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Results for “Sexual selection”

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PubMed2001

Disruptive sexual selection against hybrids contributes to speciation between Heliconius cydno and Heliconius melpomene.

Understanding the fate of hybrids in wild populations is fundamental to understanding speciation. Here we provide evidence for disruptive sexual selection against hybrids between Heliconius cydno and Heliconius melpomene. The two species are sympatric across most of Central and Andean South America, and coexist despite a low level of hybridization. No-choice mating experiments show strong assortative mating between the species. Hybrids mate readily with one another, but both sexes show a reduction in mating success of over 50% with the parental species. Mating preference is associated with a shift in the adult colour pattern, which is involved in predator defence through Müllerian mimicry, but also strongly affects male courtship probability. The hybrids, which lie outside the curve of protection afforded by mimetic resemblance to the parental species, are also largely outside the curves of parental mating preference. Disruptive sexual selection against F(1) hybrids therefore forms an additional post-mating barrier to gene flow, blurring the distinction between pre-mating and post-mating isolation, and helping to maintain the distinctness of these hybridizing species.

Animals↗

PubMed2003

Mutation and sexual selection: a test using barn swallows from Chernobyl.

Secondary sexual characters have been hypothesized to be particularly susceptible to the deleterious effects of mutation because the expression of such characters is usually influenced by many more metabolic pathways than are ordinary morphological characters. We tested this hypothesis using the elevated mutation rates in the barn swallow (Hirundo rustica) of the Chernobyl region of Ukraine as a model system. A great deal is known about the relative importance of different characters for male mating success in this species. The importance of phenotypic characters for male mating success was quantified based on a long-term study of a Danish breeding population, by expressing phenotypic differences between mated and unmated males as the difference between log-transformed mean values. For field samples from Ukraine we likewise expressed the difference in male phenotype between individuals living in a relatively uncontaminated area and individuals from the Chernobyl region as the difference between log-transformed mean values. The standardized difference in male phenotype between the two regions in Ukraine for the 41 different characters was strongly positively correlated with the standardized difference in male phenotype between mated and unmated males from Denmark. The standardized difference in male phenotype between the two regions in Ukraine was significantly positively associated with sexual size dimorphism. However, the standardized difference in male phenotype between mated and unmated males was a much better predictor of standardized difference in male phenotype between the two regions in Ukraine than was the standardized difference in sexual size dimorphism, expressed as the difference between log-transformed mean values for males and females. These findings are consistent with the hypothesis that traits most important for sexual selection are particularly susceptible to the effects of deleterious mutations.

Abnormalities, Radiation-Induced↗

PubMed2006

Phenotypic plasticity, sexual selection and the evolution of colour patterns.

When a population comes to occupy a new environment, phenotypically plastic responses alter the distribution of phenotypes, and hence affect both the direction and the intensity of selection. Rates of evolution can be accelerated or retarded compared to what would happen in the absence of plasticity. Plastic responses in one trait result in novel selection pressures on other traits, and this can lead to evolution in completely different directions than predicted in the absence of plasticity. In this paper I use the concept of the adaptive surface in order to identify conditions under which the various different outcomes are expected. I then discuss differences between sexually and naturally selected traits. Sexually selected traits are often expected to be plastic in their expression, with individuals in high condition developing greater elaboration. As examples of sexually selected traits I review the evolution of colour patterns in birds with a view to assessing the magnitude of plastic responses in their development, and to ask how such responses may have influenced genetic evolution. The common colour pigments in birds are carotenoids and melanins. Both are used in social signaling, and consequently are expected to evolve to be phenotypically plastic indicators of an individual's quality. Perhaps partly because they are condition indicators, the quantity of carotenoids in the plumage can be strongly influenced by diet. Examples are described where alterations of carotenoids in the diet are thought to have altered the phenotype, driving genetic evolution in novel directions. Melanin patterns seem to be less affected by diet, but the intensity of melanization after moult is affected by social interactions during the moult and by raising birds in humid conditions. Hormonal manipulations can have dramatic effects on both the kinds of melanin produced (eumelanin or phaeomelanin) as well as the patterns they form. Differences between species in melanin patterns resemble differences produced by environmental manipulations, as well as those produced by simple modulations of parameters in computer simulations of pattern formation. While phenotypic plasticity is one way that genetic change in plumage patterns (and other traits) could be driven, there are others, including the appearance of major mutations and selection on standing variation whose distribution is not altered in the new environment. I consider some evidence for the different alternatives, and ask when they might lead to qualitatively different evolutionary outcomes.

