Males and females share most of their genomes and express many of the same traits, yet the sexes often have markedly different selective optima for these shared traits. This sexually antagonistic (SA) selection generates intralocus sexual conflict that is thought to be resolved through the evolution of sexual dimorphism. However, we currently know little about the prevalence of SA selection, the components of fitness that generate sexual antagonism, or the relationship between sexual dimorphism and current SA selection. We reviewed published studies to address these questions, using 424 selection estimates representing 89 traits from 34 species. Males and females often differed substantially in the direction and magnitude of selection on shared traits, although statistically significant SA selection was relatively uncommon. Sexual selection generated stronger sexual antagonism than fecundity or viability selection, and these individual components of fitness tended to reinforce one another to generate even stronger sexual antagonism for net fitness. Traits exhibiting strong sexual dimorphism exhibited greater SA selection than did weakly dimorphic traits, although this pattern was not significant after we controlled for the inclusion of multiple traits nested within species. Our results suggest that intralocus sexual conflict often may persist despite the evolution of sexual dimorphism.

It is commonly argued that sexual size dimorphism (SSD) in lizards has evolved in response to two primary, nonexclusive processes: (1) sexual selection for large male size, which confers an advantage in intrasexual mate competition (intrasexual selection hypothesis), and (2) natural selection for large female size, which confers a fecundity advantage (fecundity advantage hypothesis). However, outside of several well-studied lizard genera, the empirical support for these hypotheses has not been examined with appropriate phylogenetic control. We conducted a comparative phylogenetic analysis to test these hypotheses using literature data from 497 lizard populations representing 302 species and 18 families. As predicted by the intrasexual selection hypothesis, male aggression and territoriality are correlated with SSD, but evolutionary shifts in these categorical variables each explain less than 2% of the inferred evolutionary change in SSD. We found stronger correlations between SSD and continuous estimates of intrasexual selection such as male to female home range ratio and female home range size. These results are consistent with the criticism that categorical variables may obscure much of the actual variation in intrasexual selection intensity needed to explain patterns in SSD. In accordance with the fecundity advantage hypothesis, SSD is correlated with clutch size, reproductive frequency, and reproductive mode (but not fecundity slope, reduced major axis estimator of fecundity slope, length of reproductive season, or latitude). However, evolutionary shifts in clutch size explain less than 8% of the associated change in SSD, which also varies significantly in the absence of evolutionary shifts in reproductive frequency and mode. A multiple regression model retained territoriality and clutch size as significant predictors of SSD, but only 16% of the variation in SSD is explained using these variables. Intrasexual selection for large male size and fecundity selection for large female size have undoubtedly helped to shape patterns of SSD across lizards, but the comparative data at present provide only weak support for these hypotheses as general explanations for SSD in this group. Future work would benefit from the consideration of alternatives to these traditional evolutionary hypotheses, and the elucidation of proximate mechanisms influencing growth and SSD within populations.

It is commonly argued that sexual size dimorphism (SSD) in lizards has evolved in response to two primary, nonexclusive processes: (1) sexual selection for large male size, which confers an advantage in intrasexual mate competition (intrasexual selection hypothesis), and (2) natural selection for large female size, which confers a fecundity advantage (fecundity advantage hypothesis). However, outside of several well-studied lizard genera, the empirical support for these hypotheses has not been examined with appropriate phylogenetic control. We conducted a comparative phylogenetic analysis to test these hypotheses using literature data from 497 lizard populations representing 302 species and 18 families. As predicted by the intrasexual selection hypothesis, male aggression and territoriality are correlated with SSD, but evolutionary shifts in these categorical variables each explain less than 2% of the inferred evolutionary change in SSD. We found stronger correlations between SSD and continuous estimates of intrasexual selection such as male to female home range ratio and female home range size. These results are consistent with the criticism that categorical variables may obscure much of the actual variation in intrasexual selection intensity needed to explain patterns in SSD. In accordance with the fecundity advantage hypothesis, SSD is correlated with clutch size, reproductive frequency, and reproductive mode (but not fecundity slope, reduced major axis estimator of fecundity slope, length of reproductive season, or latitude). However, evolutionary shifts in clutch size explain less than 8% of the associated change in SSD, which also varies significantly in the absence of evolutionary shifts in reproductive frequency and mode. A multiple regression model retained territoriality and clutch size as significant predictors of SSD, but only 16% of the variation in SSD is explained using these variables. Intrasexual selection for large male size and fecundity selection for large female size have undoubtedly helped to shape patterns of SSD across lizards, but the comparative data at present provide only weak support for these hypotheses as general explanations for SSD in this group. Future work would benefit from the consideration of alternatives to these traditional evolutionary hypotheses, and the elucidation of proximate mechanisms influencing growth and SSD within populations.

