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Inclusive fitness

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Inclusive fitness encompasses conventional Darwinian fitness with the addition of behaviors that contribute to an organism’s individual fitness through altruism. An organism’s ultimate goal is to leave the maximum number of viable offspring possible, thereby keeping their genes present within a population. Since relatives of an organism are likely to share common genes, an organism may increase its own fitness by keeping its relatives and offspring viable. Kin selection results from this altruistic behavior towards relatives leading to increased fitness in an organism. Inclusive fitness therefore takes into account both the passing of genes from an organism to its offspring and the inheritance of the same genes among relatives and their offspring.

The most obvious examples of increased inclusive fitness can be observed in the altruistic behaviors of parents. To ensure that their genes remain in the gene pool, organisms attempt to give rise to the maximum number of offspring that are sure to survive. Once the offspring are produced, the parents’ reproductive success is determined by the number of offspring that can then procreate and carry on the family genes. Natural selection therefore favors any genes that code for behaviors that lend themselves to increased fitness. Possibly having a genetic basis, innate behaviors that cause parents to sacrifice their well-being, either in the actual birthing process or in aiding their young, increase the parents’ fitness, which makes them more reproductively successful and therefore favored by natural selection.

Increasing an organism’s inclusive fitness can also involve altruistic behavior towards relatives that have a probability of sharing common genes. For example, Belding ground squirrels give alarm calls to warn the population of ensuing dangers. By emitting the alarm, the Belding ground squirrel puts itself in increased danger by giving away its location. In the process, however, the squirrel protects its relatives that live within the population. In further studies, it has been shown that willingness of the squirrel to put itself at risk is directly proportional to how closely related it is to members of its population. Therefore, if protecting the other squirrels in the immediate area will lead to the passing on of more of the squirrel’s genes than the squirrel could leave by reproducing on its own, the squirrel is willing to sacrifice itself, which leads to greater inclusive fitness.

Evidence of Inclusive Fitness in Humans

Human behavior is generally much more complicated than other organisms making it difficult to define human behavior in general organism terms. However, evidence for human altruistic behavior leading to increased inclusive fitness has been observed. While there exists clear evidence towards increased inclusive fitness through altruistic behaviors on behalf of parents and children, much sacrificial behavior by humans is generally done in the hope of reciprocation at some point in the future. Therefore, increasing inclusive fitness in humans is not necessarily dependent upon relatedness. Rather, it is commonly based on reciprocal altruism.

Inclusive Fitness in the Family Structure

Inclusive fitness may also be applied to the familial structure. Parents are frequently self-sacrificing towards their children with the hope that children will carry on the family genes. Frequently, the amount of altruistic behavior displayed by parents to increase their inclusive fitness is related to the amount of parental investment initially involved.

It is common for some people to express concern when parental investment (parental care) is said to contribute to inclusive fitness. This concern exemplifies the surprisingly large degree of confusion and obfuscation over such a profound and fundamental concept as IF. The distinctions between, kind of beneficiaries nurtured (collateral versus descendant relatives), and, kind of fitnesses used in our parsing events in nature to understand the goings on, are orthogonal concepts. This orthogonality, can best be understood in a thought experiment in which we consider a model of a population of animals such as tangle web spiders or crocodiles in which a gene, a, codes for parental care, and its other allele, A, codes for an absence thereof, thus 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 behaviour if there is partial dominance. Among these spiders and reptiles, some species or populations exhibit parental care, whilst closely related species or populations lack it, so this is somewhat reasonable. However, other kinds of animals could be considered in which all individuals exhibit parental care, but variation among them would be in how much, or well, they do.

If we parse nature such that life begins at conception, then, other things being equal, the only differences between how well different individuals do will be based on how much care they got as pre-weaned babies, since all mothers will conceive the same number of kids, but some will take care of them, or care for them better, and thus more of them will live, but the differences in mortality will count as part of the offsprings' fitnesses. Thus the variations in fitness among the animals will be part of their L(x) curves.

