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      Life history trade-offs at a single locus maintain sexually selected genetic variation.

      Nature
      Animals, Female, Genetic Variation, Genotype, Horns, Male, Mating Preference, Animal, physiology, Phenotype, Polymorphism, Genetic, Receptors, G-Protein-Coupled, genetics, Reproduction, Selection, Genetic, Survival Analysis

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          Abstract

          Sexual selection, through intra-male competition or female choice, is assumed to be a source of strong and sustained directional selection in the wild. In the presence of such strong directional selection, alleles enhancing a particular trait are predicted to become fixed within a population, leading to a decrease in the underlying genetic variation. However, there is often considerable genetic variation underlying sexually selected traits in wild populations, and consequently, this phenomenon has become a long-discussed issue in the field of evolutionary biology. In wild Soay sheep, large horns confer an advantage in strong intra-sexual competition, yet males show an inherited polymorphism for horn type and have substantial genetic variation in their horn size. Here we show that most genetic variation in this trait is maintained by a trade-off between natural and sexual selection at a single gene, relaxin-like receptor 2 (RXFP2). We found that an allele conferring larger horns, Ho(+), is associated with higher reproductive success, whereas a smaller horn allele, Ho(P), confers increased survival, resulting in a net effect of overdominance (that is, heterozygote advantage) for fitness at RXFP2. The nature of this trade-off is simple relative to commonly proposed explanations for the maintenance of sexually selected traits, such as genic capture ('good genes') and sexually antagonistic selection. Our results demonstrate that by identifying the genetic architecture of trait variation, we can determine the principal mechanisms maintaining genetic variation in traits under strong selection and explain apparently counter-evolutionary observations.

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          Most cited references24

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          The strength of phenotypic selection in natural populations.

          How strong is phenotypic selection on quantitative traits in the wild? We reviewed the literature from 1984 through 1997 for studies that estimated the strength of linear and quadratic selection in terms of standardized selection gradients or differentials on natural variation in quantitative traits for field populations. We tabulated 63 published studies of 62 species that reported over 2,500 estimates of linear or quadratic selection. More than 80% of the estimates were for morphological traits; there is very little data for behavioral or physiological traits. Most published selection studies were unreplicated and had sample sizes below 135 individuals, resulting in low statistical power to detect selection of the magnitude typically reported for natural populations. The absolute values of linear selection gradients |beta| were exponentially distributed with an overall median of 0.16, suggesting that strong directional selection was uncommon. The values of |beta| for selection on morphological and on life-history/phenological traits were significantly different: on average, selection on morphology was stronger than selection on phenology/life history. Similarly, the values of |beta| for selection via aspects of survival, fecundity, and mating success were significantly different: on average, selection on mating success was stronger than on survival. Comparisons of estimated linear selection gradients and differentials suggest that indirect components of phenotypic selection were usually modest relative to direct components. The absolute values of quadratic selection gradients |gamma| were exponentially distributed with an overall median of only 0.10, suggesting that quadratic selection is typically quite weak. The distribution of gamma values was symmetric about 0, providing no evidence that stabilizing selection is stronger or more common than disruptive selection in nature.
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            The Lek Paradox and the Capture of Genetic Variance by Condition Dependent Traits

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              Intralocus sexual conflict.

              Intralocus sexual conflict occurs when selection on a shared trait in one sex displaces the other sex from its phenotypic optimum. It arises because many shared traits have a common genetic basis but undergo contrasting selection in the sexes. A recent surge of interest in this evolutionary tug of war has yielded evidence of such conflicts in laboratory and natural populations. Here we highlight outstanding questions about the causes and consequences of intralocus sexual conflict at the genomic level, and its long-term implications for sexual coevolution. Whereas recent thinking has focussed on the role of intralocus sexual conflict as a brake on sexual coevolution, we urge a broader appraisal that also takes account of its potential to drive adaptive evolution and speciation.
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