Social insects and social amoebae. - Centre for Ecological Sciences

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224 Raghavendra Gadagkar and J T BonnerIAs far as primitively eusocia] species are concerned, the role of parentalmanipulation or subfertility has not been adequately tested. Instead, it has beencommon to assume that caste is determined entirely in the adult stage (e.g., Brian1980; Roseler et al 1980; Wheeler 1986; Queller and Strassmann ]989; Reeve1991). There is however evidence for pre-imaginal caste bias in R. marginata. Onlyabout 50% of freshly eclosing female wasps are capable of initiating nests andlaying eggs even when they are isolated from conspecifics. Even among the egglayers there is considerable variation in the time required to initiate egg laying(Gadagkar et al 1988, 1990, 1991a). Both of these forms of pre-imaginal castebias namely, differentiation into egg layers and non egg layers and into early andlate reproducers is mediated through larval nutrition. Relative]y better nourishedlarvae develop into egg layers and early reproducers while relatively poorlynourished larvae develop into non egg layers or late reproducers (Gadagkar et al199]b).2.3 The role of mutualismCautioning against an excessive concentration on the role of genetic relatedness indriving socia] evolution, Lin and Michener (1972) drew attention to a large numberof socia] insect species where sterility is absent or incomplete and suggested thepossibility that individuals in such groups may be selected to come together formutual benefit. This theory of mutualism has often been dismissed as incapable,by definition, of explaining the evolution of a sterile worker caste because theterm mutualism suggests that both or all participants benefit. In contrast the sterileworker caste and the fertile queen are not usually thought of as ben~fiting equallyfrom the associations (e.g., Crozier 1977; Ito 1989). However this argument deservesa second examination. Consider a situation where two individuals nesting togethercan rear more brood than the sum of their individual capabilities and further thatthe roles of fertile queen and sterile worker among these two individuals is decidedby the toss of a coin, so to speak. In such a situation an average individual thatnests in the group may obtain more fitness than a solitary individual. If we replacethe concept of alleles programming individuals into workers with alleles programmingindividuals to take the risk of being part of the group, then it is possible to showthat under certain ecological conditions the 'gamblers' will be fitter than therisk-averse solitary individuals (Gadagkar 1991a). The losers in the 'gamble' willleave behind no offspring and we will see them as sterile workers. Nevertheless,it is a form of mutualism that has given rise to this situation. One advantage ofsuch a model is that it requires no assumption of increased genetic relatedness orparental manipulation. The fact that foundress associations in primitively eusocialspecies may consist of distant relatives and that workers can become queens beforethey die suggests that the possibility of direct reproduction at some future pointof time may itself drive the evolution of group living.An important assumption of this so called 'gambling hypothesis' is that the percapita productivity in the group mode is higher (or more reliable; see Wenzel andPickering 1990) than in the solitary mode. In R. marginata a solitary foundressproduces on the average no more than one or two offspring (M Shakarad and RGadagkar, in preparation) whereas a queen of a multi female colony produces onthe average, 76 offspring (Gadagkar 1990d).
Social insects and ,social amoebae 2252.4 A possible route to eusocialityAssuming that existing species of increasing social complexity correspond tosequential stages in social evolution, the evidence reviewed above suggests a possibleroute to eusociality. First, it seems reasonable to assume that the initial incentivefor group living comes from mutualism and the benefits of "gambling". This maybe called The Gambling stage (figure 2). This stage requires no pre-adaptation forintra-group manipulation or for recognition of genetic relatedness, although groupliving among kin (who might come together merely by virtue of being neighbours)will evolve more easily (West-Eberhard 1978; Schwarz 1988). The only pre-requisitefo,r the evolution of incipient societies by mutualism is a sufficiently complexbehavioural repertoire to permit the necessary interactions; solitary wasps seem topossess an appropriately diverse array of behaviours (Tinbergen 1932, 1935; Tinbergenand Kruyt 1938; Brockmann and Dawkins 1979; Brockmann et al 1979). It is easyto imagine that many presocial wasps such as those listed in West-Eberhard (1978)and described by West-Eberhard (1987) and Wcislo et al (1988) are either alreadyat this stage or are forerunners of this stage.THE ROUTETO EUSOCIALITYI THE GAMBLING STAGE II THE MANIPUlATION STAGE III THE RECOGNITION' STAGEGroup nesting is Accidental variations Genetic asymmetriesfor mutual benefit. In fertilities are created byEither all individuals exploited to usurp haplodiploidy arelay eggs or the egg egg-laying exploited by showinglayer is decided by opportunities by ? preferential altruismchance. An average some individuals or .toward close relatives.individual in the... some individuals are... Sufficient inclusivegroup does better manipulated into fitness is thusthan solitary one. subfertile and gained to compensate~g.. ~y Qresocial subordinate roles. for reduced~ ~ Qrlmitlvely ~g.. §.Q!!:!! Qrimltively opportunities foreu~ocial ~R! !!!.Q ~ ~R! ~. reproduction.~ ~ ~g.. ~y ~?Demographic factorsFigure 2. The route to eusociality, a hypothesis concerning the evolution of the highlyeusocial state from the solitary state through the Gambling, manipulation, and recognitionstages. The examples given for each stage are tentative as our knowledge of the causesand consequences of group living in most social insect groups is still sketchy. The arrowbelow indicates the increasing importance of demographic factors through the course ofthe evolution of eusociality (modified from Gadagkar 199Oc).
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