But if we parse nature such that life begins at weaning, and the pre-weaned offspring is part of the mother until weaned, sort of like a fetus, then the number of offspring weaned successfully, will be sort of a littersize, and the variations in success among individuals will be considered part of the M(x) curves.

Simply put, l(x) is the probability of still being alive at age = x, and m(x) is fecundity at age x.

These are just 2 ways of keeping track of the bookkeeping, and the animals are exactly the same regardless how we keep track of them.

However, if we regard life as beginning at weaning, the heterozygote will have the same fitness as the homozygote with the dominant gene, and fitness will be a constant function of genotype. If life begins at conception, the 3 kinds of genotyped individuals will have different fitnesses, not only from each other but from generation to generation.

Fitnesses calculated in the life-begins-at-conception world will be examples of "personal fitnesses" or reproductive successes, whereas fitnesses calculated in the life-begins-at-weaning world will be examples of "inclusive fitnesses."

Both kinds of fitnesses can be used to parse reality in models or real populations with or without altruism toward collateral relatives. An older kind of fitness, called "classical fitness", doesn't consider social interactions at all. Altruistically-reared offspring aren't counted here as incrementing the fitness of the altruist or the beneficiary, but classical fitness, is simply, a carryover from earlier days when thought didn't go this deep in this direction.

This understanding is completely identical to that of W. D. Hamilton, whose philosophy is embodied in this discussion and terminology.

As enunciated by Richard Dawkins in his 1976 book, The Selfish Gene, with personal fitness, the increments of fitness are counted with the bearers, and with inclusive fitness they are counted with the carers.

The mathematics simply becomes easier to use inclusive fitnesses, when studying model or real populations in which altruism toward collateral relatives is common or present, but the use of an inclusive fitness approach doesn't ipso facto imply collateral altruism is occurring, nor the use of personal fitnesses imply it isn't.

The size of the increment is always one in a personal-fitnesses-parsing, but a fraction between zero and one during an inclusive-fitness parsing.

Since first cousins are related by approximately 1/8 on average, raising one kid for your first cousin automatically increments your inclusive fitness by 1/8, but has a probability of 1/8 of incrementing your personal fitness by 1. This is because the probability is 1/8 your cousin will rear a kid of yours, for you, if you rear one for one of your cousins. This is not reciprocity, but rather just due to the laws of statistics.

There are complicating factors so this number will not be exactly 1/8, see below.

This concept is exactly equivalent to the law of karma in Vedic (Hindu, Buddhist, Jaine) Philosophy.

The increment to inclusive fitness is valuable for its own sake, not necessarily because it is a promise of a higher personal fitness.

This is because a cousin (say) has copies of one's own genes every bit as much as an offspring does, just not as many of them.

The mathematical account, especially as given by Hamilton (1964) can look quite daunting, however, it isn't so bad if you keep trying to read it.

it can be verbally explained however, as in: Stories with Bill Hamilton in them. However, this account was written in the spirit of the Pliny the Younger letter to Tacitus when he said "You will use the important bits, for it is one thing to write a letter, another to write history, one thing to write to a friend, another to write for the public. Farewell." Eyewitness Account of the Disaster at Pompeii. And it wasn't expected to be published verbatim in an anthology, so you will have to read around the typos.

You can also find the simulations to test these thoughts at the website, and download the package with the simulations: [1], and to decompress the source code should you ever wish to read it, using: [2]

These probabilities of reciprocity will be coefficients of relatedness in species where there is only altruism toward relatives, but when strangers are involved they can be estimates of reciprocation, which depend on being, as if, more closely related than average at the altruism influencing portions of the genome, based on past behaviour, in a stranger. Again whether personal- or inclusive fitness approaches are used affects the observees, not a jot, but the observer's comprehension a great deal.

If any of this seems weird or counter-intuitive, you might want to take a look at a less involved at-first-counterintuitive mathematical problem called the Monty Hall problem.

