Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

During speciation, populations become spatially separated from each other by biotic or abiotic factors, and this leads to genetic divergence and reproductive isolation. Here, I study the process of speciation and the patterns resulting from this process in the chorus frogs (Pseudacris). I first lay the foundation for this work by constructing phylogenies based on molecular data. I then address broad-scale questions regarding the abiotic factors thought to drive speciation. I examine evolution of reproductive signals within a phylogenetic context, and finally, I address fine-scale questions regarding the completion of reproductive isolation in contact zones between recently-evolved species. In chapter 1, I estimate the phylogenetic relationships across the genus Pseudacris. I find that several species of unclear status (regilla, cadaverina, crucifer, ocularis) belong to this genus, and that P. ocularis is the sister species of P. crucifer. In chapter 2, I examine the phylogeography of a clade within Pseudacris, the trilling chorus frogs. I find support for at least nine species and delineate their geographic distributions. In chapter 3, I test geological and climatic hypotheses proposed to drive speciation in North American flora and fauna. By estimating divergence times in the trilling chorus frogs and correlating these divergences with timing of geologic events, I find that marine inundation of the Mississippi Embayment may have caused speciation in this group. Additionally, I find that climatic events led to reduced genetic variation rather than divergence within species. In chapter 4, I study the evolution of acoustic signals of all species of Pseudacris. Using a comparative method approach, I find that physiologybased call variables are more evolutionarily labile than morphology-based call variables. In addition, I find that sympatric signals are more different than allopatric signals, suggesting that these frogs have partitioned the acoustic niche. In chapter 5, I examine evolution of reproductive isolation between two chorus frog species in sympatry. I find that male signals show a repeated pattern of divergence in sympatry, and that different axes of the signal diverge in different populations, suggesting that heterospecific overlap may lead to reproductive isolation among conspecific populations. I also find that female preferences have evolved in sympatry, suggesting that divergence in the contact zone is due to reinforcement.

Circuits underlying motor patterns of closely related species provide an ideal framework in which to study how evolution shapes behavioral variation. Male African clawed frogs (Xenopus and Silurana) advertisement call to attract female mates and silence male rivals. Males of each species produce a unique vocal pattern that serves as a species-identifier. Xenopus laevis is the most well-studied species in terms of its vocal behavior and underlying anatomy and physiology. The clade that includes X. laevis, or X. laevis senso lato, also includes 3 other species that diverged ~8.5 million years ago. All 4 of these species produce advertisement calls that include fast trills – trains of fast rate (~60 Hz) sound pulses. However, their calls differ substantially between species in measures of trill duration and period. I examined the premotor circuit underlying vocal patterning in three of these species: X. laevis, X. petersii, and X. victorianus. I used extracellular recordings to find that a premotor nucleus, DTAM, which is part of the vocal central pattern generator, is the likely source of species-variation of vocal patterns. Species-specific trill duration and period are intrinsic to the region of the hindbrain that includes DTAM. Next, I used blind whole-cell patch recordings in DTAM of X. laevis and X. petersii to examine the cells that encode trill duration and period. I identified homologous populations of premotor vocal cells in both species that code for trill duration and period in a species-specific manner. Together, these results support an autonomous role of the DTAM circuit for generation of species variation in call duration and period.