Sexual dimorphism in relation to current selection in the house finch

Abstract.— Sexual dimorphism is thought to have evolved in response to selection pressures that differ between males and females. Our aim in this study was to determine the role of current net selection in shaping and maintaining contemporary sexual dimorphism in a recently established population of the house finch ( Carpodacus mexicanus ) in Montana. We found strong differences between sexes in direction of selection on sexually dimorphic traits, significant heritabilities of these traits, and a close congruence between current selection and observed sexual dimorphism in Montana house finches. Strong directional selection on sexually dimorphic traits and similar intensities of selection in each sex suggested that sexual dimorphism arises from adaptive responses in males and females, with both sexes being far from their local fitness optimum. This pattern is expected when a recently established population experiences continuous immigration from ecologically distinct areas of a species range or as a result of widely fluctuating selection pressures, as found in our study. Strong and sexually dimorphic selection pressures on heritable morphological traits, in combination with low phenotypic and genetic covariation among these traits during growth, may have accounted for close congruence between current selection and observed sexual dimorphism in the house finch. This conclusion is consistent with the profound adaptive population divergence in sexual dimorphism that accompanied very successful colonization of most of the North America by the house finch over the last 50 years.     

The evolution of sexual size dimorphism in the house finch. II. Population divergence in relation to local selection

Recent colonization of ecologically distinct areas in North America by the house finch (Carpodacus mexicanus) was accompanied by strong population divergence in sexual size dimorphism. Here we examined whether this divergence was produced by population differences in local selection pressures acting on each sex. In a long-term study of recently established populations in Alabama, Michigan, and Montana, we examined three selection episodes for each sex: selection for pairing success, overwinter survival, and within-season fecundity. Populations varied in intensity of these selection episodes, the contribution of each episode to the net selection, and in the targets of selection. Direction and intensity of selection strongly differed between sexes, and different selection episodes often favored opposite changes in morphological traits. In each population, current net selection for sexual dimorphism was highly concordant with observed sexual dimorphism--in each population, selection for dimorphism was the strongest on the most dimorphic traits. Strong directional selection on sexually dimorphic traits, and similar intensities of selection in both sexes, suggest that in each of the recently established populations, both males and females are far from their local fitness optimum, and that sexual dimorphism has arisen from adaptive responses in both sexes. Population differences in patterns of selection on dimorphism, combined with both low levels of ontogenetic integration in heritable sexually dimorphic traits and sexual dimorphism in growth patterns, may account for the close correspondence between dimorphism in selection and observed dimorphism in morphology across house finch populations.     

Ontogenetic stage‐specific quantitative trait loci contribute to divergence in developmental trajectories of sexually dimorphic fins between medaka populations

Sexual dimorphism can evolve when males and females differ in phenotypic optima. Genetic constraints can, however, limit the evolution of sexual dimorphism. One possible constraint is derived from alleles expressed in both sexes. Because males and females share most of their genome, shared alleles with different fitness effects between sexes are faced with intralocus sexual conflict. Another potential constraint is derived from genetic correlations between developmental stages. Sexually dimorphic traits are often favoured at adult stages, but selected against as juvenile, so developmental decoupling of traits between ontogenetic stages may be necessary for the evolution of sexual dimorphism in adults. Resolving intralocus conflicts between sexes and ages is therefore a key to the evolution of age‐specific expression of sexual dimorphism. We investigated the genetic architecture of divergence in the ontogeny of sexual dimorphism between two populations of the Japanese medaka (Oryzias latipes) that differ in the magnitude of dimorphism in anal and dorsal fin length. Quantitative trait loci (QTL) mapping revealed that few QTL had consistent effects throughout ontogenetic stages and the majority of QTL change the sizes and directions of effects on fin growth rates during ontogeny. We also found that most QTL were sex‐specific, suggesting that intralocus sexual conflict is almost resolved. Our results indicate that sex‐ and age‐specific QTL enable the populations to achieve optimal developmental trajectories of sexually dimorphic traits in response to complex natural and sexual selection.     

