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Developmental plasticity can alter phenotypic development in response to changes in environmentalconditions. Recently, there is renewed interest in investigating the role of developmental plasticity in promoting divergence of sexual signals, not only because of ongoing climate change is predicted to influence sexual signaling, but also because changes in sexual signaling systems can promote signal divergence, increasing the diversity of signals. While there are examples of developmental plasticity in signals and sensory traits, few studies have examined whether such plasticity in signals and sensory traits actually translates to sexual signal divergence. Here, I use the cabbage white butterfly, Pieris rapae system to explore this idea. Cabbage whites tend to use open, brightly lit environments, but an "open niche" presents itself in the Eastern United States, where a potential novel host plant -- the invasive garlic mustard -- grows in more shady habitats. I reared cabbage white butterflies in conditions simulating open and closed light environments. Across multiple replicates of three experiments, I observed consistent and repeatable developmental plasticity in multimodal male signals, but, interestingly, inconsistent non-repeatable plasticity in female visual and olfactory sensory traits. In mating trials, females from shade-reared environments preferred males from shade-reared environments, whereas females from open environments showed no preference. This mating pattern, although asymmetric indicates that developmental plasticity can indeed act as a powerful mechanism facilitating divergence of sexual signals and could thereby contribute to early stages of population divergence and speciation by sexual selection.

Reproductive barriers drive divergence by restricting gene flow between populations. However, barriers are often semipermeable, allowing some gene flow between groups. Further, populations may vary in the number and coincidence of reproductive barriers. This dissertation focuses on patterns of gene exchange among populations of European corn borer moths (Ostrinia nubilalis) that vary in the number of acting barriers. In Chapter One, I performed an empirical test of a ‘coupling hypothesis’ of reproductive barriers, which examines how their joint effects enhance reproductive isolation and genomic divergence. I found divergence mainly restricted to barrier loci when populations differ by a single barrier, whereas the coincidence of temporal and behavioral barriers is associated with enhanced divergence. Specifically, differentiation at temporal barrier loci increases in the presence of behavioral divergence and differentiation at behavioral barrier loci increases in the presence of temporal divergence. In Chapter Two, I examine how barrier loci can guide life history variation when a species expands its range. To identify whether the genome reflects signatures of natural selection at such phenological loci, I studied the genomes of moths from populations along a latitudinal gradient in the Midwestern US. I found genome-wide signatures that were reflective of a large expansion in both range and population size and demonstrated that two key life history loci showed signatures consistent with both selection and demographic processes. Finally, in Chapter Three, I considered how introgression and recombination complicate patterns of ancestry of key barrier loci, using genealogical trees for individuals and species of the Ostrinia moth group. I developed tree sequence topologies from both genome wide markers and a set of four barrier trait loci and calculated genealogical nearest neighbor estimates, a statistic summarizing relative ancestry proportions of each individual to describe how phenotypic variation has shaped the species and population history of O. nubilalis. I showed that much of the genome reflects genealogical sharing across many populations, but genealogies at barrier loci distinguish groups. Finally, there was evidence of introgression of these barrier traits between multiple species within the Ostrinia group, suggesting a complex model for barrier trait evolution in this group.

Discrete variation in colorful sexual signals is often associated with variation in reproductive tactics or strategies. Many taxa with intrapopulation color polymorphisms also exhibit variation among populations or among closely related species in the frequency of color morphs, the number of morphs present within a population, or which particular colors are used as signals. These patterns may be important for understanding speciation because theoretical and empirical studies indicate that both color polymorphism and alternative reproductive tactics may be associated with elevated diversification rates. I studied variation in throat color, an important sexual signal, within and among populations of the mesquite lizard (Sceloporus grammicus) species complex. The S. grammicus species complex is considered a possible example of incipient speciation due to high levels of variation in chromosome number, habitat use, reproductive cycles, and life history. My first chapter confirms this variability in female life history, and the rest of my dissertation is the first research to address sexual signals in S. grammicus. I performed a broad geographic survey of throat color polymorphisms and used laboratory behavioral trials and molecular phylogenetics to investigate the meaning and evolutionary context of the polymorphisms I observed. I found that male S. grammicus within a single population exhibited either orange, yellow, and blue throat color morphs or orange, yellow, and white throat color morphs. In no case did I observe the blue male morph and the white male morph in the same population. I found that both the orange/yellow/blue and orange/yellow/white polymorphisms in males were associated with variation in male aggression level, but the meanings of particular colors were different between the two populations I studied. I also found that females performed more rejection behaviors towards allopatric males when those males were of a color morph that did not occur in the female's home population than when the allopatric male was of a color morph that did occur in the female's home population. Finally, I constructed a phylogeny of the populations I had sampled and used ancestral state reconstruction to establish that the white male morph in the S. grammicus complex has originated at least twice.

The genus Agelenopsis (Araneae, Agelenidae) is a group of morphologically similar funnel-web spiders with overlapping habitat requirements and geographic ranges. Yet, molecular evidence suggests this group has undergone recent speciation with no evidence of hybridization. In this dissertation, I explored courtship divergence as a possible explanation for the formation and coexistence of these species. Courtship behavior patterns, sequences, and vibratory signals were compared across 12 Agelenopsis species and three related outgroup species. Courtship in Agelenopsis was found to be comparatively long but not completely species-specific. To investigate mechanisms of reproductive isolation within the genus, interspecific crosses were staged in the lab between species for two closely related species pairs, one pair being sympatric in distribution and the other peripatric. Vibratory courtship did not function as a reproductive isolating mechanism for either pair. In the sympatric species pair, males displayed significantly less courtship toward heterospecific females than to conspecific females and no interspecific copulations were attempted. Females did not reject males based on their vibratory courtship, rather males rejected females, apparently based on chemical cues. In contrast, there was interspecific courtship and copulation in the peripatric species pair. However, males appeared to encounter mechanical difficulties copulating with heterospecific females and no hybrid offspring were produced. Thus, the sympatric pair was reproductively isolated by barriers acting early in the mating process, while the geographically isolated pair appeared to be isolated by barriers acting very late in the mating process. Though these results did not support a role for vibratory courtship in reproductive isolation, vibratory courtship appeared to be quite important in Agelenopsis. Males continued to court females even after females accepted them, and even behavior occurring late in the courtship sequence varied among species. In comparison to the two more distantly related outgroup species, vibratory courtship in Agelenopsis seems to be under relatively strong sexual selection for long, complex displays.