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Description
The Channel Islands, a group of eight oceanic islands in southern California, comprise a geographically simple and accessible system that can be used to study evolution and adaptive divergence. This thesis presents the results of a genetic and phenotypic assessment of an avian species, the Spotted Towhee (Pipilo maculatus), which occurs on three of the islands. Currently, one unique island subspecies, P. m. clementae, is recognized on two islands (Santa Catalina and Santa Rosa) with the mainland southern California subspecies, P. m. megalonyx, occurring on a third island (Santa Cruz). By using both traditional and contemporary genetic methods, I demonstrate the utility of next-generation sequencing for studies of Channel Islands species. With single-nucleotide polymorphism (SNP) data from across the genome, I show that the current taxonomy for Spotted Towhees on the islands based on morphology does not accurately represent the evolutionary history of this group. I found that each extant island population comprises a unique evolutionarily distinct unit worthy of taxonomic classification. Furthermore, Spotted Towhee populations show genetic division between the islands and the mainland and between all island populations. I examined phenotypic divergence of Spotted Towhees in both morphology and coloration to assess the features that have historically been used to define the island endemic subspecies. The most prominent patterns found were that Spotted Towhees on the Channel Islands have longer bills and lighter plumage, features that have been linked to adaptation to island environments. I compared phenotypic divergence to genetic divergence to examine the potential effects of selection versus drift on the divergent features. I find evidence of selection for longer bills on the islands, wider bills for females on Santa Catalina, shorter wings for males on Santa Cruz, and lighter/grayer plumage on the northern islands. This phenotypic data provides support for endemic forms of Spotted Towhees on the Channel Islands with adaptive differences from the mainland and, in combination with the genetic evidence, supports each extant island population comprising a unique subspecies endemic to only that island.