The evolution of sexual size dimorphism in the house finch. IV. Population divergence in ontogeny

Differences among taxa in sexual size dimorphism of adults can be produced by changes in distinct developmental processes and thus may reflect different evolutionary histories. Here we examine whether divergence in sexual dimorphism of adults between recently established Montana and Alabama populations of the house finch (Carpodacus mexicanus) can be attributed to population differences in growth of males and females. In both populations, males and females were similar at hatching, but as a result of sex-specific growth attained sexual size dimorphism by the time of independence. Timing and extent of growth varied between the sexes: Females maintained maximum rates of growth for a longer time than males, whereas males had higher initial growth rates and achieved maximum growth earlier and at smaller sizes than females. Ontogeny of sexual dimorphism differed between populations, but in each population, sexual dimorphism in growth parameters and sexual dimorphism at the time of nest leaving were similar to sexual dimorphism of adults. Variation in growth of females contributed more to population divergence than did growth of males. In each population, we found close correspondence between patterns of sexual dimorphism in growth and population divergence in morphology of adults: Traits that were the most sexually dimorphic in growth in each population contributed the most to population divergence in both sexes. We suggest that sex-specific expression of phenotypic and genetic variation throughout the ontogeny of house finches can result in different responses to selection between males and females of the same age, and thus produce fast population divergence in the sexual size dimorphism.     

Sexual dimorphism in bill morphology and feeding ecology in Cory's shearwater (Calonectris diomedea)

The bill is a sexually dimorphic structure in many bird species and implicated in numerous functions. Sexual differences may arise from sexual selection or ecological divergence. Here, we examined differences in bill size and shape between males and females and explored to what extent these relate to feeding ecology of each sex in Cory's shearwater (Calonectris diomedea). We applied linear measurements and geometric morphometric methods to examine sexual differences in bill size and shape. We investigated feeding ecology by tracking foraging movements during the breeding period and by analysing stable isotope signatures in blood during the breeding period and in feathers grown during the non-breeding period. Bill traits were all sexually dimorphic, both in absolute and relative terms, and scaled hypermetrically with body mass in several characters in males. However, males and females did not differ in their feeding areas or isotopic signatures and no significant correlation was observed between these traits and bill dimorphism. Therefore, we discard the foraging-niche divergence hypothesis, and suggest that sexual dimorphism in bill size in this species is more likely driven by sexual selection related to antagonistic interactions.     

Among‐population variation and correlations in sexually dimorphic traits of Silene latifolia

Abstract The degree of sexual dimorphism in a trait may be determined directly by disruptive selection, as well as by correlations with other traits under selection. We grew seeds from nine populations of the dioecious plant Silene latifolia in a common‐garden experiment to determine whether phenotypic variation and correlations existed for floral, leaf and resource allocation traits, and whether this variation had a genetic component. We also determined the traits which were sexually dimorphic, the degree of dimorphism, and whether it varied among populations. Seven traits exhibited among‐population variation and sexual dimorphism. Variation in the degree of dimorphism occurred only for two traits, suggesting that dimorphism may be evolving more slowly than trait means. Males had more, smaller flowers, shorter leaves, and allocated less of their total biomass to stems and more to leaves than females. Flower production was the most sexually dimorphic trait and was correlated with all measured traits. Most traits exhibited significant correlations between the sexes. The pattern of correlations and the degree of sexual dimorphism among traits lead us to suggest that intrasexual selection for an exaggerated floral display in males has indirectly led to sexual dimorphism in a host of other traits.     

