Geographic and genetic diversification of Lipochaeta section Aphanopappus (Asteraceae) in the hawaiian islands
Jorgensen, Stacy Marie
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The Hawaiian Islands are well known for the spectacular examples of adaptive radiation among the native flora and fauna. The conveyor-belt like movement of the Pacific tectonic plate has created a series of chronologically arranged islands, and it is the constant creation of new, isolated habitat due to active volcanism that is thought to promote speciation in the islands. In this study, I examine the diversification of the native Hawaiian plant taxa Lipochaeta section Aphanopappus (Asteraceae: Heliantheae), a small group of mostly single-island endemics, many known from a single locality; one species, L. integrifolia, is known from coastal habitats on all eight of the main islands. To examine patterns of speciation in the group, I used amplified fragment length polymorphism (AFLP) markers, which are ideal for analyzing a large number of loci sampled from throughout the genome. Ninety-five percent of the AFLP loci identified exhibited variation within or among species; surprisingly, few fixed differences among species were identified. Instead, diversification within Lipochaeta was largely attributed to variation in the frequency of fragments among species. Genetic variability, measured by the percentage of polymorphic loci and heterozygosity, varied little among species. Notable exceptions were the O‘ahu endemics L. remyi and L. tenuis, both of which had substantially less variability than other species; unlike many species in the group, however, neither are protected with threatened or endangered status. While the relationships among species occurring on the oldest islands were obscured due to contemporary hybridization, a close relationship was found among L. integrifolia, L. remyi, and L. venosa of Hawai‘i, with the latter two species most closely related. In fact, L. remyi appears to have been derived from L. venosa, opposite of the general trend of species on older islands being parental to species on younger islands. Diversification among populations of L. integrifolia was qualitatively similar to patterns of diversification among the species of L. integrifolia, L. remyi, and L. venosa, although quantitative differences were seen in species vs. population differentiation. These data support gradual models of speciation and are generally inconsistent with the predications of founder-flush models of speciation.