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Sexual selection favours traits that confer advantages in the competition for mates. In many cases, such traits are costly to produce and maintain, because the costs help to enforce the honesty of these signals and cues 1 . Some evolutionary models predict that sexual selection also produces costs at the population level, which could limit the ability of populations to adapt to changing conditions and thus increase the risk of extinction 2 – 4 . Other models, however, suggest that sexual selection should increase rates of adaptation and enhance the removal of deleterious mutations, thus protecting populations against extinction 3 , 5 , 6 . Resolving the conflict between these models is not only important for explaining the history of biodiversity, but also relevant to understanding the mechanisms of the current biodiversity crisis. Previous attempts to test the conflicting predictions produced by these models have been limited to extant species and have thus relied on indirect proxies for species extinction. Here we use the informative fossil record of cytheroid ostracods—small, bivalved crustaceans with sexually dimorphic carapaces—to test how sexual selection relates to actual species extinction. We show that species with more pronounced sexual dimorphism, indicating the highest levels of male investment in reproduction, had estimated extinction rates that were ten times higher than those of the species with the lowest investment. These results indicate that sexual selection can be a substantial risk factor for extinction. Ostracod species (small, bivalved crustaceans) with high sexual dimorphism, and therefore high male investment, had markedly higher extinction rates than low-investment species, indicating that sexual selection can be a substantial risk factor for extinction.

By quantifying the colouration of all approximately 6,000 species of passerine birds, certain life-history traits such as large body size and tropical distribution are found to increase ornamentation in both male and female birds, whereas cooperative breeding increases it in females only, and sexual selection diminishes it in females more than it increases it in males. Colour clash in male versus female birds Sexual selection theory can potentially explain the fact that male birds are usually more colourful and ornamented than females, but it does not explain why this is not always the case and why the difference between the sexes varies between species. James Dale et al . develop a method to quantify the colouration of all of the approximately 6,000 species of passerine birds to test alternative explanations. They find that patterns in the two sexes are correlated to a considerable extent, but that certain life history traits such as large body size and tropical distribution increase ornamentation in both sexes, whereas cooperative breeding increases ornamentation in females only, and sexual selection diminishes ornamentation in females more than it increases it in males. Classical sexual selection theory 1 , 2 , 3 , 4 provides a well-supported conceptual framework for understanding the evolution and signalling function of male ornaments. It predicts that males obtain greater fitness benefits than females through multiple mating because sperm are cheaper to produce than eggs. Sexual selection should therefore lead to the evolution of male-biased secondary sexual characters. However, females of many species are also highly ornamented 5 , 6 , 7 . The view that this is due to a correlated genetic response to selection on males 1 , 8 was widely accepted as an explanation for female ornamentation for over 100 years 5 and current theoretical 9 , 10 and empirical 11 , 12 , 13 evidence suggests that genetic constraints can limit sex-specific trait evolution. Alternatively, female ornamentation can be the outcome of direct selection for signalling needs 7 , 14 . Since few studies have explored interspecific patterns of both male and female elaboration, our understanding of the evolution of animal ornamentation remains incomplete, especially over broad taxonomic scales. Here we use a new method to quantify plumage colour of all ~6,000 species of passerine birds to determine the main evolutionary drivers of ornamental colouration in both sexes. We found that conspecific male and female colour elaboration are strongly correlated, suggesting that evolutionary changes in one sex are constrained by changes in the other sex. Both sexes are more ornamented in larger species and in species living in tropical environments. Ornamentation in females (but not males) is increased in cooperative breeders—species in which female–female competition for reproductive opportunities and other resources related to breeding may be high 6 . Finally, strong sexual selection on males has antagonistic effects, causing an increase in male colouration but a considerably more pronounced reduction in female ornamentation. Our results indicate that although there may be genetic constraints to sexually independent colour evolution, both female and male ornamentation are strongly and often differentially related to morphological, social and life-history variables.