When do meadow vipers (Vipera ursinii) become sexually dimorphic? – ontogenetic patterns of sexual size dimorphisms

Contrary to an increasing number of papers that document sexual dimorphism in size (and/or shape) in adults, studies dealing with sex differences in newborn and juvenile snakes are surprisingly scarce. Data about ontogenetic shifts in sexual dimorphism are generally lacking and hence, it is unclear whether sex differences are set at birth or arise post‐natally. In this study, we analyzed patterns of sexual dimorphism in body size, head dimensions and tail length (TL) among newborn, subadult and adult meadow vipers (Vipera ursinii) from the Bjelasica Mt. in Montenegro. Patterns of sexual size dimorphisms differed among traits. There was no significant difference in head dimension of males and females, but adult snakes were sexually dimorphic in body size. Sexual differences in TL were evident since birth but changed in degree throughout ontogeny. Neonate meadow vipers presented highly significant inter‐litter variation in the sexual dimorphism of all traits we have measured. Such family effects may have an important influence on extent of inter‐sexual differences in snakes and should be included in analyses of sexual dimorphism.     

Are Monomorphic Species Really Sexually Indistinguishable: No Evidence in Wild Long‐Tailed Finches (Poephila acuticauda)

Studies of sexual selection have focused mainly on dimorphic and/or polygynous species, where males, typically possess more exaggerated secondary sexual characters. However in many species, receiving far less attention, the expression of ornamental traits by females matches that in males. Several hypotheses have been proposed to explain sexual monomorphism, including mutual mate choice, genetic correlation, weak sexual selection and sexual indistinguishability. The sexual indistinguishability hypothesis suggests that sexual monomorphism is an adaption to avoid competition in monogamous flock‐living species. Based on measurements of museum skins and domesticated birds in Europe, the Australian long‐tailed finch was classified as a sexually monomorphic species, providing the best empirical support for the sexual indistinguishability hypothesis. Using both domestic and wild long‐tailed finches, we have re‐evaluated the extent to which the sexes are really indistinguishable. Morphological measurements of wing, tail, tail streamers, tarsus, bill and patch size, and colour spectrometric measurements of the yellow upper mandible and grey crown, were compared between the sexes. While the sexes are similar, males and females nonetheless differed in seven of ten traits in wild populations. In domestic populations, the sexes differed to a lesser extent but were still significantly different at three of ten traits, and discriminant analysis showed that 92% of wild individuals and 89% of domestic individuals could reliably be sexed based on just these morphological traits. Contrary to previous work, this study demonstrates that wild long‐tailed finches are sexually dimorphic, and that the similarity between males and females in this species cannot be explained by the sexual indistinguishability hypothesis.     

Maternal inheritance and rapid evolution of sexual size dimorphism: passive effects or active strategies?

Adaptive evolution is often strongly influenced by maternal inheritance that transfers the parental strategies across generations. The consequences of maternal effects for the offspring generation depend on the between-generation similarity in environments and on the evolved sensitivity of the offspring's ontogeny to maternal effects. When these factors differ between sons and daughters, maternal effects can influence the evolution of sexual dimorphism. The establishment of house finch populations across western Montana during the last 30 years was accompanied by rapid evolutionary change in sexual size dimorphism. Here I show that traits that changed the most across generations were most influenced by maternal effects in males but not females. Maternal effects differentially affected sons' and daughters' survival; greater maternal effects were commonly associated with higher survival of sons, especially when maternal and offspring environments were similar. Stronger maternal effects extended preselection phenotypic variance in morphological traits of males, thereby producing some locally adaptive phenotypes and lessening juvenile mortality. Thus, the observed sex-specific maternal effects and their contribution to the evolution of sexual size dimorphism are likely a passive consequence of the distinct sensitivity of sons and daughters to maternal adaptations to breeding in ecologically distinct parts of the house finch's expanding range.     

Is natural selection promoting sexual dimorphism in the Green-backed Firecrown Hummingbird ( Sephanoides Sephaniodes )?

In many hummingbird species there is an opposite pattern of sexual dimorphism in bill length and other morphometric measures of body size. These differences seem to be closely related with differences in foraging ecology directly associated with a different resource exploitation strategy. The aim of this study was to assess if natural selection is acting on wing length and bill size in hummingbird males and females with different resource exploitation strategies (i.e., territorial males and non-territorial females). If competition for resources promotes sexual dimorphism as a selective pressure, males should be subjected to negative directional selection pressure for wing length and no selection pressure over bill size, while females should undergo positive directional selection pressure for both bill size and wing length. The morphometric data we collected suggests that there is no selection for wing length and bill size in male hummingbirds. In contrast, our females exhibited positive directional selection for both wing length and bill size. Although we cannot reject sexual selection acting on sexually dimorphic traits, this study suggests that natural selection may promote sexual dimorphism in traits that are closely related with hummingbird foraging ecology and resource exploitation strategies.     

