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At least 19 recordsLinked to original sources

PubMed2006

Mechanisms of sexual selection: sexual swellings and estrogen concentrations as fertility indicators and cues for male consort decisions in wild baboons.

Male mate-guarding episodes ('consortships'), are taxonomically widespread, yet costly to individual males. Consequently, males should bias consortships toward females with whom the probability of conception is high. We combined data on consortships with visual scoring of sexual swellings and assays of fecal estrogen concentrations (fE) in a wild population of baboons (Papio cynocephalus) to test the hypotheses that sexual swellings are reliable indicators of (1) within-cycle timing of ovulation, (2) differences in conception probability among females that differ in maturational stage, and (3) conceptive versus non-conceptive cycles of parous females. We also evaluated whether adult males might rely on swellings or other estrogen-dependent signals (e.g., fE) for mate-guarding decisions. We found that sexual swellings reflected conception probability within and among cycles. Adult males limited their consortships to the turgescent phase of cycles, and consorted more with adult females than with newly cycling adolescents. The highest ranking (alpha) males discriminated more than did males of other ranks; they (1) limited their consortships to the 5-day peri-ovulatory period, (2) consorted more with adult than with adolescent females, and (3) consorted more with adult females on conceptive cycles than on non-conceptive cycles, all to a greater extent than did males of other ranks. Male mate choice based on sexual swellings and other estrogenic cues of fertility may result in sexual selection on these female traits and enhance dominance-based reproductive skew in males. Alpha males are the least constrained in their mating behavior and can best take advantage of these cues to mate selectively.

Animals↗

PubMed2003

Sexual selection favors female-biased sex ratios: the balance between the opposing forces of sex-ratio selection and sexual selection.

In a verbal model, Trivers and Willard proposed that, whenever there is sexual selection among males, natural selection should favor mothers that produce sons when in good condition but daughters when in poor condition. The predictions of this model have been the subject of recent debate. We present an explicit population genetic model for the evolution of a maternal-effect gene that biases offspring sex ratio. We show that, like local mate competition, sexual selection favors female-biased sex ratios whenever maternal condition affects the reproductive competitive ability of sons. However, Fisherian sex-ratio selection, which favors a balanced sex ratio, is an opposing force. We show that the evolution of maternal sex-ratio biasing by these opposing selection forces requires a positive covariance across environments between the sex-ratio bias toward sons (b) and the mating success of sons (r). This covariance alone is not a sufficient condition for the evolution of maternal sex-ratio biasing; it must be sufficiently positive to outweigh the opposing sex-ratio selection. To identify the necessary and sufficient conditions, we partition total evolutionary change into three components: (1) maternal sex-ratio bias, (2) sexual selection on sons, and (3) sex-ratio selection. Because the magnitude of the first component asymmetrically affects the strength of the second, biasing broods toward females in a poor environment evolves faster than the same degree of bias toward males in a good environment. Consequently, female-biased sex ratios, rather than male-biased sex ratios, are more likely to evolve. We discuss our findings in the context of the primary sex-ratio biases observed in strongly sexually selected species and indicate how this perspective can assist the experimental study of sex ratio evolution.

Alleles↗

PubMed2005

Experimental removal and elevation of sexual selection: does sexual selection generate manipulative males and resistant females?

Sexual conflict over reproduction can occur between males and females. In several naturally promiscuous insect species, experimental evolution studies that have enforced monogamy found evidence for sexual conflict. Here, we subjected the naturally promiscuous, sperm-heteromorphic fruit fly Drosophila pseudoobscura to enforced monogamy, standard levels of promiscuity, and elevated opportunities for promiscuity in four replicate lines. We examined the effect of male and female selection history and the proximate effect of variation in male density on female fitness parameters. We found that male density rather than male selection history explained a greater degree of female fecundity, egg hatching success, and productivity. Additionally, females selected under elevated promiscuity had greater fecundity and hatching success than did enforced monogamy females. Selection line males do not differ in their capacity to coerce females to remate, suggesting no divergence in precopulatory manipulative ability. However, these males did vary in their ability to suppress female remating, suggesting postcopulatory manipulation. These results indicate that sexual conflict can be manifested through both the proximate effects of male density and the historical levels of sexual selection and that the sexes respond differentially to these factors and further stress the multifarious channels of sexual communication that contribute to fitness.

Animals↗

PubMed2002

Sexual dimorphism in lizard body shape: the roles of sexual selection and fecundity selection.

