PHYLOGEOGRAPHY OF THE CALIFORNIA THRASHER (TOXOSTOMA REDIVIVUM) BASED ON NESTED-CLADE ANALYSIS OF MITOCHONDRIAL-DNA VARIATION

Abstract

Distribution of genealogical lineages within a species is likely the result of a complicated series of ecological and historical events. Nested-clade analysis is specifi cally designed as an objective phylogeographic approach for inferring evolutionary processes on a spatial and temporal scale for small subclades within a larger set of intraspecifi c relationships. Here, we use nested-clade analysis as well as other phylogeographic methods to investigate the evolutionary history of California Thrasher (Toxostoma redivivum) populations. Inferences resulting from nested clade analysis suggest a history that includes past fragmentation, range expansion, and isolation-by-distance. Along with root information, those inferences enable the construction of a biogeographic scenario for this species involving general southern ancestry, an early north–south division, northward range expansion, and a southward back-expansion into an already populated southern region. Isolation-by-distance is also identifi ed, particularly in southern California, indicating that gene fl ow between localities does occur but is restricted. Many conclusions drawn from this study are concordant with geologic data as well as phy- logeographic scenarios drawn for other codistributed California taxa.