Inclusive fitness
In evolutionary biology and evolutionary psychology
Evolutionary psychology
Evolutionary psychology is an approach in the social and natural sciences that examines psychological traits such as memory, perception, and language from a modern evolutionary perspective. It seeks to identify which human psychological traits are evolved adaptations, that is, the functional...

, the inclusive fitness of an organism is the sum of its classical fitness (how many of its own offspring it produces and supports) and the number of equivalents of its own offspring it can add to the population by supporting others. Advocates of inclusive fitness theory say that an organism can improve its overall genetic success by cooperative social behavior.

From the gene's point of view, evolutionary success ultimately depends on leaving behind the maximum number of copies of itself in the population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

. Until 1964, it was generally believed that genes only achieved this by causing the individual to leave the maximum number of viable offspring. However, in 1964 W. D. Hamilton proved mathematically that, because close relatives of an organism share some identical genes, a gene can also increase its evolutionary success by promoting the reproduction and survival of these related or otherwise similar individuals.

Belding's ground squirrel
Belding's Ground Squirrel
Belding's ground squirrel , also called pot gut, sage rat or picket-pin, is a squirrel that lives on mountains in the western United States. In California, it often is found at 6,500–11,800 feet in meadows between Lake Tahoe and Kings Canyon...

 provides an example. The ground squirrel gives an alarm call to warn its local group of the presence of a predator. By emitting the alarm, it gives its own location away, putting itself in more danger. In the process, however, the squirrel protects its relatives within the local group (along with the rest of the group). Therefore, if protecting the other squirrels in the immediate area will lead to the passing on of more of the squirrel’s own genes than the squirrel could leave by reproducing on its own, then natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

 will favor giving the alarm call, provided that a sufficient fraction of the shared genes include the gene(s) predisposing to the alarm call. Further study has shown that the self-reported likelihood of risking one's life to save other's life is directly a function of the degree of genetic relatedness to the helper (Burnstein et al., 1994)

Inclusive fitness is more generalized than strict kin selection
Kin selection
Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/kin selection...

, which requires that the shared genes are identical by descent. Inclusive fitness is not limited to cases where kin are involved.

Hamilton's rule

In the context of sociobiology
Sociobiology is a field of scientific study which is based on the assumption that social behavior has resulted from evolution and attempts to explain and examine social behavior within that context. Often considered a branch of biology and sociology, it also draws from ethology, anthropology,...

, which holds that some behavior can be influenced by genes and therefore can evolve by natural selection, Hamilton proposed that inclusive fitness offers a mechanism for the evolution of altruism. He claimed that this leads natural selection to favor organisms that would behave in ways that maximize their inclusive fitness.

Hamilton's rule describes mathematically whether or not a gene for altruistic behavior will spread in a population:
  • is the probability, above the population average, of the individuals sharing an altruistic gene – commonly viewed as "degree of relatedness".
  • is the reproductive benefit to the recipient of the altruistic behavior, and
  • is the reproductive cost to the altruist,

In a recent paper, Gardner et al. suggest that Hamilton's rule can be applied to multi-locus models, but that it should be done at the point of interpreting theory, rather than the starting point of enquiry. They suggest that one should “use standard population genetics, game theory, or other methodologies to derive a condition for when the social trait of interest is favored by selection and then use Hamilton’s rule as an aid for conceptualizing this result". A recent paper by Nowak
Martin Nowak
Martin A. Nowak is Professor of Biology and Mathematics and Director of the Program for Evolutionary Dynamics at Harvard University.-Career:Martin Nowak studied biochemistry and mathematics at the University of Vienna, and earned his Ph. D. in 1989, working with Peter Schuster on quasi-species...

 et al. suggested that standard natural selection theory is superior to inclusive fitness theory, stating that the interactions between cost and benefit can not be explained only in terms of relatedness. This, Nowak said, makes Hamilton's rule at worst superfluous and at and best ad hoc. Gardner in turn was critical of the paper, describing it as "a really terrible article", and along with other co-authors has written a reply, submitted to Nature.
In work prior to Nowak various authors derived different versions of a formula for r, all designed to preserve Hamilton's rule. Orlove noted that if a formula for r is defined so as to ensure that Hamilton's Rule is preserved then the approach is by definition ad hoc
Ad hoc
Ad hoc is a Latin phrase meaning "for this". It generally signifies a solution designed for a specific problem or task, non-generalizable, and not intended to be able to be adapted to other purposes. Compare A priori....

. However, he published an unrelated derivation of the same formula for – a derivation not designed to preserve Hamilton's rule – which on its own was similarly ad hoc. Orlove argued that the existence of two unrelated derivations of the formula for reduces or eliminates the ad hoc nature of the formula, and of inclusive fitness theory as well.

Inclusive fitness and altruism

The concept serves to explain how natural selection can perpetuate altruism
Altruism is a concern for the welfare of others. It is a traditional virtue in many cultures, and a core aspect of various religious traditions, though the concept of 'others' toward whom concern should be directed can vary among cultures and religions. Altruism is the opposite of...

