The molecular systematics and biogeography of the Burseraceae
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This dissertation examines the molecular phylogenetics of the Burseraceae (Frankincense & Myrrh Family), a pantropically-distributed family of trees consisting of 18 genera and ca. 700 species. Current hypotheses that explain the widespread distribution of the family by invoking Gondwanan origin and vicariant range expansion conflict with revised dates of continental rifting, which suggest that division of Gondwana predates the origin of the majority of angiosperm lineages. We tested this hypothesis and the monophyly of the pantropical tribes by analyzing nuclear and chloroplast DNA sequence data from 13 genera and 50 species using parsimony, maximum-likelihood, Bayesian inference, and a likelihood-based method of lineagedating using fossil data. Based on surprising results of the family phylogeny, the nonmonophyly of Bursera was tested further by analyzing additional nuclear and chloroplast data for more species of Bursera (36 spp.) and Commiphora (29 spp.) with and without extended outgroup sampling. We conclude that Bursera is not monophyletic and that previous phylogenetic studies that supported monophyly lacked sufficient outgroups and were affected by long branch attraction in parsimony analysis. Interspecific hybrids and stabilized hybrid taxa in Bursera are often reported but rarely tested using molecular genetic techniques. Genetic reticulation can generate new species and alter existing species’ ability to respond to natural selection. In order to test for genetic reticulation among Bursera species, the maternal and paternal parentage of three putative hybrids of Bursera was tested by comparing nuclear and chloroplast phylogenies. The hybrids were determined to be B. spinescens X B. simaruba, B. brunea X B. simaruba, and B. simaruba X B. spinescens (= B. ovata). Two hybrids appear to be F1 progeny and one appears to be a stabilized or backcross taxon. Instances of allopatric (island) speciation and hybridization between B. graveolens and the Galapagos endemic, B. malacophylla, were tested using amplified fragment polymorphism data. Hybrid origins of morphologically intermediate accessions and population structures of accessions corresponding to geographic distribution or species designation were not supported. Weak structure in distance-based analyses and significant AMOVA results suggests allelic differentiation among morphotypes but do not indicate that B. malacophylla is an independent lineage.