Genetic constraints and the evolution of display trait sexual dimorphism by natural and sexual selection

The evolution of sexual dimorphism involves an interaction between sex-specific selection and a breakdown of genetic constraints that arise because the two sexes share a genome. We examined genetic constraints and the effect of sex-specific selection on a suite of sexually dimorphic display traits in Drosophila serrata. Sexual dimorphism varied among nine natural populations covering a substantial portion of the species range. Quantitative genetic analyses showed that intersexual genetic correlations were high because of autosomal genetic variance but that the inclusion of X-linked effects reduced genetic correlations substantially, indicating that sex linkage may be an important mechanism by which intersexual genetic constraints are reduced in this species. We then explored the potential for both natural and sexual selection to influence these traits, using a 12-generation laboratory experiment in which we altered the opportunities for each process as flies adapted to a novel environment. Sexual dimorphism evolved, with natural selection reducing sexual dimorphism, whereas sexual selection tended to increase it overall. To this extent, our results are consistent with the hypothesis that sexual selection favors evolutionary divergence of the sexes. However, sex-specific responses to natural and sexual selection contrasted with the classic model because sexual selection affected females rather than males.     

Ecology,sexual dimorphism,and jumping evolution in anurans

Sexual dimorphism (SD) is a common feature of animals, and selection for sexually dimorphic traits may affect both functional morphological traits and organismal performance. Trait evolution through natural selection can also vary across environments. However, whether the evolution of organismal performance is distinct between the sexes is rarely tested in a phylogenetic comparative context. Anurans commonly exhibit sexual size dimorphism, which may affect jumping performance given the effects of body size on locomotion. They also live in a wide variety of microhabitats. Yet the relationships among dimorphism, performance, and ecology remain underexamined in anurans. Here, we explore relationships between microhabitat use, body size, and jumping performance in males and females to determine the drivers of dimorphic patterns in jumping performance. Using methods for predicting jumping performance through anatomical measurements, we describe how fecundity selection and natural selection associated with body size and microhabitat have likely shaped female jumping performance. We found that the magnitude of sexual size dimorphism (where females are about 14% larger than males) was much lower than dimorphism in muscle volume, where females had 42% more muscle than males (after accounting for body size). Despite these sometimes-large averages, phylogenetic t-tests failed to show the statistical significance of SD for any variable, indicating sexually dimorphic species tend to be closely related. While SD of jumping performance did not vary among microhabitats, we found female jumping velocity and energy differed across microhabitats. Overall, our findings indicate that differences in sex-specific reproductive roles, size, jumping-related morphology, and performance are all important determinants in how selection has led to the incredible ecophenotypic diversity of anurans.     

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.     

Sex-specific niche partitioning and sexual size dimorphism in Australian pythons (Morelia spilota imbricata)

Sexual dimorphism is usually interpreted in terms of reproductive adaptations, but the degree of sex divergence also may be affected by sex-based niche partitioning. In gape-limited animals like snakes, the degree of sexual dimorphism in body size (SSD) or relative head size can determine the size spectrum of ingestible prey for each sex. Our studies of one mainland and four insular Western Australian populations of carpet pythons ( Morelia spilota ) reveal remarkable geographical variation in SSD, associated with differences in prey resources available to the snakes. In all five populations, females grew larger than males and had larger heads relative to body length. However, the populations differed in mean body sizes and relative head sizes, as well as in the degree of sexual dimorphism in these traits. Adult males and females also diverged strongly in dietary composition: males consumed small prey (lizards, mice and small birds), while females took larger mammals such as possums and wallabies. Geographic differences in the availability of large mammalian prey were linked to differences in mean adult body sizes of females (the larger sex) and thus contributed to sex-based resource partitioning. For example, in one population adult male snakes ate mice and adult females ate wallabies; in another, birds and lizards were important prey types for both sexes. Thus, the high degree of geographical variation among python populations in sexually dimorphic aspects of body size and shape plausibly results from geographical variation in prey availability.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 113–125.     

