Inclusive Fitness Theory

At the heart of Darwinian theory is the concept of competition between individuals over limited resources. It originated from Darwinian evolutionary theory, first coined by Herbert Spencer. It is a way of describing the mechanism of natural selection. Biological fitness is defined as reproductive success.

One would assume that survival of the fittest would lead to selfish dog-eat-dog behaviour: each individual striving to maximise their reproductive success. It seemed the ultimate confirmation of Tennyson's (1850) edict: "Nature, red in tooth and claw". This should render altruism impossible.

Biologically speaking, altruism refers to self-sacrificing behaviour whereby one individual sacrifies some component of its potential reproductive value for the benefit of another: the donor incurs a reproductive cost whilst the recipient incurs a reproductive benefit. By the process of evolution, altruism would've been expected to have been selected against. It is not just humans who perform altruistic acts.

Alarm calling:

• Many species, including gofers or prairie dogs and vervet monkeys, give alarm calls upon spotting a predator. By calling, an individual increases the risk it will be detected by the predator, yet they selflessly call to warn others anyway

Kamikaze honey bees

• Guard honey bees will fight to the death to protect their hive

Childcare

• Female lions have often been observed to suckle each other's cubs

Reproductive altruism

• Many social insects (e.g many species of bees and ants) contain sterile workers who dedicate their lives to raising and protecting the offspring of a single breeding "queen". Naked mole rats also have a single breeding queen and sterile workers who help raise her young.

Hamilton (1964) argued that fitness (reproductive success) should be measured at the level of the gene, not the species, group or individual: an individual has better fitness if they pass on more copies of a gene to subsequent generations, rather than just the number of offspring produced.

Thus, individuals can increase their fitness in two ways; by doing so indirectly by aiding the reproduction of other individuals who are likely to carry the same gene, and directly by passing genes on to their own offspring. We share a higher proportion of our genes with our kin or relatives than with strangers.

Therefore, if we promote the survival of our kin (genetic relatives) we're also promoting our own long term genetic survival. Indirect and direct fitness combine to give a measure known as inclusive fitness.

Therefore, altruism could evolve under Darwinian selection if the altruists' behaviour allowed a genetic relative that shared the same gene to reproduce more than it would have done otherwise. To increase inclusive fitness, individuals should only behave altruistically toward those with whom they're likely to share a given gene with, e.g their kin. This theory is referred to as kin selection.

Hamilton's Rule: a gene for altruism will evolve whenever rB > C

• r: the coefficient of relatedness between donor and beneficiary
• B: benefit to the recipient
• C: cost to the donor

The coefficient of relatedness is the probability that the two individuals involved share the same gene by descent from a comon ancestor(s). There is a 50% chance that any on of our genes is present in our full siblings. Coefficient of relatedness with full siblings is 0.5/50%. This is also the same for our parents.

Altruistic behaviour should only occur when the rule is satisfied: when the benefit accruing to the beneficiary, devalued by the probability that the two individuals share the gene in question, is greater than the cost to the altruist. Thus if the individuals are unrelated (r = 0), then the left-hand side of the formula is always reduced to 0 and altruistic behaviour shouldn't occur. Altruistic acts are expected to occur with greater frequency between close relatives than between distant relatives because the conditions satisfying Hamilton's Rule will occur with greater frequency between individuals with a high r value.

Hoogland (1983) found that, far from calling selflessly and indiscriminately, individual praire dogs are more likely to alarm call when close relatives are nearby, and least likely to call when there are only unrelated individuals present to hear them. Praire dogs are thus prepared to take a risk if it will increase their inclusive fitness, but tend to keep quiet if this isn't the case.

Female lions within a pride also tend to be closely related, and by suckling their sisters' offspring as well as their own, they are helping to increase their inclusive fitness by promoting the survival of genes that they share with their nephews and neices.

Kin selection helps explain why altruism occurs among family members, however it cannot explain altruism to non-relatives. Trivers (1971) came up with the theory of reciprocal altruism. According to this theory, if an individual behaves altruistically but is paid back for their altruistic act at a later date, then both participants will ultimately gain a net benefit. 

Packer (1977) found that baboons engaged in reciprocal altruism. A male baboon attempting to gain access to a female from the high ranking/alpha male will often recruit another male to help him. The recruited male helps in attacks on the alpha male, while the other male mates with the female. At a later date, they swap roles, with the former beneficiary returning the favour to the former altruist. Wilkinson (1984) found reciprocal altruism in vampire bats. Bats will often regurgitate blood to starving colony mates. Later on, the bats who had been starved were likely to help out their donor if they became hungry - reciprocal partnerships betweens pairs of bats were observed.

A problem in reciprocal altruism is that, since there is a delay between the altruist helping the beneficiary and the beneficiary repaying this act, they may neglect to return the favour after they have been helped. 

Axelrod & Hamilton (1981) conducted a computer tournament of repeated social exchanges. There were 200 rounds of the game and contestants were randomly paired with one another. 14 different programmes were entered. The simplest programme won - "***-for-tat". ***-for-tat's rules were: (1) cooperate on the first move and (2) reciprocate on every move thereafter: only cheat if your opponent's previous move was to cheat.

Axelrod & Hamilton identified 3 markers of ***-for-tat's success. (1) never be the first to refuse - always start by cooperating and continue as long as the other player does. (2) refuse only after the other has refused. (3) be forgiving; as soon as a previously refusing player cooperates, start cooperating again.