Chloroplast DNA intraspecific phylogeography of plants from the Pacific Northwest of North America

Molecular studies of plants from the Pacific Northwest of North America suggest a recurrent pattern of genetic differentiation and geographic structuring. In each of five angiosperms and one fern species representing diverse life histories, cpDNA data indicate two clades of populations that are geographically structured. A northern group comprises populations from Alaska to central or southern Oregon, whereas populations from central Oregon southward to northern California form a southern group. In several of these species, a few populations having southern genotypes may have survived in glacial refugia further north in the Olympic Peninsula, Queen Charlotte Islands, and Prince of Wales Island. Allozyme data reveal a similar pattern of differentiation in several other plants from the Pacific Northwest. North-south partitioning of genotypes has also been reported for several animal species from this region. On a broader geographic scale, north-south partitioning of genotypes has also been observed in other plants from western North America having a variety of geographic distributions. Some species also display a reduction of genetic variability in the northern portion of their 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 advanced to explain the geographic structuring of genotypes. First, past glaciation may have created discontinuities in the geographic distributions of plant species, with populations surviving in several well-isolated northern and southern refugia. Following glaciation, migration of genetically differentiated, once-isolated populations resulted in the formation of a continuous geographic distribution with a major genetic discontinuity. Alternatively, plants survived and subsequently migrated northward from a southern refugium, and a genotype became fixed in one or a few populations at the leading edge of recolonization. Subsequent long-distance dispersal from this leading edge resulted in a relatively uniform northern genotype that differs from the southern genotype(s). Whatever the underlying mechanism, Pleistocence glaciation may have molded the intraspecific genetic architecture of both plants and animals from the Pacific Northwest in a geographically similar manner. Future studies should seek to obtain a comprehensive phylogeography for regions that includes a diversity of both plants and animals.