Hybrid zone movement in a pair of songbirds offers windows into speciation and extinction processes.

摘要

Hybrid zones, areas of range overlap and interbreeding between species, have been widely studied for the insights they offer into evolutionary processes. Yet, one of the most interesting features of hybrid zones remains poorly understood: their ability to move over time and space. This stems largely from the difficulty of observing hybrid zone movement, which is expected to be ephemeral, eventually halting at areas of low population density or culminating in the extinction of one of the species. Due to the paucity of empirical examples and the challenge of identifying past movement of zones that are no longer moving or are moving too slowly to be measured, the evolutionary impacts of hybrid zone movement remaining largely unexplored.;This dissertation presents novel methods for detecting current and historical hybrid zone movement, and explores the unique insights moving zones offer into speciation and extinction processes. In particular, I have focused on a pair of songbirds, hermit warbler (Dendroica occidentalis) and Townsend's warbler (D. townsendi), that hybridize where their ranges overlap in Washington and Oregon. Previous research has suggested that the more aggressive Townsend's has been pushing its hybrid zone with hermit warbler south along the Pacific coast for thousands of years, replacing hermits across up to 2,000 km of their historical range. I have used mitochondrial and nuclear genetic markers to look for evidence of this historical hybrid zone movement, and also to understand the genetic basis of the traits conferring Townsend's competitive advantage. In addition, I have conducted morphological analyses of their current hybrid zones to determine whether this movement continues today. This research has yielded several novel results and insights: that mitochondrial genetic variation of species replaced by hybrid zone movement may remain intact in inter-specific colonizing populations as signatures of historical zone movement, that nuclear genetic wakes may also be generated and can be used to understand the genetic basis of the adaptive traits driving zone movement, and that ongoing hybrid zone movement may be detected in brief windows of time and provides a rare opportunity to study competitive replacement between species.