In This Article Expand or collapse the "in this article" section Inclusive Fitness

  • Introduction
  • General Overviews
  • Development of Theory
  • The Price Equation
  • Hamilton’s Rule
  • Cooperation between Nonrelatives
  • Kin Discrimination
  • Ecology
  • Study Systems
  • Assumptions and Alternative Viewpoints

Evolutionary Biology Inclusive Fitness
Charlie K. Cornwallis
  • LAST REVIEWED: 13 January 2014
  • LAST MODIFIED: 13 January 2014
  • DOI: 10.1093/obo/9780199941728-0033


Cooperation is abundant throughout the natural world and exists at all biological levels, from genes forming genomes to individuals collaborating in societies. Nature documentaries are frequently packed with stunning examples, from kamikaze bees stinging intruders to save the lives of their nest mates to meerkat helpers feeding the pups of others. However, beneath this appearance of kindness lies one of the most challenging issues for evolutionary theory. The problem is that natural selection favors genes that increase an organism’s ability to survive and reproduce and so how can behavior that benefits others ever evolve? To simplify this problem the complex spectrum of social behaviors can be broken down into pair-wise interactions and classified according to the direct fitness benefits (number of offspring an individual produces stripped of social interactions) and costs to the actors and recipients involved. This leads to four types of behavior: Selfishness (benefit to actor, cost to the recipient) and mutually beneficial interactions (benefit to actor, benefit to the recipient) are easily understood as they increase the direct fitness of the actor. Altruism (cost to the actor, benefit to the recipient) and spite (cost to the actor, cost to the recipient), on the other hand, present an evolutionary paradox—how can a gene that is disadvantageous to an individual spread in a population? Darwin realized this problem, but it wasn’t until 1963–1964, when William (Bill) D. Hamilton produced his benchmark papers, that it became clear how actions that decrease direct fitness can evolve through natural selection. Hamilton coined the term “Inclusive Fitness” to emphasize that the quantity that individuals attempt to maximize is not simply direct fitness, but also something called indirect fitness—the effect individuals have on the number of offspring everybody else in the population produces weighted by their relatedness. Inclusive fitness theory remains one of the most active areas of evolutionary research and provides an extremely important tool for understanding both the process and purpose of evolution.

General Overviews

The literature on inclusive fitness is vast, so my recommendations are a few key texts that span the life of the field. There are many more overviews available, and these often appear in the reference lists of the highlighted texts or in other sections of this article. Although the concepts behind inclusive fitness were discussed prior to Hamilton, it was his 1964 contributions (see Hamilton 1964) that provided the foundations of all later work, representing perhaps the most significant contribution to evolutionary biology since Darwin to this day. Amazingly, Hamilton wrote these papers during his PhD at a time when there was still great confusion about what the unit of selection was—genes, individuals, groups, or species? It took over a decade for the magnitude of Hamilton’s contribution to be realized, but after this lag research on inclusive fitness grew in three main directions: theory, work on social insects, and work on cooperatively breeding vertebrates (but see Study Systems). Although on the surface inclusive fitness theory is quite intuitive, there are many pitfalls. Grafen 1984 clarifies some of the misconceptions and gives directions on how theory can be best used in empirical testing. Trivers 1985 shows how inclusive fitness theory can be used to explain a wide range of evolutionary problems and provides some of the earlier examples across a broad range of taxa. Frank 1998 gives a detailed and easy-to-read account of the somewhat daunting array of concepts and tools used to develop inclusive fitness theory, and Wilson 1975 provides an extensive account of the wonderful natural history of different animal societies. Over the decades since its conception much confusion has arisen over inclusive fitness theory and the study of cooperation, particularly across scientific disciplines. West, et al. 2007 provides a roadmap to navigate through this labyrinth. For general up-to-date reviews of both theory and empirical work see Bourke 2011 and Davies, et al. 2012.

  • Bourke, A. F. G. 2011. Principles of social evolution. Oxford: Oxford Univ. Press.

    A recent review of inclusive fitness theory that shows how it can explain the organization of life through “the major evolutionary transitions” (see Major Evolutionary Transitions).

  • Davies, N. B., J. R. Krebs, and S. A. West. 2012. An Introduction to Behavioural Ecology. 4th ed. Oxford: Wiley-Blackwell.

    The latest edition in the series gives an up-to-date account of inclusive fitness theory illustrated with lots of examples. This series of books has formed the backbone of behavioral ecology undergraduate teaching since the 1980s, and it is well worth looking back through past editions.

  • Frank, S. A. 1998. Foundations of social evolution. Princeton, NJ: Princeton Univ. Press.

    A lucid review of the mathematical methods used to construct inclusive fitness theory. Downloadable from Frank’s website.

  • Grafen, A. 1984. Natural selection, kin selection and group selection. In Behavioural ecology: An evolutionary approach. Edited by J. R. Krebs and N. B. Davies, 62–84. Oxford: Blackwell Scientific.

    Outlines what inclusive fitness theory is and how to apply it. Thirty years on this still provides key insight.

  • Hamilton, W. D. 1964. The genetical evolution of social behaviour I, II. Journal of Theoretical Biology 7:1–52.

    DOI: 10.1016/0022-5193(64)90038-4

    These papers are the conception of inclusive fitness theory following Hamilton’s 1963 short note. The second paper proposes the haplodiploidy hypothesis for the evolution of eusocial insects.

  • Trivers, Robert. 1985. Social evolution. Menlo Park, CA: Benjamin/Cummings.

    This book covers the theoretical basis of inclusive fitness theory and gives great insight into the biological problems it can be applied to, illustrated with weird and wacky examples.

  • West, S. A., A. S. Griffin, and A. Gardner. 2007. Social semantics: Altruism, cooperation, mutualism, strong reciprocity and group selection. Journal of Evolutionary Biology 20:415–432.

    DOI: 10.1111/j.1420-9101.2006.01258.x

    This paper clarifies the confusion that has accumulated over decades of research on social evolution. Helps unify different fields by clearly stating how different terms are used in different disciplines.

  • Wilson, E. O. 1975. Sociobiology: The new synthesis. Cambridge, MA: Harvard Univ. Press.

    When this book was published it became famous for the controversy it caused over its gene-centered discussion of human evolution. However, it is much more than that, providing a comprehensive overview of social evolution at that time. Re-released in 2000 with an extra section on human genetics and neuroscience.

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