Male genitalia show considerable morphological variation among animals with internal fertilization and exhibit a high level of evolvability in lizards. Studies have suggested that sexual selection may be driving hemipenial evolution against natural selection and pleiotropy. Given the direct interaction of male and female genitals, coevolution of the aforementioned is posited by several hypotheses of genital evolution. However, there are only a few studies on female genitalia morphology, resulting in a lack of coevolution description and understanding. Studies of allometric patterns have filled some gaps by answering questions about male genital evolution and could prove a powerful tool in clarifying coevolution between male and female genitals. Here, we studied the genital morphology of Tropidurus torquatus. This Tropidurus lizard species is an emerging Neotropical lizard model organism notable for having enlarged hemipenial lobes in contrast with other tropidurid species. In this study, we analyzed hemipenial development in early and late stages, describing both morphological variation and ontogenetic allometric pattern. We used quantitative traits to describe male and female genital morphology, examining their static allometric patterns and correspondence. Our study provides a quantitative discussion on the evolution of lizard genitals, suggesting that sexual selection plays an important role in genital evolution in Tropidurus lizards.

Animal communication is an intriguing topic in evolutionary biology. In this comprehensive study of visual signal evolution, we used a phylogenetic approach to study the evolution of the flash communication system of North American fireflies. The North American firefly genus Photinus contains 35 described species with simple ON–OFF visual signals, and information on habitat types, sympatric congeners, and predators. This makes them an ideal study system to test hypotheses on the evolution of male and female visual signal traits. Our analysis of 34 Photinus species suggests two temporal pattern generators: one for flash duration and one for flash intervals. Reproductive character displacement was a main factor for signal divergence in male flash duration among sympatric Photinus species. Male flash pattern intervals (i.e., the duration of the dark periods between signals) were positively correlated with the number of sympatric Photuris fireflies, which include predators of Photinus. Females of different Photinus species differ in their response preferences to male traits. As in other communication systems, firefly male sexual signals seem to be a compromise between optimizing mating success (sexual selection) and minimizing predation risk (natural selection). An integrative model for Photinus signal evolution is proposed.

Sexual reproduction is ubiquitous in the natural world, suggesting that sex must have extensive benefits to overcome the cost of males compared to asexual reproduction. One hypothesized advantage of sex with strong theoretical support is that sex plays a role in removing deleterious mutations from the genome. Theory predicts that transitions to asexuality should lead to the suppression of recombination and segregation and, in turn, weakened natural selection, allowing for the accumulation of slightly deleterious mutations. We tested this prediction by estimating the dN/dS ratios in asexual vertebrate lineages in the genus Aspidoscelis using whole mitochondrial genomes from seven asexual and five sexual species. We found higher dN/dS ratios in asexual Aspidoscelis species, indicating that asexual whiptails accumulate nonsynonymous substitutions due to weaker purifying selection. Additionally, we estimated nucleotide diversity and found that asexuals harbor significantly less diversity. Thus, despite their recent origins, slightly deleterious mutations accumulated rapidly enough in asexual lineages to be detected. We provide empirical evidence to corroborate the connection between asexuality and increased amino acid substitutions in asexual vertebrate lineages.

Background The theory of trade-off suggests that limited resources should lead to trade-off in resource intensive traits such as, immunity related and sexually selected traits in males. Alternatively, sexual exaggerations can also act as an honest indicator of underlying immunocompetence, leading to positive correlations between these traits. Evidences in support of either hypothesis in invertebrates are equivocal. Whereas several studies have addressed this question, few have used naturally occurring pathogens and realized post infection realized immunity (e.g., survivorship) to assay the fitness correlations between these two sets of traits. Results Adopting an experimental evolution approach, we evolved replicate populations of Drosophila melanogaster under high and low sexual selection regimes for over a hundred generations and found the following in virgin and mated males in three separate assays: Post infection survivorship against two natural pathogens - Pseudomonas entomophila (Pe) and Staphylococcus succinus (Ss): Mated males survived better against Pe, but were no different than virgins against Ss. Bacterial clearance ability against a third natural pathogen Providencia rettgeri (Pr): Mated males had significantly lower CFUs than virgins. However, sexual selection history had no effect on realized immunity of either virgin or mated males. Conclusion We show that while mating can affect realized immunity in a pathogen specific way, sexual selection did not affect the same. The results highlight that complex polygenic traits such as immunity and reproductive traits not necessarily evolve following a binary trade-off model. We also stress the importance natural pathogens while studying sexual selection-immunity correlations.

