One of the important questions for ecologists interested in species interactions is whether the interactions that are observed today represent long-term associations. This question is particularly pertinent to mutualistic interactions, where all involved species benefit from the interaction because it clarifies the extent to which long-term co-evolution would be possible. Here, we perform an analysis of the distribution of evolutionary lineages of the epiphytic lace lichen, Ramalina menziesii, and we then compare the patterns found in the lichen with those in one of its host trees, California valley oak (Quercus lobata). The distribution of Ramalina menziesii and Quercus lobata overlaps in the eastern part of the Coastal Ranges and both species occur more widely where the other species is not found. We sampled R. menziesii at 50 sites, and Q. lobata at 40 sites. At 25 of these sites, both the lichen and the oak were collected. For R. menziesii, we used nuclear sequences from three low-copy genes in betatubulin, elongation factor 1-alpha, and glyceraldehyde 3-phosphate dehydrogenase (gapC), and a fourth unidentified locus. For Q. lobata, we used microsatellite genetic markers from 7 nuclear and 6 chloroplast loci. Our preliminary analysis indicates that both species have a high genetic diversity, with a few geographical regions that seem to be centers of biodiversity. In the talk, we will present the statistical evidence to test whether the colonizing histories of these two species show similar patterns or whether the evidence suggests that the current mutualistic interaction is recent in origin.