Some people would consider IF fundamentally important, which it is, but for a reason, some of them would question and many wouldn't, which is that they believe that the ultimate test for whether some one is a true mutualist, i. e., a true friend, would be based on whether they were increasing your inclusive fitness. Others might say it is based on whether they are increasing your personal fitness. Sometime increasing one would decrease the other, but advocates for these ideas would say someone was purloining you if they were decreasing your fitness. Some wound argue it doesn't matter which kind of fitness they choose to increase as long as it is one of them and consistent. And many would argue such a value system shouldn't be based on fitness at all.

Complicating factors would include, what if you include the meme as well as the gene in your argument?

What about conflicts within individuals between genes, between memes, and between genes and memes?

From thoughts along these lines we might derive the test as to whether humans are spiritual beings, or mere automata. If some person or organisation of persons had an endeavour which generated a pollutant, a chemical, or waste product laden with parasites or pathogens.

In order to clean up their operation they have two choices:

  • 1. nutralise the pollutant, rendering it harmless, or
  • 2. dilute it so thoroughly as to equally distribute it in the atmosphere, or ocean, where it would harm people slightly, or alternatively, not so slightly but randomly and rarely, but the harm or risk would be equally distributed around the world.

In order to be consistent, a believer in Inclusive fitness, in the sense as a utility function, but one with a moral imperative, would argue that if people based the choice of pollution control on which was cheaper, then the chooser would be an automaton without a spirit. Similarly, if a person were willing to pay extra to go for the nutralisation option, then spiritual humanity exists.

Considering IF at all opens moral dilemmas in slightly, but only slightly, less grand questions, like, "Do you inform a person who has been sterilised that it isn't so bad, if their identical twin is still alive?" If you don't tell them they may die forlorn. If you do what would it do to the twin's marriage?

Some people might look down on celibate people as inferior; a positive spin-off of IF Theory is that their ammunition evaporates under it.

Another positive spin-off of IF would be to challenge those among us who would have considered people from India as savages, or primitive, and Karma as a mumbo-jumbo concept, but praise Bill Hamilton as if he were Einstein or Darwin, which he deserves, but there is no place for putting down people of foreign cultures. William Provine, In a lecture, and an attempt to present a cynical view of the cosmos, told his audience that only one to 3 of them would have their bloodlines extant in a thousand years thence, due to random events. IF Theory shows this doesn't matter another jot. The explanation is simply that if bad luck eliminates some bloodlines, it will enhance others, and if it is truly random, if you lose your bloodline, other ones carrying the same genes in the same numbers will exist somewhere. And if you believe genes don't matter, then there is no reason to get upset from the onset.

Another spin-off of IF Theory is parent-offspring conflict as discovered by Robert L. Trivers in 1974 and popularised and reviewed by Dawkins in The Selfish Gene.

Basically, a parent is trying to maximise its number of grandchildren, but one of its offspring would give up the chance to have n kids only if the benefit to one of its siblings for doing so provided more than 2n nieces or nephews. So if the benefit to cost ratio is between 2 and 1 there is a conflict.

Trivers predicts that if the parent can't be around forever to coerce an offspring into being a worker, aka a helper, at a sibling's nest, then if the benefit to cost ratio is between 2 and 1, the parent needs to get it higher than 2, so the offspring will be a voluntary worker.

Since increasing the productivity of the more productive child is impractical, the parents' best bet is to lower the reproductive capability of the offspring, without harming its somatic skills, such as walking or finding food.

Trivers argues that if Sigmund Freud had lived after 1964, he would have explained intra-family conflict, and castration complex as resulting from resource sharing issues, and economics rather than sexual or incestuous jealousy.

See also

Also, applications to the study of human behavior:


  • 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 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 (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 beween 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, U.K.

Further reading

  • Helle, S., Lummaa, V. & Jokela J. (2004). Selection for increased brood size in pre-industrial humans. Evolution, 52, 430-436. Full text

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