Sexual dimorphism is widespread in lizards, with the most consistently dimorphic traits being head size (males have larger heads) and trunk length (the distance between the front and hind legs is greater in females). These dimorphisms have generally been interpreted as follows: (1) large heads in males evolve through male-male rivalry (sexual selection); and (2) larger interlimb lengths in females provide space for more eggs (fecundity selection). In an Australian lizard (the snow skink, Niveoscincus microlepidotus), we found no evidence for ongoing selection on head size. Trunk length, however, was under positive fecundity selection in females and under negative sexual selection in males. Thus, fecundity selection and sexual selection work in concert to drive the evolution of sexual dimorphism in trunk length in snow skinks.

Animals↗

PubMed1997

Stress, developmental stability and sexual selection.

Sexual selection may give rise to increases in the general level of stress experienced by individuals, either because intense directional selection reduces the ability of individuals to control the stable development of their phenotype, or because extravagant secondary sexual characters on their own impose stress on their bearers. Sexual selection often acts against individuals with asymmetric or otherwise deviant phenotypes, particularly if such phenotypic deviance occurs in secondary sexual characters. A small number of studies suggests that such characters also are more susceptible to the disruptive effects of deviant environmental conditions than are ordinary morphological characters. Plants often show extensive phenotypic asymmetry, and pollinators avoid asymmetric flowers, either because they are generally less attractive or provide fewer pollinator rewards. Floral symmetry may give rise to sexual selection with direct or indirect fitness benefits, as in animals. Sexual selection in animals may result in selection for relatively larger male body size, an overall increase in body size of a lineage and an increased risk of extinction (Cope's rule). Reduced stress resistance associated with intense sexual selection may contribute to this trend.

Animals↗

PubMed1998

Sex roles and sexual selection.

Sexual selection has been portrayed as acting predominantly on males who compete with each other over copulatory access to females; selection was considered to be driven by females choosing between males at the pre- or postcopulatory level. However, a broader view of sexual selection is now emerging. Examining male discrimination between females and female-female competition has been beneficial in identifying factors influencing the direction and strength of sexual selection. Furthermore, consideration of processes such as sexual coercion or genetic incompatibility, which indirectly influence an individual's set of copulation partners, gamete set or their offspring success, has helped to clarify the ways in which sexual selection may operate. Moreover, there is increasing evidence that not all copulations translate directly to paternity and that paternity does not necessarily translate into successful offspring. Postcopulatory and postfertilization mechanisms that influence not only paternity share but offspring recruitment now require further consideration. The benefits to each sex of copulating with particular partners or with more than one partner remains an area of debate. More carefully designed studies which eliminate alternative possibilities or quantify the relative importance of different selective pressures will also benefit from considering that not all copulations function solely to inseminate or receive sperm. It is also now clear that not all individuals of one sex follow the same strategy. Examining the variation between individuals in reproductive behaviour, fertilization success and offspring success will be important in establishing the selective pressures and mechanisms underlying the operation of sexual selection. (c) 1998 The Association for the Study of Animal Behaviour.

Journal Article↗

PubMed2000

Estimating sexual selection and sexual isolation effects from mating frequencies.

Sexual selection (defined as the change in genotypic or phenotypic frequencies of mated versus total population frequencies) and sexual isolation (defined as the deviation from random mating in mated individuals) show different evolutionary consequences and partially confounded causes. Traditionally, the cross-product estimator has been used to quantify sexual selection, whereas a variety of indexes, such as Yule V, Yule Q, YA, joint I, and others have been used to quantify sexual isolation. Because the two types of estimators use different scales, the effects of both processes cannot be monitored simultaneously. We describe three new related statistics that quantify both sexual selection (PSS) and sexual isolation (PSI) effects for every mating pair combination in polymorphic traits, as well as measure their combined effects (PTI = PSI x PSS). The new statistics have the advantage of providing information on every mating pair combination, quantifying the effects of sexual selection and isolation in the same units, and detecting asymmetry in sexual isolation. The ability of the new statistics to ascertain the biological causes of sexual selection and sexual isolation are investigated under different models involving distinct marginal frequencies, mate propensity, and mate choice coefficients. We also studied the use of classical isolation indexes applied on PSI coefficients, instead of on raw data. The use of the classical indexes applied to PSI coefficients considerably reduces the statistical bias of the estimates, revealing the good estimation properties of the new statistics.