Sexual size dimorphism in red-necked phalaropes and functional significance of nonsexual bill structure variation for feeding performance

Sexual size dimorphism is widespread in shorebirds, yet no tests of the assumption that such size dimorphism extends to functionally significant dimensions of the bill exist. This report presents tests of: (1) the assumption that sexual size dimorphism extends to the feeding structures in sexually size dimorphic bird, and (2) the hypothesis that bill-size variation influences feeding performance in Phalaropus lobatus, the red-necked phalarope. Discriminant function analysis revealed that the sexes of this species can be distinguished on the basis of five body size/bill length variables, but with low accuracy in sexing of females because of misclassification of small females as males. In the shorebird literature, the assumption is generally made that in the absence of selection to the contrary, bill size scales to body size and hence sexual size dimorphism extends to bill size. However, discriminant function analysis of measures from red-necked phalaropes failed to separate the sexes on the basis of either external or internal bill dimensions other than length. Nonetheless, internal dimensions of the upper jaw combined with exposed culmen length explained 86% of the variance in feeding performance of phalaropes; high feeding performance depends on a wide, shallow, complex internal bill structure. This study provides evidence that internal bill dimensions determine feeding performance in a manner consistent with the mechanics of surface tension transport of prey. These results suggest that some dimensions of bill size may be constrained by performance demands and demonstrate that variation in bill morphology has functional consequences. © 1996 Wiley-Liss, Inc.     

Phenotypic correlates of male reproductive success in western gorillas

Sexual selection is thought to drive the evolution of sexually dimorphic traits that increase male reproductive success. Despite a large degree of sexual dimorphism among haplorhine primates, phenotypic traits that may influence the reproductive success of males are largely unstudied due to long life spans and the difficulties in quantifying such traits non-invasively. Here we employ digital photogrammetry of body length and crest size, as well as ranking of the gluteal muscle size, to test whether these sexually dimorphic traits are associated with long-term measures of male reproductive success in western gorillas. Among 19 adult male gorillas monitored for up to 12.5 years, we found that all three phenotypic traits were positively correlated with the average number of mates per male, but only crest size and gluteal muscle size were significantly correlated with offspring survival and the annual rate of siring offspring that survive to weaning age. We discuss why such sexually dimorphic traits might be under ongoing selection in gorillas and other species.     

Using photogrammetry and color scoring to assess sexual dimorphism in wild western gorillas (Gorilla gorilla)

Investigating sexual dimorphism is important for our understanding of its influence on reproductive strategies including male-male competition, mate choice, and sexual conflict. Measuring physical traits in wild animals can be logistically challenging and disruptive for the animals. Therefore body size and ornament variation in wild primates have rarely been quantified. Gorillas are amongst the most sexually dimorphic and dichromatic primates. Adult males (silverbacks) possess a prominent sagittal crest, a pad of fibrous and fatty tissue on top of the head, have red crest coloration, their saddle appears silver, and they possess a silverline along their stomach. Here we measure levels of sexual dimorphism and within-male variation of body length, head size, and sexual dichromatism in a population of wild western gorillas using photogrammetry. Digital photogrammetry is a useful and precise method to measure sexual dimorphism in physical traits yielding sexual dimorphism indices (ISD), similar to those derived from traditional measurements of skeletal remains. Silverbacks were on an average 1.23 times longer in body length than adult females. Sexual dimorphism of head size was highest in measures of crest size (max ISD: 60.4) compared with measures of facial height (max ISD: 24.7). The most sexually dimorphic head size measures also showed the highest within-sex variation. We found no clear sex differences in crest coloration but there was large sexual dichromatism with high within-male variation in saddle coloration and silverline size. Further studies should examine if these sexually dimorphic traits are honest signals of competitive ability and confer an advantage in reproductive success.     

