Phylogenetic systematics, historical biogeography, and the evolution of vocalizations in Nearctic toads (Bufo)

The evolution of mating signals has long interested biologists because changes in mating signal production and/or reception can lead to reproductive isolation and speciation. Here, I examine the evolution of the male mating signal (the advertisement call) and the female preference for this call in the Western Toad, Bufo boreas. Call surveys and a morphological study for the occurrence of vocal sacs, which are necessary for producing these calls, reveal that only populations in the northeastern corner of this species’ range produce long, high-amplitude advertisement calls. This is the first study to report among-population variation in the presence of the major mating signal in any animal. Although populations vary in whether or not males call, phonotaxis tests demonstrate that female B. boreas in calling and non-calling populations have the preference for this call. Phylogenetic analyses indicate that the call was lost in the ancestor to modern B. boreas and then secondarily re-evolved in the ortheastern populations. Bufo boreas is one of many toad species that inhabits the Nearctic region. I use phylogenetic analyses of large and small subunit mitochondrial ribosomal DNA sequences to examine the phylogenetic relationships among Nearctic toad species and test previously proposed biogeographic hypotheses for the colonization history of the Nearctic region. This work indicates that the Nearctic Bufo are monophyletic and result from a single colonization event from the Neotropics. Further, fossil and paleogeographic data suggest that this colonization occurred prior to the formation of a contiguous land bridge between the Neotropic and Nearctic regions. Many of the individuals examined in the Nearctic toad study had previously been sequenced for the same gene region. A surprising number of errors were found in the earlier sequences and attributed to the method of sequence generation. In my final chapter, I review the causes and consequences of sequencing error and present a novel method that uses sequence conservation information to detect errors. This approach is exemplified with the unique dataset of replicated sequences, and resources for easily implementing this approach are made available on the Comparative RNA Web Site (http://www.rna.ccbb.utexas.edu/).