Subfertility and the evolution of eusociality by kin selection

Abstract

The role of subfertility in the evolution of eusociality in Hymenoptera was examined using deterministic computer simulations of two models of facultative altruism, in which only subfertile females become altruists. The altruism allele was dominant. If altruism is not facultative, then altruism is selected against under the conditions modelled unless altruists have better brood-rearing ability than non-altruists. In Model 1, subfertile females give up all their offspring, and raise siblings. This allows selection for altruism even when altruists are less efficient workers than non-altruists, provided that the ratio of the number of siblings they raise to the number of offspring raised by subfertile non-altruists, exceeds a threshold value ( K min. K min > 1, and increases as the frequency of subfertile females increases. In Model 2, altruists raise as many siblings as they can, without giving up any offspring. This behaviour is always selected because there is no cost. Little variability in fertility is required, and workers do not have to be highly related to the individuals they raise, which indicates that this model may have relevance to the initial evolution of sociality. The results suggest that eusociality evolved via fixation of facultative alleles, followed by selection on queens to exploit this by raising more subfertile daughters in their early broods (parental manipulation). Presentday caste determination mechanisms reflect these hypothesized early stages. However, facultative altruism can have no role in the evolution of sociality if there is insufficient variability in fertility, or if ability, not fertility, limits brood size in subfertile females.