Evolution on islands: Dispersal, speciation, and hybridization in Scaevola (Goodeniaceae) in the Hawaiian Archipelago

Examining the patterns of dispersal of plants to islands and their subsequent speciation helps to understand the diversity of island life. Scaevola (Goodeniaceae), which has dispersed throughout the Pacific Basin, radiating on many high island groups, provides an ideal system for studying these patterns. Data from the Internal Transcribed Spacer (ITS) shows that Scaevola (excluding Scaevola collaris and including Diaspasis filifolia) represents a monophyletic genus. The genus is Australian in origin with at least six separate dispersal events from Australia. Four of these dispersals resulted in single extra-Australian species. The remaining two were larger radiations that were each related to one of the widely dispersed strand species, S. taccada and S. plumieri. Remarkably, three of the six dispersals established species on the remote Hawaiian Archipelago. One of these dispersal events to the Hawaiian Islands, which resulted in eight endemic species, was examined further.;Unfortunately, the phylogenetic history of closely related plant species has been difficult to infer because of a lack of well-characterized genes that show sufficient sequence variation. Large nuclear intron regions were therefore sought out, and primers were designed for introns in Nitrate Reductase (NIA) and LEAFY (LFY). These regions were compared to Glyceraldehyde 3-Phosphate Dehydrogenase (G3PDH) and ITS, to produce gene genealogies for the seven extant Hawaiian endemic diploid species. Multiple accessions from these four regions were sequenced from each of these Scaevola species. The sequences from all four loci were congruent across five of the seven species (excluding S. kilaueae and S. procera). When all four regions were combined the relationships among these five species were fully resolved even though they were only partially resolved by the individual gene regions. The alleles from S. procera and S. kilaueae were incongruent among loci. It was hypothesized that S. procera is the result of an ancestral homoploid hybridization event between S. gaudichaudii and S. mollis and that S. kilaueae may be the result of hybridization between S. coriacea and S. chamissoniana on the island of Hawaii. Additionally, a hybrid zone between S. gaudichaudiana and S. mollis appears to be more recent and may be compounded by the introduction of a generalist bee, Apis mellifera.