Animals↗

PubMed2006

No evidence of current sexual selection on sexually dimorphic traits in a bird with high variance in mating success.

Sexual dimorphism, particularly in ornamental traits, is likely to have arisen by sexual selection. Most empirical and theoretical studies of sexual dimorphism assume that ongoing sexual selection also maintains the dimorphism. Over four seasons, I measured the sexual selection acting on three sexually dimorphic attributes (epaulet size, body size, and the blackness of the body plumage) of male red-winged blackbirds and found no consistent directional or stabilizing selection on any of them. Correlational selection was also negligible. I used path analysis to explore potential relationships in more detail but found no direct or indirect effects of male traits on either within- or extrapair success. Males who were resident on the marsh for more years had higher within-pair success, primarily because they spent more of the season on their territory. Experimental manipulations of epaulet size and color and the extent of nonblack feathers in the black body plumage had no detectable effect on the number of within-pair mates, paternity, or the number of extrapair offspring sired in nearby territories. These results combine with data from other studies of red-winged blackbirds to suggest that, despite high variation in male mating success and hence a strong opportunity for sexual selection, several morphological attributes that differ between the sexes and vary among males are not under current sexual selection. The possible explanations for why add complexity to our understanding of how sexual selection operates.

Animals↗

PubMed1988

Frequency-dependent sexual selection.

Sexual selection by female choice is expected to give rise to a frequency-dependent sexual advantage in favour of preferred male phenotypes: the rarer the preferred phenotypes, the more often they are chosen as mates. This 'rare-male advantage' can maintain a polymorphism when two or more phenotypes are mated preferentially: each phenotype gains an advantage when it is rarer than the others; no preferred phenotype can then be lost from the population. Expression of preference may be complete or partial. In models of complete preference, females with a preference always mate preferentially. Models of partial preference are more realistic: in these models, the probability that a female mates preferentially depends on the frequency with which she encounters the males she prefers. Two different 'encounter models' of partial preference have been derived: the O'Donald model and the Charlesworth model. The encounter models contain the complete preference model as a limiting case. In this paper, the Charlesworth model is generalized to allow for female preference of more than one male phenotype. Levels of frequency dependence can then be compared in the O'Donald and Charlesworth models. The complete preference model and both encounter models are formulated in the same genetical terms of preferences for dominant and recessive male phenotypes. Polymorphic equilibria and conditions for stability are derived for each of the three models. The models are then fitted to data of frequencies of matings observed in experiments with the two-spot ladybird. The complete preference model gives as good a fit as the encounter models to the data of these and other experiments. The O'Donald and Charlesworth encounter models are shown to produce a very similar frequency-dependent relation. Generally, as females become less choosy, they express their preference with more dependence on male frequency, whereas the resulting selection of the males becomes less frequency dependent. More choosy females are more constant in expressing their preference, producing greater frequency dependence in the selection of the males.

Animals↗

PubMed2006

JMATING: a software for the analysis of sexual selection and sexual isolation effects from mating frequency data.

BACKGROUND: Many different sexual isolation and sexual selection statistics have been proposed in the past. However, there is no available software that implements all these statistical estimators and their corresponding tests for the study of mating behaviour. RESULTS: JMATING is an easy-to-use program developed in Java for the analysis of mating frequency data to study sexual selection and sexual isolation effects from laboratory experiments as well as descriptive studies accomplished in the wild. The software allows the re-organization of the data previous to the analysis, the estimation of the most important estimators, and a battery of complementary statistical tests. CONCLUSION: JMATING is the first complete and versatile software for the analyses of mating frequency data. It is available at http://www.uvigo.es/webs/c03/webc03/XENETICA/XB2/JMsoft.htm and requires the Java runtime environment.

Animals↗

PubMed2004

Pleiotropy and the genomic location of sexually selected genes.

Sexual selection drives the evolution of traits involved in the competition for mates. Although considerable research has focused on the evolution of sexually selected traits, their underlying genetic architecture is poorly resolved. Here I address the pleiotropic effects and genomic locations of sexually selected genes. These two important characteristics can impose considerable constraints on evolvability and may influence our understanding of the process of sexual selection. Theoretical models are inconsistent regarding the genomic location of sexually selected genes. Models that do not incorporate pleiotropic effects often predict sex linkage. Conversely, sex linkage is not explicitly predicted by the condition-dependent model (which considers pleiotropic effects). Evidence largely based on reciprocal crosses supports the notion of sex linkage. However, although they infer genetic contribution, reciprocal crosses cannot identify the genes or their pleiotropic effects. By surveying the genome of Drosophila melanogaster, I provide evidence for the genomic location and pleiotropic effects of 63 putatively sexually selected genes. Interestingly, most are pleiotropic (73%), and they are not preferentially sex linked. Their pleiotropic effects include fertility, development, life span, and viability, which may contribute to condition and/or fitness. My findings may also provide evidence for the capture of genetic variation in condition via the pleiotropic effects of sexually selected genes.