. If there is an '"altruism gene"' (or complex of genes) that influences an organism's behavior to be helpful and protective of relatives and their offspring, this behavior also increases the proportion of the altruism gene in the population, because relatives are likely to share genes with the altruist due to common descent. In formal terms, if such a complex of genes arises, Hamilton's rule (rb>c) specifies the selective criteria (in terms of cost, benefit and relatedness) for such a trait to increase in frequency in the population. Hamilton noted that inclusive fitness theory does not by itself predict that a species will necessarily evolve such altruistic behaviors, since an opportunity or context of interaction between individuals is more primary necessary requirement for any social interaction to occur in the first place. As Hamilton put it, “Altruistic or selfish acts are only possible when a suitable social object is available. In this sense behaviours are conditional from the start.” (Hamilton 1987, 420). In other words, whilst inclusive fitness theory specifies a set of necessary criteria for the evolution of altruistic traits, it does not specify a sufficient condition for their evolution in any given species. More primary necessary criteria include the existence of gene complexes for altruistic traits in gene pool, as mentioned above, and especially that "a suitable social object is available", as Hamilton noted. Paul Sherman, who has contributed much research on the ground squirrels mentioned above, gives a fuller discussion of Hamilton's latter point:
The occurrence of sibling cannibalism in several species underlines the point that inclusive fitness theory should not be understood to simply predict that genetically related individuals will inevitably recognize and engage in positive social behaviors towards genetic relatives. Only in species that have the appropriate traits in their gene pool, and in which individuals typically interacted with genetic relatives in the natural conditions of their evolutionary history will social behavior potentially be elaborated, and consideration of the evolutionarily typical demographic composition of grouping contexts of that species is thus a first step in understanding how selection pressures upon inclusive fitness have shaped the forms of its social behavior. Dawkins gives a simplified illustration:
Evidence from a variety of species including primates and other social mammals suggests that contextual cues (such as familiarity) are often significant proximate mechanisms mediating the expression of altruistic behavior, regardless of whether the participants are always in fact genetic relatives or not. This is nevertheless evolutionarily stable since selection pressure acts on the typical conditions, not on the rare occasions where actual genetic relatedness differs from that normally encountered (see Sherman above). Inclusive fitness theory thus does not imply that organisms evolve to direct altruism towards genetic relatives. Many popular treatments do however promote this interpretation, as illustrated in a recent review:
Such misunderstandings of inclusive fitness' implications for the study of alturism, even amongst professional biologists utilizing the theory, are widespread, prompting prominent theorists to regularly attempt to highlight and clarify the mistakes . Here is one recent example of attempted clarification from West et al. (2010):

Green-Beard effects

see also: kin recognition
Kin recognition
Kin recognition is animals' abilities to distinguish between close genetic kin and non-kin. In evolutionary biology and in psychology, such capabilities are presumed to have evolved to serve the adaptive functions of kin altruism and inbreeding avoidance...

As well as interactions within reliable contexts of genetic relatedness, altruists may also have some way to recognize altruistic behavior in unrelated individuals and be inclined to support them. As Dawkins points out in The Selfish Gene (Chapter 6) and The Extended Phenotype, this must be distinguished from the green-beard effect
Green-beard effect
The gene-centered view of evolution postulates that natural selection will increase the frequency of those genes whose phenotypic effects ensure their successful replication...

The proposal that altruists who support other altruists who are not their kin will encourage the evolution of altruism requires that altruists recognize and choose to support others predisposed toward altruism whom they have detected by their past altruistic behavior, not on the observation of some temporarily correlated characteristic (e.g., altruists have green beards). If the green beard effect were the mechanism, some non-altruistic individuals would evolve to mimic the label and would receive the benefits of support from altruists. This would happen quickly due to crossing over of chromosomes; it would not require waiting for the rare event of a mutation. The mimics would receive the benefits but would not incur the costs of caring for others, and so would out-compete the true altruists. Green-Beard transfers thus have a negative affect on the evolution of altruistic behaviors.

Inclusive fitness and parental care

Some might express concern that parental investment (parental care) is said to contribute to inclusive fitness. The distinctions between the kind of beneficiaries nurtured (collateral versus descendant relatives) and the kind of fitnesses used (inclusive versus personal) in our parsing of nature are orthogonal concepts. This orthogonality can best be understood in a thought experiment: Consider a model of a population of animals such as crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

s or tangle web spider
Tangle web spider
Theridiidae is a large family of spiders, also known as the tangle-web spiders, cobweb spiders and comb-footed spiders. The diverse family includes over 2200 species in over 100 genera) of three-dimensional space-web-builders found throughout the world...

s. Some species or populations of these spiders and reptiles exhibit parental care, while closely related species or populations lack it. Assume that in these animals a gene, called a, codes for parental care, and its other allele, called A, codes for an absence thereof. The aa homozygotes care for their young, and AA homozygotes don't, and the heterozygotes behave like aa homozygotes if a is dominant, and like AA homozygotes if A is dominant, or exhibit some kind of intermediate behavior if there is partial dominance. Other kinds of animals could be considered in which all individuals exhibit parental care, but variation among them would be in the quantity and quality thereof.

