Plant-feeding insects account for about one fourth of macroscopic biodiversity. This study aims to document factors contributing to this diversity by investigating phylogenetic relationships within a large radiation of herbivorous insects, Phytomyza leaf-mining flies (Diptera: Agromyzidae).

After a brief introduction (Chapter 1), a general overview of phylogenetic patterns in phytophagous insects is presented, based on over 200 phytophagous insect phylogenies from the recent literature (Chapter 2).  A few salient results include 1) host use conservatism at the family level predominates, with shifts occurring at about 5% of speciation events; 2) host shifts are a major contributor to speciation, occuring in about half of 145 speciation events tabulated; 3) insect-host associations mostly reflect colonization of already diversified host plant clades; and 4) variation in diversification rates is not yet well-documented for phytophagous insects, except at the broadest scale.

Chapter 3 is a phylogenetic study of the genus Phytomyza sensu lato, using over 3,000 nucleotides of DNA sequence data from three genes.  Results indicate that the genus Chromatomyia, considered by some as synonymous with Phytomyza, is in fact polyphyletic and nested within Phytomyza.  Possible parallelism in a biological trait (internal pupation in leaf tissue) which is one of the defining traits of species in the former Chromatomyia is discussed.  In addition, the internal classification of Phytomyza is assessed and revised insofar as the data permit.

Divergence times for the Agromyzidae, and also for Phytomyza and related genera, were estimated using a molecular phylogeny calibrated by three agromyzid fossils (Chapter 4).  Results suggest that the temperate Phytomyza group of genera originated in the relatively warm Eocene epoch.  Ranunculaceae, a primitive plant family, is inferrred as the ancestral host for a clade including most Phytomyza species, but is probably secondary to feeding on more derived plant families ("asterid clade").  Ten clades were identified for comparison of diversification rates between Ranunculaceae- and asterid-feeding lineages, which showed that asterid-feeding clades exhibit higher rates of diversification.  Phytomyza originated approximately at the early Oligocene global cooling event, but contrary to expectations, diversification significantly slowed during the Oligocene cool period, when suitable habitats for Phytomyza were presumably widespread.