The evolution of insect biodiversity.

Insects comprise over half of all described animal species. Together with the Protura (coneheads), Collembola (springtails) and Diplura (two-pronged bristletails), insects form the Hexapoda, a terrestrial arthropod lineage characterised by possessing six legs. Exponential growth of genome-scale data for the hexapods has substantially altered our understanding of the origin and evolution of insect biodiversity. Phylogenomics has provided a new framework for reconstructing insect evolutionary history, resolving their position among the arthropods and some long-standing internal controversies such as the placement of the termites, twisted-winged insects, lice and fleas. However, despite the greatly increased size of phylogenomic datasets, contentious relationships among key insect clades remain unresolved. Further advances in insect phylogeny cannot rely on increased depth and breadth of genome and taxon sequencing. Improved modelling of the substitution process is fundamental to countering tree-reconstruction artefacts, while gene content, modelling of duplications and deletions, and comparative morphology all provide complementary lines of evidence to test hypotheses emerging from the analysis of sequence data. Finally, the integration of molecular and morphological data is key to the incorporation of fossil species within insect phylogeny. The emerging integrated framework of insect evolution will help explain the origins of insect megadiversity in terms of the evolution of their body plan, species diversity and ecology. Future studies of insect phylogeny should build upon an experimental, hypothesis-driven approach where the robustness of hypotheses generated is tested against increasingly realistic evolutionary models as well as complementary sources of phylogenetic evidence.