Evolutionary and ecological influences on color pattern variation in the Australian common froglet, Crinia signifera

Elucidation of mechanisms that generate and maintain population-level phenotypic variability provides insight into processes that influence within-species genetic divergence. Historically, color pattern polymorphisms were used to infer population-level genetic variability, but recent approaches directly capture genetic variability using molecular markers. Here, I clarify the relationship between genetic variability and color pattern polymorphism within and among populations using the Australian common froglet, Crinia signifera. To illustrate genetic variability in C. signifera, I used phylogenetic analysis of mitochondrial DNA and uncovered three ancient geographically restricted lineages whose distributions are consistent with other southeastern Australian species. Additional phylogeographic structure was identified within the three ancient lineages and was consistent with geographic variation in male advertisement calls. Natural selection imposed by predators has been hypothesized to act on black-and-white ventral polymorphisms in C. signifera, specifically through mimicry of another Australian frog, Pseudophryne. I used clay replicas of C. signifera to test whether predators avoid black-and-white coloration. In fact, black-and-white replicas were preferentially avoided by predators in some habitats, but not in others, indicating that differential selection among habitats plays a role in maintaining color pattern polymorphism. When black-and-white color patterns in a sample of C. signifera populations were compared with those in sympatric Pseudophryne, several color pattern characteristics were correlated between the species. Furthermore, where C. signifera and Pseudophryne are sympatric, color patterns are more similar compared to those in allopatry. Extensive phylogenetic variability suggests that phylogenetic history and genetic drift may also influence C. signifera color pattern. Fine-scale phylogenetic analysis uncovered additional genetic diversity within lineages and low levels of introgression among previously identified clades. Measures of color pattern displayed low levels of phylogenetic signal, indicating that relationships among individuals only slightly influence color patterns. Finally, simulations of trait evolution under Brownian motion illustrated that the phylogeny alone cannot generate the pattern of variation observed in C. signifera color pattern. Therefore, this indicates a minimal role for genetic drift, but instead supports either the role of stabilizing selection due to mimicry, or diversifying selection due to habitat differences, in color pattern variation in C. signifera.