If we consider a lifecycle as extending from conception to conception, and an animal is an offspring of parents with poor parental care, the higher mortality with poor care could be considered a dimunition of the offspring's expected fitness.

Alternatively, if we consider the lifecycle as extending from weaning to weaning, the same mortality woulkd be considered a dimunition in the parents' fecundity, and therefore a dimunition of the parent's fitness.

In Hamilton's paradigm fitnesses calculated according to in the weaning to weaning perspective are inclusive fitnesses, and fitnesses calculated in the conception to conception perspective are personal fitnesses. This distinction is independent of whether the altruism involved in child rearing is toward descendents or toward collateral relatives, as when aunts and uncle rear their nieces and nephews.

Inclusive fitness theory was developed in order to better understand collateral altruism, but this does not mean that it is limited to collateral altruism. It applies just as well to parental care. Which perspective we choose does not affect the animals but just our understanding.

See also

  • Gene-centered view of evolution
    Gene-centered view of evolution
    The gene-centered view of evolution, gene selection theory or selfish gene theory holds that evolution occurs through the differential survival of competing genes, increasing the frequency of those alleles whose phenotypic effects successfully promote their own propagation, with gene defined as...

  • Evolutionary psychology
    Evolutionary psychology
    Evolutionary psychology is an approach in the social and natural sciences that examines psychological traits such as memory, perception, and language from a modern evolutionary perspective. It seeks to identify which human psychological traits are evolved adaptations, that is, the functional...

  • Criticism of evolutionary psychology
  • Kin selection
    Kin selection
    Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/kin selection...

  • Reproductive success
    Reproductive success
    Reproductive success is defined as the passing of genes onto the next generation in a way that they too can pass those genes on. In practice, this is often a tally of the number of offspring produced by an individual. A more correct definition, which incorporates inclusive fitness, is the...

  • r/K selection theory
    R/K selection theory
    In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity or quality of offspring...


  • Campbell, N., Reece, J., et al. 2002. Biology. 6th ed. San Francisco, California. pp. 1145–1148.
  • Rheingold, Howard, “Technologies of cooperation” in Smart Mobs. Cambridge, MA : Perseus Publishing, 2002 (Ch. 2:pp 29–61)
  • Dawkins, Richard C. 1976 The Selfish Gene, Oxford University Press (Discussion of carers and bearers in relation to inclusive and personal fitnesses, and the bugbear of parental investment as part of inclusive fitness occurs herein)
  • Hamilton, W. D. 1964 The Genetical Evolution of Social Behaviour I and II, J. Theor. Biol. v7, pp 1–16, and 17-52
  • Hamilton, W. D. 1975, Innate Social Aptitudes of Man: an Approach from Evolutionary Genetics, in Robin Fox
    Robin Fox
    Robin Fox is an Anglo-American anthropologist who has written on the topics of marriage, human and primate kinship systems, and evolutionary anthropology and sociology. He was born in Yorkshire. He founded the department of anthropology at Rutgers University in 1967 and remained a professor there...

    (ed.), Biosocial Anthropology, Malaby Press, London, 133-153 (IF including altruism to fellow altruists among strangers discussed herein)
  • Hamilton, W. D. Narrow Roads of Geneland I and II, 1995 Freeman I 2001 Oxford Press II (biography of WDH and anthology of his writings)
  • Orlove, M. J. 1975 A Model of Kin Selection not Invoking Coefficients of Relationship J. Theor. Biol. v49 pp289–310 (Isomorphism between Karma and Kin Theories discussed herein)
  • Orlove, M. J. 1979 A Reconciliation of Inclusive Fitness and Personal Fitness Approaches: a Proposed Correcting Term for the Inclusive Fitness Formula, J. Theor. Biol. v81 pp577–586 (Karma-Theory/Kin-Theory equivalence moves from conjecture to theorem status here)
  • Trivers, R. L. 1971 The Evolution of Reciprocal Altruism, Quarterly Review of Biology 46: 35-57
  • Trivers, R. L. 1972 Parental Investment and Sexual Selection in B. Campbell (ed.), Sexual Selection and the Descent of Man, 1871-1971 (pp. 136–179) Chicago, Il: Aldine
  • Trivers, R. L. 1974 Parent/Offspring Conflict, American Zoologist, 14 249-264 (Bigtime importance of If in understanding intra-family conflict)
  • Sherman, P.W. 2001. “Squirrels” (pp. 598–609, with L. Wauters) and “The Role of Kinship” (pp. 610–611) in Encyclopedia of Mammals, D.W. Macdonald (Ed.). Andromeda, UK.

External links

  • Test simulations:
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