TIME SINCE CONTACT AND GENE FLOW MAY EXPLAIN VARIATION IN HYBRID FREQUENCIES AMONG THREE DENDROICA TOWNSENDI × D. OCCIDENTALIS (PARULIDAE) HYBRID ZONES

Abstract

Hermit Warblers (Dendroica occidentalis) and Townsend’s Warblers (D. townsendi) meet and hybridize in three replicate zones in Washington and Oregon. Using male specimens, we show that the ratio of hybrid to parental phenotypes varies systematically across these three zones: there were 60.4% hybrids among 96 males from the Olympic zone, 43.3% hybrids among 411 males from the Washington Cascade zone, and 27.8% hybrids among 209 males from the Oregon Cascade zone. We found no evidence of habitat or climatic factors driving these systematic differences in hybrid frequencies, and we argue that they are related to differences among these zones in the immigration of pure parentals and time since contact. Building on earlier hypotheses about massive movement of the coastal zones (Rohwer and Wood 1998), we can infer that the Olympic and Washington Cascade zones are old and of similar age. The Washington Cascade zone, but not the Olympic zone, receives Townsend’s immigrants from the Rocky Mountains through the Okanogan high- lands, as shown by the linkage analyses in Rohwer et al. (2001). This immigration apparently accounts for the lower frequency of hybrids in the Washington Cascades. The Washington Cascade and Oregon Cascade zones are both situated adjacent to forest corridors that connect them to the Rocky Mountains to the east, so both of these zones should receive Townsend’s immigrants from the east (linkage analyses have yet to be done for the Oregon Cascades because the mitochondrial DNA haplo- types for these specimens have not been determined). The Washington Cascade zone, however, is old, whereas the Oregon Cascade zone appears to have been established only recently in response to anthropogenic habitat changes, which accounts for its low frequency of hybrids. We are unaware of any other replicate set of hybrid zones where differences in time since contact and dispersal have been related to variation in hybrid frequencies. El Tiempo Transcurrido desde el Contacto y el Flujo Genético Podrían Explicar la Variación en la Frecuencia de los Híbridos entre Tres Zonas de Hibridación entre Dendroica townsendi y D. occidentalis (Parulidae)