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Eusociality in Isoptera (termites) converges along many lines with colony organization and highly social behavior in the phylogenetically distinct insect order Hymenoptera (ants, bees, wasps). Unlike the haplodiploid Hymenoptera, however, both sexes of Isoptera are diploid. Termite families thus lack asymmetric degrees of genetic relatedness generated by meiosis and fertilization, so explanations for eusocial evolution based on such asymmetries are not applicable to Isoptera. The evolution of eusociality in termites likely occurred in small families in which most helpers retained developmental flexibility and reproductive options. A suite of ecological and life-history traits of termites and their ancestors may have predisposed them toward eusocial evolution. These characteristics include familial associations in cloistered, food-rich habitats; slow development; overlap of generations; monogamy; iteroparity; high-risk dispersal for individuals; opportunities for nest inheritance by offspring remaining in their natal nest; and advantages of group defense. Such life-history components are particularly persuasive as fostering social evolution because many are present in a broad group of eusocial taxa, including Hymenoptera, beetles, aphids, thrips, naked mole rats, and shrimp. The evolution of eusociality in Isoptera likely evolved in response to a variety of contributing elements and the selective pressures that they generated.

We present new hypotheses and report experimental evidence for powerful selective forces impelling the evolution of both eusociality and the soldier caste in termites. Termite ancestors likely had a nesting and developmental life history similar to that of the living family Termopsidae, in which foraging does not occur outside the host wood, and nonsoldier helpers retain lifelong options for differentiation into reproductives. A local neighborhood of families that live exclusively within a limited resource results in interactions between conspecific colonies, high mortality of founding reproductives, and opportunities for accelerated inheritance of the nest and population by offspring that differentiate into nondispersing neotenic reproductives. In addition, fertile reproductive soldiers, a type of neotenic previously considered rare and docile, frequently develop in this intraspecific competitive context. They can be highly aggressive in subsequent interactions, supporting the hypothesis that intercolonial battles influenced the evolution of modern sterile termite soldier weaponry and behaviors.

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