Macro and micro evolutionary determinants of diversity in cecidomyiid flies

Understanding how diversification proceeds during adaptive radiation requires studies of diversity at multiple levels (within species, between species, and above the species level). Adaptive radiation involves both the radiation of species from a common ancestor and partitioning of environments by those species through ecological divergence. Phytophagous insects comprise the bulk of the world’s biological diversity, and understanding the evolutionary processes that drive their diversification is a central theme in evolutionary biology. The ecologically specialized relationship between gall-inducing phytophagous insects and their host plants makes them ideal systems for examining causal mechanisms of evolutionary diversification. Gall midges (Diptera: Cecidomyiidae) are especially useful among gall-inducing insects because they are diverse, disperse over large distances, and have radiated among a variety of host-plant species; taxonomic data also show that many genera exhibit large groups of closely-related species on single host-plant species. This thesis examines the macro- and micro-evolutionary diversification of host-associated species of gall-inducing midges. Increases in cecidomyiid diversity between host-plant taxa were associated with increases in ecological opportunity, plant lineage age, and plant architectural complexity and decreases in plant insularity. Diversification of Rhopalomyia gall midges within plant family (Asteraceae) results from a combination of host-plant shifts and within host-plant speciation. Diversification of Asphondylia gall midges within a single host-plant species results from within host-plant speciation. Speciation without a host shift in both Asphondylia and Rhopalomyia is associated with shifts among plant parts and shifts among time periods indicating that such shifts may be general mechanisms facilitating divergence within a single host-plant species. Divergence population genetics supports inferences of large ancestral population size and gene flow during divergence between a species pair shifted in life-history timing; and small ancestral population size and no gene flow during divergence in a species pair displaying divergence in plant-part use. Comparative analyses of Asphondylia wing length and ovipositor length suggest strong divergent selection on ovipositor length accompanies evolutionary shifts between host-plant parts. Studies of other radiations of cecidomyiids combined with analyses of genes putatively involved in the evolution of reproductive isolation will provide a more complete understanding of the evolutionary processes involved in cecidomyiid diversification.