The Hawaiian Islands have been recognized as an ideal place to study evolutionary processes due to their remote location, multitude of ecological niches and diverse biota. As the oldest and largest radiation in the Hawaiian Islands the Hawaiian Drosophilidae have been the focus of decades of evolutionary research and subsequently the basis for understanding how much of the diversity within these islands and other island systems have been generated. This dissertation revolves around the diversification of a large clade of Hawaiian Drosophila, and examines the molecular evolution of this group at several different temporal scales. The antopocerus, modified tarsus, ciliated tarsus (AMC) clade is a group of 90 described Drosophila species that utilize decaying leafs as a host substrate and are characterized by a set of diagnostic secondary sexual characters: modifications in either antennal or tarsal morphologies. This research uses both phylogenetic and population genetic methods to study how this clade has evolved at increasingly finer evolutionary scales, from lineage to population level.
The first chapter resolves the relationships within the AMC lineage, delimiting species group and subgroup relationships for the first time. This work complements recent phylogenetic studies focused on other lineages of Hawaiian Drosophila, most notably of the picture wing clade. This dissertation presents the most comprehensively sampled data set for the AMC in terms of both species and phylogenetic characters. The AMC clade is strongly supported as monophyletic and relationships among of the five major lineages in this radiation are proposed. Molecular dating analyses indicate a rapid radiation occurred about four million years ago, giving rise to all the extent lineages of AMC species. Interestingly, the AMC does not strictly follow the progression rule common to many other Hawaiian taxa. By comparing this lineage to other Hawaiian Drosophila lineages it is hypothesized that an initial burst of speciation occurred following colonization of the main Hawaiian Islands ands its magnitude was correlated with sexual selection within each group.
Focusing more specifically on the spoon tarsus subgroup within the AMC, I redescribe this subgroup in chapter two. The species boundaries in this group are discussed in light of diagnostic secondary sexual characters of males. Drosophila septuosa Hardy is regarded as a junior synonym of Drosophila percnosoma Hardy. A new species, Drosophila kikalaeleele, is described. Drosophila fastigata Hardy, a species endemic to O`ahu, is added to the species subgroup, bringing the total number of known species to 12. An updated key to species is provided to the spoon tarsus subgroup. I employ several methods to delimit relationships at the species/population interface to get a more refined view of the evolution of this subgroup in chapter three. Eight of the twelve species in this subgroup are found only on the Island of Hawaii, suggesting that they have diverged within the past 500,000 years. This rapid diversification has made determining the relationships within this group difficult. We find widespread agreement between phylogenetic estimates derived from different methods. Notably, our analyses suggest that the spoon tarsus subgroup, as currently defined, is not monophyletic.
The last chapter in this dissertation explores how biogeography influences microevolution within the spoon tarsus species, Drosophila waddingtoni, to understand the forces that drive macroevolution within the AMC clade. Previous chapters show that D. waddingtoni originated on the youngest island, Hawaii, and subsequently colonized the older islands of the Maui Nui complex, the opposite direction predicted by the progression rule. The recent origin of Hawaii suggests that this species will provide unique insight into the dynamics of recent island colonization events. Three nuclear and three mitochondrial genes are used to study gene flow and divergence following founder events. Biogeographic patterns, coupled with divergence time estimates suggest that the back colonization to Maui Nui occurred within the past 250,000 years and has since not led to significant population differentiation. These data also demonstrate that while migration between islands is possible and present in this species, it is not frequent enough to homogenize populations. Biogeography is identified as being a major driver in the diversification of this species.
The AMC clade is a useful tool in understanding diversification in the Hawaiian Islands and rapid radiations in general. By examining this lineage from several different evolutionary timescales this dissertation research elucidates the processes involved in generating and maintaining diversity at all taxonomic levels. This research is also useful in extrapolating how evolution drives diversity in other Hawaiian Drosophilidae clades and other native Hawaiian lineages as well.