Adaptation, Physiological↗

PubMed1997

Sexual selection constrained by internal fertilization in the livebearing fish Xenotoca eiseni

Fish of the family Goodeidae have an advanced form of viviparity but males lack a specialized copulatory organ. Goodeids show reduced sexual-size dimorphism compared with the other livebearing families of the order Cyprinodontiformes. I investigated some mechanisms of sexual selection acting on body size in the goodeid fish Xenotoca eiseni. Both males and females strongly prefer mates of their own size. Comparison of mating activities in pairs with various degrees of size difference between males and females showed that matched pairs copulated more successfully. Similar mate preference in the two sexes thus appears to be the consequence of the primitive method of internal fertilization that requires the partners to be exactly synchronized during sperm transfer. Competition for access to a female was intense and body size determined the hierarchy in a small group. However, unless the size of the female was close to that of the dominant male, assortative mate preferences prevailed and the large-male advantage in competition was offset. Reduced size dimorphism in X. eiseni and in other goodeids may be explained by constraints on the action of sexual selection imposed by the need for effective fertilization.Copyright 1997 The Association for the Study of Animal BehaviourCopyright 1997The Association for the Study of Animal Behaviour.

Journal Article↗

PubMedDate not supplied

Differential regulation of msx genes in the development of the gonopodium, an intromittent organ, and of the "sword," a sexually selected trait of swordtail fishes (Xiphophorus).

The possession of a conspicuous extension of colored ventral rays of the caudal fin in male fish of swordtails (genus Xiphophorus) is a prominent example for a trait that evolved by sexual selection. To understand the evolutionary history of this so-called sword molecularly, it is of interest to unravel the developmental pathways responsible for extended growth of sword rays during development of swordtail males. We isolated two msx genes and showed that they are differentially regulated during sword outgrowth. During sword growth in juvenile males, as well as during testosterone-induced sword development and fin ray regeneration in the sword after amputation, expression of msxC is markedly up-regulated in the sword forming fin rays. In contrast, msxE/1 is not differentially expressed in ventral and dorsal male fin rays, suggesting a link between the development of male secondary sexual characters in fins and up-regulation of msxC expression. In addition, we showed that msx gene expression patterns differ significantly between Xiphophorus and zebrafish. We also included in our study the gonopodium, a testosterone-dependent anal fin modification that serves as a fertilization organ in males of live-bearing fishes. Our finding that increased levels of msxC expression are associated with the testosterone-induced outgrowth of the gonopodium might suggest either that at least parts of the signaling pathways that pattern the evolutionary older gonopodium have been coopted to evolve a sexually selected innovation such as the sword or that increased msxC expression may be inherent to the growth process of long fin rays in general.

Amino Acid Sequence↗

PubMed2001

Sexual selection of multiple handicaps in the red-collared widowbird: female choice of tail length but not carotenoid display.

Although sexual selection through female choice explains exaggerated male ornaments in many species, the evolution of the multicomponent nature of most sexual displays remains poorly understood. Theoretical models suggest that handicap signaling should converge on a single most informative quality indicator, whereas additional signals are more likely to be arbitrary Fisherian traits, amplifiers, or exploitations of receiver psychology. Male nuptial plumage in the highly polygynous red-collared widowbird (Euplectes ardens) comprises two of the commonly advocated quality advertisements (handicaps) in birds: a long graduated tail and red carotenoid coloration. Here we use multivariate selection analysis to investigate female choice in relation to male tail length, color (reflectance) of the collar, other aspects of morphology, ectoparasite load, display rate, and territory quality. The order and total number of active nests obtained are used as measures of male reproductive success. We demonstrate a strong female preference and net sexual selection for long tails, but marginal or no effects of color, morphology, or territory quality. Tail length explained 47% of male reproductive success, an unusually strong fitness effect of natural ornament variation. Fluctuating tail asymmetry was unrelated to tail length, and had no impact on mating success. For the red collar, there was negative net selection on collar area, presumably via its negative relationship with tail length. None of the color variables (hue, chroma, and brightness) had significant selection differentials, but a partial effect (selection gradient) of chroma might represent a color preference when tail length is controlled for. We suggest that the red collar functions in male agonistic interactions, which has been strongly supported by subsequent work. Thus, female choice targets only one handicap, extreme tail elongation, disregarding or even selecting against the carotenoid display. We discuss whether long tails might be better indicators of genetic quality than carotenoid pigmentation. As regards the evolution of multiple ornaments, we propose that multiple handicap signaling is stable not because of multiple messages but because of multiple receivers, in this case females and males.

