Investigating Processes Speciation at Multiple Scales in Chorus Frogs (Pseudacris)

Key to the study of evolutionary biology is understanding the process through which new species form and are maintained. Reinforcement, the process through which prezygotic isolation between species is enhanced upon secondary contact due to low hybrid fitness, is considered by many to be the final stage in the speciation process. One common consequence of this process is a pattern of reproductive character displacement between two closely related species wherein the traits of individuals living in sympatry (co-occurring with other species) are more different than the traits of individuals from allopatric populations (not occurring with other species). Many examples of this pattern and process are documented in natural populations, but there is still a lot to learn about this process. The goal of my dissertation is to enhance our understanding of reinforcement, which is fundamental to the development and maintenance of new species. Anuran amphibians (frogs and toads) are an ideal system for studying this topic because male advertisement calls and female preferences for these calls are an important reproductive isolating mechanism that is easily measured. Additionally, male calls vary with temperature and female preferences also adjust to allow them to continue to recognize and locate males. However, it is unknown whether temperature during breeding affects the level of reproductive isolation between taxa, thereby contributing to completion of speciation, or conversely, the collapse of species boundaries. In my dissertation, I have integrated ecological, genomic, and behavioral data to investigate processes of speciation at multiple scales in North American chorus frogs (Pseudacris). At the broad scale, I used a large genomic dataset collected across the central United States to determine species boundaries and what factors attribute to maintenance of these boundaries. In this region, four putative species of chorus frogs (Pseudacris triseriata, P. feriarum, P. maculata, and P. fouquettei) have overlapping ranges, but the taxonomy of this group is complicated owing to similar morphologies and behaviors. I identified five lineages across this region following large geographic boundaries—such as rivers, elevation, and glacial boundaries—and found that each species pair diverges behaviorally, ecologically, or both. Additionally, I identified two hybrid zones between P. maculata and P. triseriata with evidence of behavioral divergence. I then narrowed focus into on a single hybrid zone at two timepoints to determine how hybrid zones change over time and whether genetic clines match phenotypic clines in reproductive behaviors. For this chapter, I focused on a narrow region in the Southeastern United States where the upland chorus frog (P. feriarum) experiences costly hybridization with the southern chorus frog (P. nigrita), resulting in the reinforcement of male signals and female preferences in sympatry. I found that character displacement in male advertisement calls—such that P. feriarum produce faster calls in sympatry compared to allopatry—has become more pronounced where populations that formerly had an intermediate phenotype now display the allopatric phenotype. Additionally, the transition between the sympatric and allopatric phenotypes has shifted southward—farther into sympatry. Finally, I focused solely on two populations of P. feriarum—allopatry and sympatry—to determine whether environmental temperature affects reproductive isolation. Males from the population that co-occurs with P. nigrita have more variation in how their calls vary with temperature. Additionally, despite the two species having more similar advertisement calls at cold temperatures compared to warm temperatures, females from the population that co-occurs with P. nigrita can still distinguish between the two species. However, they take twice as long to make a decision, which has important implications for the rate of hybridization as environmental conditions change. Together, these findings help us to understand what factors are important in processes of speciation and emphasize the importance of considering variable factors, such as environmental temperature. In conclusion, my research provides valuable insights into processes of speciation at multiple scales in North American trilling chorus frogs (Pseudacris). I show that both ecological and behavioral divergence play a role in maintaining boundaries at the coarse scale. I then provide evidence for hybrid zone movement and steepening in a reinforcement contact zone between P. feriarum and P. nigrita. Finally, by investigating the role of temperature in reproductive behaviors, I show that sympatry in P. feriarum has increased plasticity of male advertisement call relative to allopatry and that temperature does not affect species discrimination for sympatric female P. feriarum, though it may affect hybridization rates due to differences in sound localization ability at warm versus cold temperatures.