SOCIAL EVOLUTION - HAS NATURE EVER REWOUND THE TAPE

Social insects such as ants, bees, wasps and termites exhibit extreme forms of altruism where some individuals remain sterile and assist oth er individuals in reproduction. Hamilton's inclusive fitness theory pr ovides a powerful framework for investigating the evolution of such al truism. Using the paper wasp Ropalidia marginata, we have quantified a nd delineated the role of ecological, physiological, genetic and demog raphic factors in social evolution. An interesting feature of the mode ls we have developed is their symmetry so that either altruism or self ishness can evolve, depending on the numerical values of various param eters. This suggests that selfish/solitary behaviour must occasionally re-emerge even from the eusocial state, It is useful to contemplate e xpected intermediate states during such potential reversals. We can pe rhaps envisage three successive steps in such a hypothetical process: i) workers revolt against the hegemony of the queen and challenge her status as the sole reproductive, ii) workers stop producing queens and one or more of them function as egg layers (functional queen/s) capab le of producing both haploid as well as diploid offspring and iii) soc ial evolution reverses completely so that a eusocial species becomes s olitary, at least facultatively. It appears that the third step, namel y transition from eusociality to the solitary state, is rare and has b een restricted to transitions from the primitively eusocial state only . The absence of transitions from the highly eusocial state to the sol itary state may be attributed to a number of 'preventing mechanisms' s uch as (a) queen control of workers (b) loss of spermathecae and abili ty to mate (c) morphological specialization (d) caste polyethism and ( e) homeostasis, which must each make the transition difficult and, tak en together, perhaps very difficult. However, the discovery of a trans ition from the highly eusocial to the solitary state can hardly he rul ed out, given that little or no effort has gone into its detection. In this paper I discuss social evolution and its possible reversal and c ite potential examples of stages in the transition from the social to the solitary.