Animals↗

PubMed2003

Shall we dance or shall we fight? Using DNA sequence data to untangle controversies surrounding sexual selection.

Population and evolutionary genetics studies have largely benefitted from advances in DNA manipulation and sequencing, as well as DNA data analysis techniques. Molecular evolution studies of male reproductive genes show a pattern of rapid evolution shaped, in some cases, by an adaptive selective process. Despite the large body of data on male reproductive genes, the female side of the story has remained unexplored. The few cases of female egg receptors analyzed also show rapid evolution. However, to disentangle between competing hypotheses on how selection operates on male x female molecular interaction leading to fertilization, we need to find male and female molecules that are partners in fertilization. A conflict model of sexual selection (similar to a host-parasite model) assumes a male-driven system where females are being forced under suboptimal conditions. This predicts that the amount of divergence at a female receptor depends on the amount of divergence among the male reproductive proteins that it binds (i.e., males are leading). Under a classical model of runaway sexual selection, female protein receptors might be the key to the rapid molecular changes observed in male reproductive proteins and higher divergence should be expected among female receptors than among their respective male binding proteins.

Animals↗

PubMed2000

Sexual selection and the evolution of exclusive paternal care in arthropods.

Internal fertilization and anisogamy are thought to impede the evolution of exclusive paternal care by reducing paternity assurance and increasing male promiscuity. The potential role of sexual selection in easing these constraints is currently being examined in vertebrates but has not been seriously studied in most arthropods. To distinguish the effects of sexual from natural selection on the evolution of arthropod paternal care, I tested predictions of the state of several life history and behavioural traits under both forms of selection across all known taxa with exclusive paternal care. The results suggest parallels between prezygotic nuptial gifts and exclusive postzygotic paternal care and support the hypothesis that, in arthropods, male behaviours that enhance female reproductive success either directly by releasing females from the fecundity constraints of maternal care (enhanced fecundity hypothesis) or indirectly by identifying mates with superior genes (handicap principle) are traits on which sexual selection has acted. Under such conditions males willing to guard young become preferred mates for gravid females and enjoy greater promiscuity than males unable or unwilling to guard. Females use nest construction or the act of guarding another female's eggs as honest signals of paternal intent and quality. Copyright 2000 The Association for the Study of Animal Behaviour.

Journal Article↗

PubMed2004

Sexual selection and genital evolution.

Genitalia are conspicuously variable, even in closely related taxa that are otherwise morphologically very similar. Explaining genital diversity is a longstanding problem that is attracting renewed interest from evolutionary biologists. New studies provide ever more compelling evidence that sexual selection is important in driving genital divergence. Importantly, several studies now link variation in genital morphology directly to male fertilization success, and modern comparative techniques have confirmed predicted associations between genital complexity and mating patterns across species. There is also evidence that male and female genitalia can coevolve antagonistically. Determining mechanisms of genital evolution is an important challenge if we are to resolve current debate concerning the relative significance of mate choice benefits and sexual conflict in sexual selection.

Journal Article↗

PubMed2005

Long-term decline in a red-winged blackbird population: ecological causes and sexual selection consequences.

Habitat loss and large-scale climate phenomena are widely implicated as causing decline in animal populations. I examined how both factors contributed to a precipitous decline in an Ontario red-winged blackbird (Agelaius phoeniceus) population using 16 years of data collected between 1974 and 1995. The decline was manifested as an almost 50% reduction in mean harem size, which reduced the opportunity for sexual selection threefold. Regional hay production, which should affect recruitment into the study population, also declined substantially. Correlation between blackbirds and hay may be coincidental, however, because annual changes in harem size were not associated with annual changes in hay production. This study coincided with an unprecedented positive phase of the North Atlantic Oscillation (NAO). Changes in harem size were correlated with winter NAO index values, suggesting that winter mortality contributed to the population decline. Positive correlation between harem size change and male return rates also supported the winter mortality hypothesis. Continued declines will cause this blackbird population to change from socially polygynous to socially monogamous. Study of red-winged blackbird winter ecology is needed to identify the proximate causes of mortality, whereas breeding studies can explore the consequences of relaxed sexual selection.

Animals↗

PubMed2006

Sexual selection and the evolution of brain size in primates.

