UC Santa Cruz

The field of eco-evolutionary dynamics has highlighted the myriad ways that ecological and evolutionary processes are intertwined. One key revelation emerging from this framework is the importance of intraspecific variation, which can have ecological effects comparable to or greater than the effects of species incidence and abundance. Despite the rapid expansion of research highlighting the consequences of intraspecific variation, most work in this area has focused on traits shaped primarily by natural selection, excluding sexual selection, a potent driver of phenotypic diversity. In this dissertation, I studied the evolutionary drivers and ecological consequences of intraspecific variation in sexually selected traits, using Western Mosquitofish (Gambusia affinis) as a focal organism. Mosquitofish mating behavior is characterized by male sexual harassment, in which males actively attempt copulations with unreceptive females. Although ambient factors (e.g., temperature) that drive plastic variation in this behavior have been described previously, few have examined the evolutionary drivers of variation in mating behavior, especially with respect to abiotic factors. Temperature is among the most influential abiotic factors because it governs the rates of all biological processes, influencing the physiology, morphology, and behavior of organisms. In Chapter 1, I used common rearing and behavioral assays to ask whether divergent average temperatures generated systematic differences in the thermal plasticity of mating behavior among six populations of mosquitofish. While harassment was sensitive to ambient temperature, peaking at intermediate temperatures and declining at thermal extremes, I found no evidence that divergent source temperatures could explain evolved behavioral differences among populations, highlighting the inherent challenges in identifying the drivers of intraspecific diversity. In Chapter 2, I used a free-swimming paired choice behavioral assay to determine whether differences in male harassment could explain population divergence in female gravid spots, which likely serve as a sexual signal due to the established correlation between gravid spot size and female fertility. Male preferences for gravid spots differed among populations but were inversely correlated with female gravid spot sizes within populations, an outcome in direct opposition to theoretical predictions regarding the coevolution of mate preferences and traits. In Chapter 3, I tested how intraspecific variation in male sexual harassment behavior influences aquatic ecosystems. Sexual harassment intensity modulated the effects of mosquitofish introduction for zooplankton communities, demonstrating the ecological importance of sexually selected traits. In summary, my dissertation shows how sexual selection, a ubiquitous evolutionary force, is fertile ground for exploring the interplay of ecological and evolutionary processes and warrants further attention in the framework of eco-evolutionary dynamics.