Animals↗

PubMed2005

Experimental sexual selection in Chlamydomonas.

Sexual and asexual lines of the unicellular chlorophyte Chlamydomonas reinhardtii were propagated for about 100 sexual cycles and 1000 vegetative cycles in contrasted environments, liquid and solid growth media, in order to generate divergent natural and sexual selection. Sexual lines were transferred by many zygotes or by a single zygote in each sexual generation. By the end of the experiment zygote production was in the order sexual mass-transfer>sexual single-zygote>asexual>ancestor. The direct response to sexual selection was large, with zygote production increasing by about two orders of magnitude, mainly because mating had become spontaneous instead of being invoked by nitrogen starvation. Asexual lines became sexually sterilized by the fixation of a single mating type. Sexual selection caused a radical shift in the gender system, with homothallism spreading to high frequency in all sexual lines of this normally heterothallic species. This may have been caused by the transposition of a mating-type gene to an autosome. No substantial degree of environment-specific mating evolved, however, and thus no sexual isolation indicative of incipient speciation. It is possible that selection experiments of this kind are unlikely to induce sexual isolation because mating-type genes evolve in a saltatory fashion.

Animals↗

PubMed2005

Environmental origins of sexually selected variation and a critique of the fluctuating asymmetry-sexual selection hypothesis.

Identifying sources of phenotypic variability in secondary sexual traits is critical for understanding their signaling properties, role in sexual selection, and for predicting their evolutionary dynamics. The present study tests for the effects of genotype, developmental temperature, and their interaction, on size and fluctuating asymmetry of the male sex comb, a secondary sexual character, in Drosophila bipectinata Duda. Both the size and symmetry of elements of the sex comb have been shown previously to be under sexual selection in a natural population in northeastern Australia. Two independent reciprocal crosses were conducted at 25 degrees and 29 degrees C between genetic lines extracted from this population that differed in the size of the first (TC1) and third (TC3) comb segments. These temperatures are within the documented range experienced by the species in nature. Additive and dominance genetic effects were detected for TC1, whereas additive genetic, and Y-chromosomal effects were detected for TC3. TC2 and TC3 decreased sharply with increasing temperature, by 10% and 22%, respectively. In contrast, positional fluctuating asymmetry (PFA) significantly increased with temperature, by up to 38%. The results (1) document an important source of environmental variance in a sexual ornament expected to reduce trait heritability in field populations, and thus act to attenuate response to sexual selection, (2) suggest that variation in ornament size reflects differences in male condition; and (3) support the general hypothesis that asymmetry in a sexual ornament is indicative of developmental instability arising from environmental stress. The "environmental heterogeneity" (EH) hypothesis is proposed, and supportive evidence for it presented, to explain negative size-FA correlations in natural populations. Data and theory challenge the use of negative size-FA correlations observed in nature to support the FA-sexual selection hypothesis, which posits that such correlations are driven by differences in genetic quality among individuals.

Analysis of Variance↗

PubMed2001

Sexual selection and the maintenance of sexual reproduction.

The maintenance of sexual reproduction is a problem in evolutionary theory because, all else being equal, asexual populations have a twofold fitness advantage over their sexual counterparts and should rapidly outnumber a sexual population because every individual has the potential to reproduce. The twofold cost of sex exists because of anisogamy or gamete dimorphism-egg-producing females make a larger contribution to the zygote compared with the small contribution made by the sperm of males, but both males and females contribute 50% of the genes. Anisogamy also generates the conditions for sexual selection, a powerful evolutionary force that does not exist in asexual populations. The continued prevalence of sexual reproduction indicates that the 'all else being equal' assumption is incorrect. Here I show that sexual selection can mitigate or even eliminate the cost of sex. If sexual selection causes deleterious mutations to be more deleterious in males than females, then deleterious mutations are maintained at lower equilibrium frequency in sexual populations relative to asexual populations. The fitness of sexual females is higher than asexuals because there is no difference in the fecundity of sexual females and asexuals of the same genotype, but the equilibrium frequency of deleterious mutations is lower in sexual populations. The results are not altered by synergistic epistasis in males.

