Dartmouth professor Ryan Calsbeek and post-doctorate Robert Cox recently found that forces like sexual selection, fecundity or viability selection, and sexual dimorphism have differing levels of effect on sexual antagonism (SA), or the shaping of differences between males and females in nature. The finding was published earlier this month in The American Naturalist.
Males and females share a large portion of their genome and express many of the same phenotypic traits and yet, the sexes have often very different fitness optima for these shared traits. This sexual antagonism generates intralocus sexual conflict, which occurs when selection at a locus favors different alleles in males versus females since genes that are beneficial for males may prove detrimental for females. Sexual dimorphism occurs anytime the sexes differ in some trait.
“There has a been a lot of recent interest in how males and females can evolve away from each other given the shared genome and our goal was to understand the importance that natural selection plays in this process. We are very interested in how natural and sexual selection operate in the wild and we just don’t know a lot about how frequently these selection pressures differ between the sexes. Our motivation was to summarize all of these data in one place to make the information available to a wide range of scientists in evolutionary biology,” said Calsbeek in an email to DUJS.
After reviewing published studies of 424 selection estimates representing 89 traits from 34 species, the research team found that the median strength of SA selection was 0.3 across all traits when considering only those instances in which the strength of SA was at least 0.1 in each sex (to prevent bias that overestimates the strength of selection). This result reveals that SA selection is common (though not ubiquitous) and often quite strong in wild populations.
The research team further found that sexual selection, which acts on an adult organism’s ability to successfully reproduce, generated stronger sexual antagonism than viability selection, which is the selection of an organism to survive and reach adulthood before it can reproduce. These individual components of fitness had a synergistic effect and reinforced one another to generate even stronger sexual antagonism for net fitness, which underscores the importance of integrating multiple selective episodes to estimate net lifetime fitness when assessing SA selection in wild populations.
Through a paired analysis comparing juvenile viability selection with adult reproductive selection within a subset of species for which both estimates were available, the research team found significantly stronger SA directional selection during adult reproduction. This result supports the tendency for increasing SA selection as an organism’s development progresses.
Further, the research team found that traits exhibited greater SA selection than did weakly dimorphic traits, though this pattern was no longer significant after the inclusion of multiple traits within species were controlled. These results suggest that intralocus sexual conflict often may persist despite the evolution of sexual dimorphism.
In response to a question of whether he was satisfied with his results, Calsbeek said, “Never. Our goal is to understand the evolution of sexual conflict as well as its resolution (i.e., we want to know how sexual antagonism puts pressure on the sexes, but we also want to understand how natural populations cope with these pressures). This paper is only a first step towards that understanding.”