Reproductive competition among males has long been considered a powerful force in the evolution of primates. The evolution of brain size and complexity in the Order Primates has been widely regarded as the hallmark of primate evolutionary history. Despite their importance to our understanding of primate evolution, the relationship between sexual selection and the evolutionary development of brain size is not well studied. The present research examines the evolutionary relationship between brain size and two components of primate sexual selection, sperm competition and male competition for mates. Results indicate that there is not a significant relationship between relative brain size and sperm competition as measured by relative testis size in primates, suggesting sperm competition has not played an important role in the evolution of brain size in the primate order. There is, however, a significant negative evolutionary relationship between relative brain size and the level of male competition for mates. The present study shows that the largest relative brain sizes among primate species are associated with monogamous mating systems, suggesting primate monogamy may require greater social acuity and abilities of deception.

Animals↗

PubMed1998

Sexual selection, parasites and bower height skew in a bower-building cichlid fish.

The good genes mechanism of sexual selection predicts that secondary sexual ornaments may reliably reveal a males resistance to parasites. We studied correlates of courtship and spawning success in a species of Copadichromis, a lekking cichlid fish from Lake Malawi, Africa, which builds sand bowers. We present the first evidence of a negative relationship between the structure of an extended phenotypic character (height skew of sand bowers) and male parasite load. Males that spawned had significantly fewer dilepipid cestodes in their livers than males that did not spawn, never before demonstrated in a lekking species of fish. Furthermore, males that spawned had significantly heavier gonads than 'unsuccessful males. We also found significant correlations between relative liver weight and some measures of reproductive success. This may indicate females are choosing to mate with males in better condition. Copyright 1998 The Association for the Study of Animal Behaviour

Journal Article↗

PubMed2005

Experimental evidence for multivariate stabilizing sexual selection.

Stabilizing selection is a fundamental concept in evolutionary biology. In the presence of a single intermediate optimum phenotype (fitness peak) on the fitness surface, stabilizing selection should cause the population to evolve toward such a peak. This prediction has seldom been tested, particularly for suites of correlated traits. The lack of tests for an evolutionary match between population means and adaptive peaks may be due, at least in part, to problems associated with empirically detecting multivariate stabilizing selection and with testing whether population means are at the peak of multivariate fitness surfaces. Here we show how canonical analysis of the fitness surface, combined with the estimation of confidence regions for stationary points on quadratic response surfaces, may be used to define multivariate stabilizing selection on a suite of traits and to establish whether natural populations reside on the multivariate peak. We manufactured artificial advertisement calls of the male cricket Teleogryllus commodus and played them back to females in laboratory phonotaxis trials to estimate the linear and nonlinear sexual selection that female phonotactic choice imposes on male call structure. Significant nonlinear selection on the major axes of the fitness surface was convex in nature and displayed an intermediate optimum, indicating multivariate stabilizing selection. The mean phenotypes of four independent samples of males, from the same population as the females used in phonotaxis trials, were within the 95% confidence region for the fitness peak. These experiments indicate that stabilizing sexual selection may play an important role in the evolution of male call properties in natural populations of T. commodus.

Analysis of Variance↗

PubMed1998

Females prefer leading males: relative call timing and sexual selection in katydid choruses.

Females of several acoustic insects and anurans have recently been shown to orient preferentially to the leading of two identical male calls presented in close succession. We studied this phenomenon via two-choice phonotaxis trials in the neotropical katydid Neoconocephalus spiza and found that females avoided male calls occurring during an interval beginning immediately after the onset of a leading call. This avoidance occurred whether or not the following call was overlapped by the leader, indicating that the mechanism of preference for leading calls may represent a psychoacoustic precedence effect rather than simple call masking. We also found that females preferred leading calls as long as the amplitude of the following call was not more than 2 dB higher than the leader. Under certain circumstances, preferences for leading calls remained when females were presented with a sequence of four calls. Thus, preferences for leading calls probably influence a male's attractiveness and mating success in complex natural choruses: females are expected to orient towards males producing leading calls regardless of call length (and the probability of overlapping a following call), when the leading call is up to 1.5 times as distant as a follower, and when succeeded by multiple followers. We suggest that female preferences for leading male calls represent sensory biases that originated in contexts unrelated to sexual selection. None the less they currently play an important role in sexual selection and influence individual male signalling behaviour, and thus chorus structure. Copyright 1998 The Association for the Study of Animal Behaviour.

Journal Article↗

PubMed2000

Fixation of new alleles and the extinction of small populations: drift load, beneficial alleles, and sexual selection.

