HOW DISCRETE ARE OAK SPECIES - INSIGHTS FROM A HYBRID ZONE BETWEEN QUERCUS-GRISEA AND QUERCUS-GAMBELII

The white oaks Quercus gambelii and Q. grisea overlap in distribution in New Mexico and Arizona. Within the region of overlap, there are num erous instances of contact between the two taxa. in some areas of cont act morphologically, intermediate trees are common, whereas in others, morphologically intermediate trees are rare or absent. We describe a set of RAPD markers that distinguish between the two species and use t hese markers to examine patterns of gene exchange in an area of contac t in the San Mateo Mountains of New Mexico. The markers are highly coi ncident with morphology and confirm that hybridization between the two species takes place. Despite the occurrence of hybrids, both species remain distinct, even in areas of sympatry, and marker exchange appear s to be limited.