Eusocial Insects as a Model for Understanding Altruism, Cooperation, and Levels of Selection
- LAST MODIFIED: 26 August 2020
- DOI: 10.1093/obo/9780199941728-0127
- LAST MODIFIED: 26 August 2020
- DOI: 10.1093/obo/9780199941728-0127
If the logic of natural selection is applied strictly at the level of individual production of offspring, sterile workers in insect societies are enigmatic. How can natural selection ever produce individuals that refrain from reproduction, and how are traits of such individuals that never produce offspring scrutinized and changed through natural selection? The solution to both questions is found in the family structures of insect societies. That is, the sterile helper individuals are evolutionary altruists that give up their own reproduction and instead are helping their kin reproduce and proliferate shared genes in the offspring of the fertile queen. Selection in such cases is not just a matter of individual’s direct reproduction, and instead of own offspring, the currency of the evolutionary success of sterile individuals is inclusive fitness. The concept of inclusive fitness and the process of kin selection are key to understanding the magnificent cooperation we see in insect societies, and reciprocally, insect societies are key case studies of inclusive fitness logic. In extreme cases, such as the highly advanced and sophisticated societies of ants, honeybees, and termites, the division of labor and interdependence of colony members is so complete, that it is justified to talk about a new level of evolutionary individuality. Such increases in the hierarchical complexity of life are called major transitions in evolution. We see adaptations of the colony, rather than individuals, in, e.g., their communication and group behaviors. The division of labor between morphologically differentiated queens and workers is analogous to germline-soma separation of a multicellular organism, justifying the term superorganism for the extreme cases of social lifestyle. Alongside these extreme cases, there is enormous diversity in the social lifestyles across social insect taxa, which provides a window into the balance of cooperation and conflict, and individual reproduction and helping others, in social evolution. Over the last decades, social insect research has been an area where the theoretical and empirical understanding have been developed hand in hand, together with examples of wonderful natural history, and has tremendously improved our understanding of evolution.
General Overview and Brief History
Ever since Darwin, insect societies with a sterile worker caste have presented a challenge to evolutionary thinking. Darwin 1859 worried how natural selection can act on traits of such non-reproductive individuals, and he foresaw the formal solution in realizing how selection can be applied at the family, rather than individual level. The other evolutionary enigma of insect societies, the apparent selfless cooperation by the sterile workers, is a major theme in evolution that has fascinated many of evolutionary biology’s great minds, as covered in Dugatkin 2006. W. D. Hamilton was the first to formalize the solution to the evolutionary enigma of social insects by applying the logic of inclusive fitness as outlined in Hamilton 1996. That is, instead of the number of offspring produced by an individual, we must also consider the effects an individual has on reproduction by relatives, as they are genetically more similar to the actor than random members of the population. Hamilton’s rule presented in this work elegantly captures the conditions for evolution of altruism. The rule states that an individual’s altruism, i.e., forgoing own reproduction in order to rear offspring of others, pays off when the benefits of help to the recipients (b), weighed by the genetic relatedness between recipient and altruist (r), outweighs the costs (c), i.e., the reproductive gains the helper is giving up by foregoing own reproduction. That is, rb > c. Bourke 2011 explains that congruently with Hamilton’s rule, advanced insect societies where non-reproductive workers help queens reproduce, are families, and one of the prime examples of kin selection at work. The most highly social lifestyles are seen in termites and social Hymenoptera, such as ants, corbiculate bees and yellowjacket wasps, introduced in Hölldobler and Wilson 2009. These most extreme examples are undisputable cases of evolutionary altruism and a major transition in evolution. The diversity of social evolution outside social insects and comparative understanding of the general principles across taxa are provided in, e.g., Bourke 2011.
Bourke, A. F. G. 2011. Principles of social evolution. Oxford: Oxford Univ. Press.
An accessible overview of the basics of social evolution theory that also summarizes briefly the evolution of sociality in insects, and their importance as a model system for social evolution and inclusive fitness.
Darwin, C. R. 1859. On the origin of species by means of natural selection, or preservation of favoured races in the struggle for life. London: John Murray.
The foundation of modern evolutionary thinking. Darwin famously discusses the sterile castes of social insects as one of the important challenges he faced when formulating his theories. He offers applying selection thinking at the family level as the solution.
Dugatkin, L. A. 2006. The altruism equation: Seven scientists search for the origins of goodness. Princeton, NJ: Princeton Univ. Press.
A historical account on the contrast between altruism and selfishness as central themes in evolutionary thinking, outlined through key characters such as Darwin, Haldane, Hamilton, and Price.
Hamilton, William Donald. 1996. Narrow roads of gene land: Evolution of social behaviour. Oxford: Oxford Univ. Press.
A compendium (first of three parts) of the works of W. D. Hamilton, with original papers as well as eloquent and insightful autobiographical essays on the processes behind the papers and how the ideas evolved. Includes the key papers from 1963, 1964, and 1970 laying the foundation for inclusive fitness thinking. The individual papers are cited under The Status of Inclusive Fitness Theory as an Explanation for Social Evolution, Altruism and Cooperation, The Role of Haplodiploidy in Evolution of Insect Societies, and Kin and Nestmate Recognition.
Hölldobler, B., and E. O. Wilson. 2009. The superorganism: The beauty, elegance, and strangeness of insect societies. New York: W. W. Norton.
An overview of social behaviors and natural history of insect societies, including both adaptive considerations, ecological understanding, and mechanistic detail. Note that the section on theory of social evolution does not reflect the mainstream view on the role of inclusive fitness theory, but is rather controversial.
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