With a small effective population size, random genetic drift is more important than selection in determining the fate of new alleles. Small populations therefore accumulate deleterious mutations. Left unchecked, the effect of these fixed alleles is to reduce the reproductive capacity of a species, eventually to the point of extinction. New beneficial mutations, if fixed by selection, can restore some of this lost fitness. This paper derives the overall change in fitness due to fixation of new deleterious and beneficial alleles, as a function of the distribution of effects of new mutations and the effective population size. There is a critical effective size below which a population will on average decline in fitness, but above which beneficial mutations allow the population to persist. With reasonable estimates of the relevant parameters, this critical effective size is likely to be a few hundred. Furthermore, sexual selection can act to reduce the fixation probability of deleterious new mutations and increase the probability of fixing new beneficial mutations. Sexual selection can therefore reduce the risk of extinction of small populations.

Alleles↗

PubMed1999

Density-dependent variation in lifetime breeding success and natural and sexual selection in Soay rams.

Variation in male lifetime breeding success (LBS) is central to understanding selection, yet it has rarely been measured in natural populations of large mammals. Here, we first describe variation in the opportunity for selection in cohorts of Soay rams (Ovis aries) on the archipelago of St. Kilda, Scotland, that were born during years of varying population density. Variation in LBS is closely coupled with demography, as rams born in years of low density following population crashes enjoy greater LBS than do those born in high-density years. Paradoxically, the opportunity for selection was greatest in the largest cohorts, those born in years of high population density, owing to low juvenile breeding success and overwinter survival. Variation in longevity and the contribution of nonbreeders were the most important components of the total variance in LBS in cohorts born in years of high density, while variation in fecundity was more important in cohorts born in low-density years. The opportunity for sexual selection is thus stronger in cohorts born in low-density years, as many rams in these cohorts survive to compete for mates as adults in subsequent ruts. Variation in population density in the year of birth also influenced the intensity of selection. Individuals born in years of high population density underwent strong natural selection in favor of longer hindlimbs over their first winter. In contrast, in cohorts born in low-density years, there was no natural selection on hindlimb in the first year of life. Longer hindlimbs were associated with increased fecundity over the entire lifetime of individuals born in low-density years. Natural and sexual selection thus act on the same trait in the same direction at different life-history stages in Soay rams, depending on the population density experienced in the year of birth.

Journal Article↗

PubMed2006

Early maternal investment in mice: no evidence for compatible-genes sexual selection despite hybrid vigor.

Confronting a recently mated female with a strange male can induce a pregnancy block ('Bruce effect'). The physiology of this effect is well studied, but its functional significance is still not fully understood. The 'anticipated infanticide hypothesis' suggests that the pregnancy block serves to avoid the cost of embryogenesis and giving birth to offspring that are likely to be killed by a new territory holder. Some 'compatible-genes sexual selection hypotheses' suggest that the likelihood of a pregnancy block is also dependent on the female's perception of the stud's and the stimulus male's genetic quality. We used two inbred strains of mice (C57BL/6 and BALB/c) to test all possible combinations of female strain, stud strain, and stimulus strain under experimental conditions (N(total) = 241 mated females). As predicted from previous studies, we found increased rates of pregnancy blocks if stud and stimulus strains differed, and we found evidence for hybrid vigour in offspring of between-strain mating. Despite the observed heterosis, pregnancies of within-strain matings were not more likely to be blocked than pregnancies of between-strain matings. A power analysis revealed that if we missed an existing effect (type-II error), the effect must be very small. If a female gave birth, the number and weight of newborns were not significantly influenced by the stimulus males. In conclusion, we found no support for the 'compatible-genes sexual selection hypotheses'.

Animals↗

PubMed2006

ESS gene expression of X-linked imprinted genes subject to sexual selection.

We define ESS (Evolutionary Stable Strategy) conditions for the evolution of genomic imprinting at an X-linked locus. The system analysed is designed for mammalian imprinting in which X-linked genes typically undergo random X-inactivation and lack Y-linked homologues. We consider two models that map cellular gene expression to fitness in females subject to random X-inactivation. In the first model, female fitness is simply a function of the average gene expression across all cells. In the second model, each cell contributes independently to fitness, and female fitness is assessed as the average of these contributions across all cells. In both models, imprinting readily evolves when sexual selection favours different levels of gene expression in the two sexes. Imprinting is beneficial as it improves adaptation in both sexes. There are limits to the improvement in adaptation when sexual selection is strong and favours greater gene expression in males (the heterogametic sex). We also consider the consequences of an active Y-linked homologue on the evolution of imprinting. Our analysis suggests that restrictive conditions apply for the evolution of polymorphic ESSs at an X-linked imprinted loci.

Animals↗