Different Evolutionary Conditions for Worker and Soldier Castes: Genetic Systems Explaining Caste Distribution among Eusocial Insects

Although evolution of sterile castes in social insects has been extensively studied, the differences between worker and soldier castes have not been addressed sufficiently. It is suggested here that the evolution of worker and soldier castes must be considered in terms of different trade-offs, specifically, the different relations between benefits and costs of sterility. An individual should be a sterile worker if the genetic gain of raising its sibling is higher than the genetic cost of losing its own offspring, while an individual should be a sterile soldier if the sacrifice of its own life is genetically compensated for by survival of other family members. Using these different conditions for evolution of sterility, caste distribution among existing eusocial insects can be explained by a difference in genetic systems: Aphids and a parasitic wasp with asexual reproduction have only the soldier caste; wasps, bees, and ants with male-haploidy have mainly the worker caste; and termites with diploidy have dominantly the soldier caste.