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ment (Wilson, 1971). Although there has been much theoretical research on the evolutionary forces that may select for eusociality (Strassmann and Queller, 2007; Nowak et al., 2010), less is known about the actual molecular mechanisms involved in transitions from solitary to social living and in the maintenance and elaboration of eusociality in insects (C. R. Smith et al., 2008).

The social insects provide a powerful comparative framework for investigating mechanisms involved in eusocial evolution. Eusociality has arisen independently at least 12 times in the insects (Cameron and Mardulyn, 2001; Brady et al., 2006; Hines et al., 2007; Cardinal et al., 2010), and eusocial insects have all converged on the following three characteristics: reproductive division of labor, cooperative brood care, and overlapping generations (Michener, 1974). Additionally, despite sharing this core set of traits, there are many differences among eusocial lifestyles, which may be related to ecological, phylogenetic, or other factors specific to particular eusocial lineages (Wilson, 1971). By comparing across social insect lineages, it is possible to both search for common mechanisms of eusocial evolution and explore how eusociality evolves under different conditions.

Analysis of adaptive evolution at the molecular level can yield great insights into the mechanisms underlying the evolution of complex phenotypes, such as eusociality. Genomic sequence provides a molecular record of how natural selection has shaped an organism’s evolutionary history (Clark, 2006). Several methods have been developed for comparing genes and genomes to identify molecular signatures of adaptation. These methods were largely developed during the pregenomic era (Li, 1997) but gain enormous power when large genomic datasets are available, particularly for sets of closely related and phenotypically variable species (Clark et al., 2003; Drosophila 12 Genomes Consortium, 2007). For example, comparisons of primate genomes have identified adaptive genetic changes involved in the evolution of brain size in humans (Pollard et al., 2006), and comparisons of drosophilid genomes have shed light on the ecological pressures that shaped speciation in this group (Clark et al., 2003).

Here, we review some of the first contributions of molecular evolutionary research to our understanding of eusocial evolution in insects. This research has focused on the most well-studied social insects, which include several eusocial lineages within the order Hymenoptera, the ants, bees, and wasps, and the one eusocial lineage in the order Blattodea, the termites (Fig. 8.1). Some studies have performed targeted molecular evolutionary analyses of candidate genes that have been particularly valuable in species for which large amounts of genomic sequence are not yet available. Others have focused on comparative analyses of

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