Ecology of Social Evolution

pp 37-56

The Ecology of Altruism in a Clonal Insect

  • Nathan PikeAffiliated withDepartment of Zoology, Oxford University
  • , William A. FosterAffiliated withDepartment of Zoology, University of Cambridge

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Social aphids are an ideal animal group in which to demonstrate the relative importance of ecological versus genetic factors in the evolution of sociality for several reasons: (1) Since aphids are clonal, the genetics of a colony is starkly simple (the aphids are either from the same clone or they are not) and, in principle, easy to measure; (2) Because good phylogeni es are available for several aphid taxa and there is clear evidence that there have been more independent origins of sociality in aphids than in any other social clade, it is possible to test for associations between ecological factors and the evolution of sociality; (3) Recent developments, in the understanding of the mechanism of the proximate control of soldier development, in the genetic basis of sociality, and in models of social evolution, make the social aphids an ideal group for experimental work on the evolution of social behavior; (4) The social aphids are of special ecological interest because they include the only organisms that have evolved sterile castes in societies that do not occupy some kind of nest (the secondary-host generations of the Hormaphidinae).

The ecological context of altruism in social aphids has been shown to be quite intricate since it is now clear that colony defense is not the only costly behavior that they perform: they also have vital roles in keeping the colony clean, migrating to new colonies, and repairing their nest. Numerous ecological factors are highly pertinent in aphid social evolution including (1) the fact that all social aphids have at some stage in their life cycle a valuable and defensive fortress in the form of a plant gall, (2) population size and density, (3) birth rate, (4) the level of exposure to specialized predation, and (5) variation in the level of tending provided by ants. Kin selection in social aphids has given rise not only to a range of elaborate adaptations in behavior and morphology but also to impressive short-term flexibility in social investment. For example, in species that have specialized defenders that can mature to make a direct contribution to their colony’s fitness, defense investment can be increased both through heightened production of defenders at birth and prolongation of the defender stage.

We demonstrate that ecological factors are essential in any attempt to understand the role of kin selection in the evolution of social behavior in a group of organisms: ecology determines the extent to which groups consist of related individuals and the context in which these individuals can give and receive help.