The Evolution of Eusociality

Abstract

Eusociality is characterized by overlapping adult generations, cooperative brood care, and more or less nonreproductive workers or helpers (96, 151). Evolutionists have long recognized that the presence of sterile workers is difficult to explain by traditional individual selection (128). Darwin said of the workers in eusocial insects that they posed special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory . . . [because] being sterile, they cannot propagate their kind (44, p. 236). Darwin proposed that workers evolve through selection at the level of the colony, a mechanism that is probably important in several respects (39, 109). Processes at the colony level, however, do not fully explain why individuals give up reproduction and hence the possibility of passing on their genes via offspring. Williams & Williams (149) and Hamilton (63, 65) showed that genetic related­ ness between the workers and the reproductives they help raise may resolve the apparent paradox. The most extreme cases of eusociality are species with highly specialized, permanently sterile castes of workers and soldiers. Sterile castes have evolved in the Hymenoptera (ants, bees, and wasps), Isoptera (termites), and apparently in one rodent (the naked mole-rat). Eusociality has arisen many times indepen­ dently in Hymenoptera (albeit not always to the point of permanent worker sterility), and most of the discussion of the evolution of eusociality has focused on that order. This review begins with an evaluation of the various hypotheses proposed to explain the evolution of eusociality in Hymenoptera and Isoptera. A compara­ tive analysis of preconditions that are important for the evolution of insect