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.