Graduation date: 2008
A fossil-calibrated phylogenetic framework based on exemplars from each of the four taxonomic sections within Pinus was created using multiple nuclear and chloroplast loci. Calibration at the well-defined subgeneric split within Pinus with either fossil leaves and cones (ca. 45 million years ago) or fossil wood (ca. 85 million years ago) provides a reasonable starting point. Despite a wide difference in rates between loci, this work infers a moderate tempo of mutation rate in Pinus, and concludes that the within-locus rate variation and the leaves vs. wood question are both minor factors in comparison to the enormous effects of incorrect fossil/node association.
Next, I used full taxonomic sampling with multiple individuals per species and two nuclear loci to create independent phylogenies for 18 species in subsection Ponderosae (Section Trifoliae, subgenus Pinus). I estimate that some species have enormous effective population sizes (> 105 individuals). This factor and an inferred origin 15 million years ago contribute to the lack of species-level coalescence and the incongruence between Ponderosae gene trees. Pinus jeffreyi is allied with Sabinianae, as proposed from terpene biochemistry. I observed one instance where reticulate evolution is likely. Pinus coulteri accessions resolve with Sabinianae at one locus, but in a monophyletic clade sister to other Ponderosae at the other locus, suggesting that this species originated as a diploid hybrid. Similar incongruence between loci for accessions of the P. ponderosa/P. washoensis complex could also be the result of introgression.
Using this phylogenetic framework, I focused on three sympatric Ponderosae (P. washoensis, P. ponderosa, and P. jeffreyi) to evaluate the genetic distinctiveness and specific status of P. washoensis. Population-level sampling with faster-evolving nuclear microsatellite loci reveals a clear divergence between P. jeffreyi and the P. ponderosa/P. washoensis complex. Using allele frequencies, I observed a weak cluster of traditional P. washoensis with high-elevation putative P. washoensis populations from Oregon, but other methods show no differentiation between P. ponderosa and P. washoensis. Nuclear admixture and chloroplast haplotype analysis suggest a low level of introgression between P. jeffreyi and P. ponderosa/P. washoensis, providing an important source of migrants into the genetic milieu of these species.