Sexual size dimorphism in northern giant petrels: ecological correlates and scaling

Northern giant petrels ( Macronectes halli ) are among the largest and most sexually size dimorphic species of seabirds, with females being only 80% the mass of males. Both sexes scavenge on seal and penguin carrion in the sub-Antarctic ecosystem, but during the breeding season females also feed extensively on other marine food resources and show more pelagic habits than males. The outstanding sexual segregation in foraging and feeding ecology in northern giant petrels suggests that mechanisms maintaining sexual size dimorphism by ecological factors may be operating. I evaluated this possibility by examining ecological correlates with body size and by static allometry analyses. Fledging sex ratio in four consecutive years did not depart from parity. There was no assortative mating by size neither association between the male size with the breeding performance. By contrast, smaller females raised their chick in better condition. Moreover, bill size showed a size dimorphism beyond that expected by body size dimorphism, i.e. when controlling for body mass, males showed relatively longer bill than females. This trait did not deviate from isometry with respect to body size and its phenotypic variability was low, suggesting that the disproportionately large bill of males is related to their more scavenging life style compared to females. In general, the increase and maintenance of sexual size dimorphism in giant petrels is more consistent with an ecological causation rather than a result of sexual selection.     

Seasonal dimorphism in the horny bills of sparrows

Bill size is often viewed as a species‐specific adaptation for feeding, but it sometimes varies between sexes, suggesting that sexual selection or intersexual competition may also be important. Hypotheses to explain sexual dimorphism in avian bill size include divergence in feeding niche or thermoregulatory demands, intrasexual selection based on increased competition among males, or female preference. Birds also show seasonal changes in bill size due to shifts in the balance between growth rate and wear, which may be due to diet or endogenous rhythms in growth. Insight into the function of dimorphism can be gained using the novel approach of digital x‐ray imaging of museum skins to examine the degree to which the skeletal core or the rhamphotheca contribute to overall dimorphism. The rhamphotheca is ever‐growing and ever‐wearing, varying in size throughout life; whereas the skeletal core shows determinant growth. Because tidal marsh sparrows are more dimorphic in bill size than related taxa, we selected two marsh taxa to investigate dimorphism and seasonality in the size of the overall bill, the skeletal core, and the rhamphotheca. Bill size varied by sex and season, with males having larger bills than females, and bill size increasing from nonbreeding to breeding season more in males. Skeletal bill size varied with season, but not sex. The rhamphotheca varied primarily with sex; males had a larger rhamphotheca (corrected for skeletal bill size), which showed a greater seasonal increase than females. The rhamphotheca, rather than the skeletal bill, was responsible for sexual dimorphism in overall bill size, which was particularly well developed in the breeding season. The size of the rhamphotheca may be a condition‐based character that is shaped by sexual selection. These results are consistent with the evidence that bill size is influenced by sexual selection as well as trophic ecology.     

Sexual, fecundity, and viability selection on flower size and number in a sexually dimorphic plant

The evolution of sexual dimorphism will depend on how sexual, fecundity and viability selection act within each sex, with the different forms of selection potentially operating in opposing directions. We examined selection in the dioecious plant Silene latifolia using planted arrays of selection lines that differed in flower size (small vs. large). In this species, a flower size/number trade-off exists within each sex, and males produce smaller and more numerous flowers than females. Moreover, floral traits are genetically correlated with leaf physiology. Sexual selection favoring males in the small-flower line occurred via greater overlap in the timing of flower output between males from this line and females. Fecundity selection favored males with high flower production, as siring success was proportionate to pollen production. Viability selection opposed sexual selection, favoring males from the large-flower line. In females, fecundity and viability selection operated in the same direction, favoring those from the large-flower line via greater seed production and survival. These results concur with the pattern of floral sexual dimorphism. Together with previous results they suggest that the outcome of the different forms of selection will be environmentally dependent, and therefore help to explain variation among populations in sexually dimorphic traits.