-dependent selection has important implications for the evolution of sexual dimorphism in attractive traits. First, it suggests that only certain conditions generate male-biased selection and, thus, could lead to selection-driven male-biased petal size dimorphism. Second, it suggests that flexible pollinator foraging may be an important mechanism by which sex ratio influences selection on attractive traits. Finally, it implies that variation in sex ratio could limit the evolution of sexual dimorphism and/or could maintain genetic variation in attractive traits.     

Human skin-color sexual dimorphism: a test of the sexual selection hypothesis

Applied to skin color, the sexual selection hypothesis proposes that male preference for light-skinned females explains the presence of light skin in areas of low solar radiation. According to this proposal, in areas of high solar radiation, natural selection for dark skin overrides the universal preference of males for light females. But in areas in which natural selection ceases to act, sexual selection becomes more important, and causes human populations to become light-skinned, and females to be lighter than males. The sexual selection hypothesis proposes that human sexual dimorphism of skin color should be positively correlated with distance from the equator. We tested the prediction that sexual dimorphism should increase with increasing latitude, using adult-only data sets derived from measurements with standard reflectance spectrophotometric devices. Our analysis failed to support the prediction of a positive correlation between increasing distance from the equator and increased sexual dimorphism. We found no evidence in support of the sexual selection hypothesis.     

Determinants of sex allocation in a gynodioecious wild strawberry: implications for the evolution of dioecy and sexual dimorphism

One evolutionary pathway from plants with combined male and female functions (hermaphroditism) to those with separate sexes (dioecy) involves females coexisting with hermaphrodites (gynodioecy). The research presented here explores sex allocation in Fragaria virginiana (a gynodioecious wild strawberry), within the context of theory on the gynodioecy–dioecy transition. By growing clonally replicated plants in the greenhouse and surveying six populations in situ, I evaluated the effects of plant size, genotype, sexual identity, population of origin and female frequency on sex allocation. I found significant positive effects of plant size on most sex allocation traits studied. In addition to strong sex-specific allocation patterns, I found significant broad-sense heritabilities for all traits, suggesting that plants could respond to selection. Moreover, there was a negative genetic correlation between pollen production and fruit set per flower within hermaphrodites, lending support to a basic assumption of sex allocation theory. On the other hand, several sex allocation traits, namely pollen and ovules per flower in hermaphrodites, were positively genetically correlated, suggesting that they may act to constrain the evolution of sexual dimorphism. Populations differed in the frequency of females, and females were more prevalent on sites with lower soil moisture and where hermaphrodites were least likely to produce fruit, suggesting that females’ seed fitness relative to that of hermaphrodites may be strongly environment-dependent in this species.