INTRAGROUP CONFLICTS AND THE EVOLUTION OF STERILE CASTES IN TERMITES

A hypothesis is proposed to account for the onset of a helper phenotype in the ancestors of termites, for the regular expression of this phenotype by a fraction of the individuals composing the colony, and for the subsequent development and diversification of sterile castes (soldiers and workers). This hypothesis rests on the gradual enhancement of behavioral and physiological traits preexisting in noneusocial ancestors. Ancestors of termites probably lived in parent-offspring groups, in which parents assumed reproduction while older offspring helped to raise younger siblings. I demonstrate that conflicts (in particular, competition among helpers to reach the alate stage) are expected within such groups. The ancestral character of such conflicts in termites is substantiated by empirical evidence for their existence in all major groups of extant lower termites, in which they can still play an important role in caste determination. I suggest that such conflicts produced individuals with lessened abilities for alate development, which would invest preferentially in siblings rather than in their own regeneration. Selection could then favor any increase in the efficiency of this helper phenotype at carrying out social tasks and a fine-tuning of the switch mechanism between the two phenotypes-the helper and the sexual nymph-as to fit Hamilton's rule as closely as possible. This process could have led to the regular expression of nondispersive phenotypes (i.e., the onset of permanent helper castes), followed by a wide diversification of the helpers, as observed nowadays in the higher termites.