Evolutionary history and gene flow of an endemic island oak: Quercus pacifica

PREMISE OF THE STUDY: Understanding historical patterns of colonization and subsequent gene flow clarifies the evolutionary origins and history of endemic island species. METHODS: Here we use DNA microsatellite markers to characterize the genetic structure of the island endemic species Quercus pacifica K. Nixon & C.H. Mull., found on three of the California Channel Islands, and to examine its relationship to two mainland oaks, Q. berberidifolia and Q. dumosa. KEY RESULTS: We found that Q. pacifica is a genetically cohesive and differentiated evolutionary lineage, diverging from mainland scrub oaks in the Pleistocene with little subsequent introgression. Genetic differentiation of Q. pacifica among islands is small but significant. Both recent and historical gene flow were surprisingly high considering the disjunct distribution of Q. pacifica on islands separated by as much as 125 km of open ocean. Gene flow estimates were highest between the two northern islands and from the northern islands to Santa Catalina. While there is no evidence of recent bottlenecks, historical bottlenecks are indicated on each of the islands. CONCLUSIONS: The genetic cohesiveness of the Q. pacifica species suggests allopatric speciation on the islands with subsequent gene flow that has maintained genetic continuity over great distances.