Background Genes that encode proteins associated with sperm competition, fertilization, and sexual conflicts of interest are often among the most rapidly evolving parts of animal genomes. One family of sperm-expressed genes ( Zp3r , C4bpa ) in the mammalian gene cluster called the regulator of complement activation (RCA) encodes proteins that bind eggs and mediate reproductive success, and are therefore expected to show high relative rates of nonsynonymous nucleotide substitution in response to sexual selection in comparison to other genes not involved in gamete binding at fertilization. We tested that working hypothesis by using phylogenetic models of codon evolution to identify episodes of diversifying positive selection. We used a comparative approach to quantify the evidence for episodic diversifying selection acting on RCA genes with known functions in fertilization (and sensitivity to sexual selection), and contrast them with other RCA genes in the same gene family that function in innate immunity (and are not sensitive to sexual selection). Results We expected but did not find evidence for more episodes of positive selection on Zp3r in Glires (the rodents and lagomorphs) or on C4BPA in Primates, in comparison to other paralogous RCA genes in the same taxon, or in comparison to the same orthologous RCA gene in the other taxon. That result was not unique to RCA genes: we also found little evidence for more episodes of diversifying selection on genes that encode selective sperm-binding molecules in the egg coat or zona pellucida ( Zp2 , Zp3 ) in comparison to members of the same gene family that encode structural elements of the egg coat ( Zp1 , Zp4 ). Similarly, we found little evidence for episodic diversifying selection acting on two other recently discovered genes ( Juno , Izumo1 ) that encode essential molecules for sperm–egg fusion. Conclusions These negative results help to illustrate the importance of a comparative context for this type of codon model analysis. The results may also point to other phylogenetic contexts in which the effects of selection acting on these fertilization proteins might be more readily discovered and documented in mammals and other taxa.

Sexual selection in mammals has resulted in the evolution of sexual size dimorphism (SSD), with males usually being the larger sex. Comparative analyses indicate that the evolution of SSD is associated with the evolution of male-biased mortality, suggesting a possible causal link between the two. Here, we use a comparative approach to investigate the possible role of parasites in generating this relation. We show that there is a robust association between male-biased parasitism and the degree of sexual selection, as measured by mating system (monogamous or polygynous) and by the degree of SSD. There is also a positive correlation, across taxa, between male-biased mortality and male-biased parasitism. These results are consistent with the hypothesis that parasites contribute to the observed association between SSD and male-biased mortality.

Objective The purpose of this study was to segregate the genetic lines responsible for the orange area of coloration in males and the response to orange coloration exhibited by females in the guppy ( Poecilia reticulata ) through artificial selection. This study is part of a project that uses QTL-seq to search for candidate genes involved in male orange coloration and female response to male coloration. We created two lines: high-selected lines of males having large areas of orange spots and of females with high response to male orange coloration; and low-selected lines of males having small areas of orange spots and of females with low response to male orange coloration. Results The male orange area and the female response became significantly different between high- and low-selected lines after three generations of artificial selection. This indicates that the differences in the frequencies of alleles at loci affecting the orange area and the female response between the lines increased over the generations through selection.

Male sexually selected signals can indicate competitive ability by honestly signaling fitness-relevant traits such as condition or performance. However, behavior can also influence contest outcomes; in particular, boldness often predicts dominance rank and mating success. Here, we sought to determine whether male ornament size is associated with consistent individual differences in boldness in water anoles . We measured the relative size of the dewlap, a flap of skin under the chin that is a sexually selected ornament in lizards, and tested for associations with responses to a novel and potentially risky environment: time to emerge from a refuge into an arena and number of head scans post-emergence. We found that individuals consistently differed in both time to emerge and head scanning (i.e., individual responses were repeatable), and that dewlap size was negatively related to number of head scans. This suggests that ornament size could indicate male boldness if scanning represents antipredator vigilance. We found that males that had larger relative dewlaps were also in better body condition, but boldness (i.e., head scanning) was not related to condition. Lastly, we found consistent differences in behavior between trials, showing that anoles were becoming habituated or sensitized to the testing arena. Overall, our study shows that in addition to indicating condition and performance, dewlap size could also honestly indicate male boldness in lizards.