Most studies interpret reptilian sexual size dimorphism (SSD) as a means to reducing resource competition by way of sexual selection, fecundity selection, and natural selection. This chapter assesses the importance of these processes using data on 832 species of snakes, lizards, and turtles. The data reveal allometry consistent with Rensch's rule in most, but not all reptilian taxa, and support the hypothesis that sexual selection for large male size has influenced the evolution of reptile SSD. However, more data on male combat and territoriality are needed to test more fully this hypothesis. Although fecundity increases with female body size in many reptiles, comparative data provide only weak support for the fecundity advantage of large female size. The chapter concludes that further progress in assessing the relative importance of different selective processes in reptiles will require studies that more fully integrate evolutionary hypotheses with knowledge of proximate physiological and developmental mechanisms.

Tropical ectotherms are thought to be especially vulnerable to climate change because they are adapted to relatively stable temperature regimes, such that even small increases in environmental temperature may lead to large decreases in physiological performance. One way in which tropical organisms may mitigate the detrimental effects of warming is through evolutionary change in thermal physiology. The speed and magnitude of this response depend, in part, on the strength of climate-driven selection. However, many ectotherms use behavioral adjustments to maintain preferred body temperatures in the face of environmental variation. These behaviors may shelter individuals from natural selection, preventing evolutionary adaptation to changing conditions. Here, we mimic the effects of climate change by experimentally transplanting a population of Anolis sagrei lizards to a novel thermal environment. Transplanted lizards experienced warmer and more thermally variable conditions, which resulted in strong directional selection on thermal performance traits. These same traits were not under selection in a reference population studied in a less thermally stressful environment. Our results indicate that climate change can exert strong natural selection on tropical ectotherms, despite their ability to thermoregulate behaviorally. To the extent that thermal performance traits are heritable, populations may be capable of rapid adaptation to anthropogenic warming. Bahamas | thermoregulationA nthropogenic climate change may be the single most dramatic physical change our planet has experienced during human history (1). In the tropics, where species are adapted to relatively stable climates, the impacts of climate change are predicted to be especially severe (2-5) (but see refs. 6, 7). Because many species maintain a body temperature (T b ) that is already close to their thermal limits, even small increases in environmental temperature (T e ) may produce large decreases in physiological performance that could push populations toward extinction (4).In tropical environments, evolutionary adaptation may be one of the most important mechanisms by which populations can avoid extinction (8). As climates shift, fitness for many species will become increasingly linked to variation in traits important for performance in a warmer and more thermally variable world (4). Numerous theoretical, laboratory, and field studies demonstrate a capacity for rapid evolution on time scales similar to those over which global warming is predicted to occur (9-11). Indeed, recent work on lizards (12) and butterflies (13) has revealed rapid shifts in thermal physiology that appear to be directly associated with changing thermal environments. Despite mounting evidence that the capacity for evolution may fundamentally alter extinction probabilities in the face of anthropogenic climate change, the potential for rapid evolution is usually not considered in models that attempt to predict the impact of climate change on biological populations (8).The rate at which ...

Summary 1.A central tenet of life-history theory is that investment in reproduction compromises survival. However, the underlying physiological mechanisms that link reproduction to survival are poorly understood, particularly in wild populations. 2. Previous experiments in the brown anole lizard (Anolis sagrei) show that the elimination of reproduction via surgical ovariectomy results in a dramatic increase in the survival of wild females. We hypothesized that this trade-off reflects underlying differences in energy allocation between reproduction and physiological processes that influence survival. 3. To test this hypothesis, we compared ovariectomized (OVX) females to reproductive controls (SHAM) with respect to four physiological parameters that are thought to influence survival: energy storage, haematocrit, immune function and parasitemia. 4. Consistent with previous studies, we found that OVX females exhibited increased survival and growth relative to reproductive SHAM females. At the end of the breeding season, OVX also exceeded SHAM with respect to energy storage, haematocrit and immune response to phytohemagglutinin challenge. 5. Contrary to our predictions, OVX were more likely than SHAM to exhibit high levels of parasitemia. However, growth and parasite load were positively correlated in OVX and negatively correlated in SHAM, suggesting that reproductive investment may compromise parasite tolerance rather than parasite resistance. 6. Collectively, our results provide direct experimental evidence that reproductive investment affects several key physiological traits that likely interact to influence survival in wild populations.

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The integration of macroevolutionary pattern with developmental mechanism presents an outstanding challenge for studies of phenotypic evolution. Here, we use a combination of experimental and comparative data to test whether evolutionary shifts in the direction of sexual size dimorphism (SSD) correspond to underlying changes in the endocrine regulation of growth. First, we combine captive breeding studies with mark‐recapture data to show that male‐biased SSD develops in the brown anole lizard (Anolis sagrei) because males grow significantly faster than females as juveniles and adults. We then use castration surgeries and testosterone implants to show that castration inhibits, and testosterone stimulates, male growth. We conclude by reviewing published testosterone manipulations in other squamate reptiles in the context of evolutionary patterns in SSD. Collectively, these studies reveal that the evolution of SSD has been accompanied by underlying changes in the effect of testosterone on male growth, potentially facilitating the rapid evolution of SSD.