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 numerous instances of contact between the two taxa. In some areas of contact 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 these markers to examine patterns of gene exchange in an area of contact in the San Mateo Mountains of New Mexico. The markers are highly coincident 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 appears to be limited.