CHLOROPLAST DNA INTRASPECIFIC PHYLOGEOGRAPHY OF PLANTS FROM THE PACIFIC-NORTHWEST OF NORTH-AMERICA

Molecular studies of plants from the Pacific Northwest of North Americ a suggest a recurrent pattern of genetic differentiation and geographi c structuring. In each of five angiosperms and one fern species repres enting diverse life histories, cpDNA data indicate two clades of popul ations that are geographically structured. A northern group comprises populations from Alaska to central or southern Oregon, whereas populat ions from central Oregon southward to northern California form a south ern group. In several of these species, a few populations having south ern genotypes may have survived in glacial refugia further north in th e Olympic Peninsula, Queen Charlotte Islands, and Prince of Wales Isla nd. Allozyme data reveal a similar pattern of differentiation in sever al other plants from the Pacific Northwest. North-south partitioning o f genotypes has also been reported for several animal species from thi s region. On a broader geographic scale, north-south partitioning of g enotypes has also been observed in other plants from western North Ame rica having a variety of geographic distributions. Some species also d isplay a reduction of genetic variability in the northern portion of t heir range compared to the south. The data suggest strongly that past glaciation profoundly influenced the genetic architecture of the flora and fauna of the Pacific Northwest. Two alternative hypotheses are ad vanced to explain the geographic structuring of genotypes. First, past glaciation may have created discontinuities in the geographic distrib utions of plant species, with populations surviving in several well-is olated northern and southern refugia. Following glaciation, migration of genetically differentiated, once-isolated populations resulted in t he formation of a continuous geographic distribution with a major gene tic discontinuity. Alternatively, plants survived and subsequently mig rated northward from a southern refugium, and a genotype became fixed in one or a few populations at the leading edge of recolonization. Sub sequent long-distance dispersal from this leading edge resulted in a r elatively uniform northern genotype that differs from the southern gen otype(s). Whatever the underlying mechanism, Pleistocence glaciation m ay have molded the intraspecific genetic architecture of both plants a nd animals from the Pacific Northwest in a geographically similar mann er. Future studies should seek to obtain a comprehensive phylogeograph y for regions that includes a diversity of both plants and animals.