Natural hybridization has the potential to be a major evolutionary force by producing new genetic combinations that can colonize habitats not available to their parents. Abies is a group of approximately 40 species that are known to hybridize readily and produce seeds with varying degrees of viability. Within the last century, a number of natural hybridization studies of North American Abies have been conducted, but the majority of these were either artificial crossing experiments or morphological studies. We have initiated a molecular genetic analyses of putative hybrid zones involving two western North American species; A. grandis, from mesic forests of the Cascades and northern Rocky Mountains, and A. concolor, from more xeric forests in the Sierras and southern Rockies. Putative hybrids have been documented in a broad zone in the southern Cascades, and in an extensive area of the central Rockies. The latter hybrid zone, as inferred from morphology, is approximately 400km north of the current range of A. concolor. Preliminary analysis of chloroplast DNA revealed that the hybrid zone is polymorphic for both A. grandis and A. concolor haplotypes, confirming the hybrid nature of these populations. Nuclear microsatellites will be employed to quantify the degree of hybridity across the region. The broad hybrid zone represents the southern portion of what is considered the range of A. grandis, and has a different climate than the rest of the species. We hypothesize that ancient hybridization has lead to the introgression of ecological traits that have allowed A. grandis to colonize warmer and drier habitats. We will test this hypothesis using stable isotopes to measure water use efficiency of pure and hybrid individuals of varying genetic composition.

Key words: Abies, hybridization, plastid DNA, western North America