Animals↗

PubMed1977

Mating advantage of rare males in models of sexual selection.

Models of sexual selection in polygynous species of animals have been derived on the assumption that some females have preferences to mate with males with particular genotypes. The mating advantage gained by the males is always frequency-dependent because the preferred males take part in the same number of preferential matings when they are rare as when they are common; individually therefore, they mate more often when they are rare. Frequency-dependent sexual selection has been demonstrated in many experiments with Drosophila: rare males take part in a higher proportion of matings than their frequency as available mates. Ehrman and Spiess explained this phenomenon by frequency-dependence either in female preference or in male courtship. This explanation, which is difficult to interpret in behavioural terms, may not be necessary, however, because constant female preferences would entail frequency-dependent selection among the males. I show here that a simple model of constant preferences for particular phenotypes or genotypes is sufficient to explain a large body of data on frequency-dependent sexual selection in Drosophila.

Alleles↗

PubMed2003

A sexual selection theory longitudinal analysis of sexual segregation and integration in early adolescence.

The three objectives in this longitudinal study were motivated by sexual selection theory. The theory specifies the role of sexually segregated groups and the effects of dominance in male groups and relational/indirect aggression in female groups for heterosexual relationships. Using a multi-method, multi-informant, longitudinal design we studied youngsters (N=138) across their first two years of middle school. First, we examined the nature of change in segregation and dating popularity across two years during early adolescence. Second, a model derived from sexual selection theory is tested to explain the ways in which boys and girls are nominated for hypothetical dates (dating popularity). Third, we examined the role of "poke and push courtship" behavior in boys' and girls' dating popularity. Results indicate that although groups did not become more integrated with time, changes in peer group sexual integration co-varied dynamically with dating popularity. Secondly, dominance-related strategies were more important for boys than girls in dating popularity whereas indirect, or relational, aggression strategies were more important for girls than boys. Third, "poke and push courtship" behaviors did not influence peer group integration or dating.

Adolescent↗

PubMed2003

Sexually selected nest-building--Pomatoschistus minutus males build smaller nest-openings in the presence of sneaker males.

Both natural selection and sexual selection may act on nest-building. We tested experimentally how different regimes of egg-predation and male-male competition influence nest-building before mating, using the marine fish sand goby, Pomatoschistus minutus. Males with sneaker males present built the smallest nest-openings, smaller than males held alone or with Pomatoschistus microps males (which may predate eggs and compete over nest-sites but not compete over fertilizations). Males with visual access to other nest-building males tended also to build smaller openings than males held alone or with P. microps. Males with egg-predators present built nests with openings not differing significantly from any other treatment. Our results indicate that the small nest-openings found in the sneaker male treatment are sexually selected through protection against sneaking or by female choice. Across treatments, time span before a male started to build his nest also explained variation in nest-opening width; males starting late built larger nest-openings.

Adaptation, Physiological↗

PubMed1985

Quantitative genetic models of sexual selection.

Quantitative genetic models of sexual selection have disproven some of the central tenets of both the handicap mechanism and the 'sexy son' hypothesis. These results suggest that the 'good genes' approach to sexual selection may generally lead to erroneous results. Runaway sexual selection seems possible under a wide variety of circumstances. Quantitative genetic models have revealed runaway processes for sexually selected attributes expressed in both sexes and for attributes of parental care. Furthermore, the runaway could occur simultaneously in a series of populations that straddle an environmental gradient. While the models support the feasibility of runaway processes, empirical studies are needed to evaluate whether runaways actually happen. Estimates of critical genetic parameters are particularly needed, as well as measures of natural and sexual selection acting on the same population. The models also show that sexual selection has tremendous potential to produce population differentiation, particularly in epigamic traits. Differentiation is promoted by indeterminancy of evolutionary outcome, transient differences among populations during the final slow approach to equilibrium, sampling drift among equilibrium populations, and the tendency of sexual selection to amplify geographic variation arising from spatial differences in natural selection. Recent work with two- and three-locus models of sexual selection has produced results that parallel the results of the polygenic models. Thus the feature of indeterminate equilibria (outcome dependent on initial conditions) is common to both types of model.

Animals↗