Inclusive Fitness: Meaning and Development

Evolutionary biology's concept of inclusive fitness explains how an organism's genetic success is influenced not only by its reproduction but also by the reproduction of closely related organisms. Based on the idea that an organism's fitness is affected not only by how well it reproduces on its own but also by how well it can pass on its genes to the offspring of near relatives.

According to the evolutionary theory of inclusive fitness, a living person's genetic characteristics can boost their and their near relatives' chances of having children. Implications show that individuals in various species frequently work together to raise young, defend the group, and collect food, which explains why they are willing to risk their own lives or sacrifice their reproductive success.

Who Developed the Concept of Inclusive Fitness?

Although Hamilton is credited with developing inclusive fitness theory, it is worth noting that J.B.S. Haldane, an evolutionary biologist, had come close to developing the same theory in the 1930s. Haldane had written down his idea on a beer mat in a London pub, stating that he would risk his life for at least an identical twin or eight cousins. While Haldane briefly outlined his idea in the mid-1950s, he did not fully develop it, and it was not until a decade later that Hamilton's work revolutionized evolutionary thinking.

Components of Inclusive Fitness

It consists of two components −

Kin Selection

The portion of the theory concentrates on the effectiveness of an entity and near relatives in reproducing. It suggests that because relatives share a part of a person's genes, those relatives' procreation can pass on that person's genes. As a result, they encourage near relations to live long lives and procreate, which can improve a person's inclusive fitness.

This theory also suggests that people prefer to cooperate with those genetically similar to them because this will improve their inclusive fitness. This theory has significant consequences for the evolution of social behaviour and cooperation. Additionally, since social behaviour benefits close relations, it may be easier for social behaviour to evolve in animals where people are closely related.

Kin selection can be further divided into two classifications −

  • Direct Fitness − The achievement of one's reproduction is referred to as direct fitness. How many of an individual's progeny can propagate themselves determines it.

  • Indirect Fitness − The effectiveness of a person's genetic relations in reproducing is called indirect fitness. It is determined by how many of an individual's genetic relations children live to reproduce themselves.

Personal Fitness

It describes an entity's capacity to endure and procreate in its surroundings. An individual's qualities, such as bodily characteristics, behaviour, and the ability for resource competition, can also impact their fitness level. The number of children a person has who go on to propagate themselves can also be used to measure it.

The Role of Altruism and Sociality in Inclusive Fitness

Today, researchers interested in animal behaviour view animals as nepotistic strategists rather than individual strategists. According to ethnologist John Alcock, "Hamilton's explanation for altruism rests on the premise that the unconscious goal of reproduction, from an evolutionary perspective, is to propagate one's distinctive alleles." Alcock clarifies that ethologists do not require animals to be aware of their actions towards relatives, but rather it is enough that they act accordingly. In natural conditions, immature animals usually need help more than their mature relatives. Therefore, it is common to see older or less vulnerable relatives providing aid to younger, more vulnerable individuals in the animal kingdom.

"sociality" describes a species' propensity to create social organizations, such as ants, bees, and people. According to this theory, social behaviour can arise in closely related groups that share a significant percentage of their DNA, making it more advantageous for them to cooperate and boost their overall reproductive success. A form of a societal structure known as eusociality divides people into castes for reproductive and non-reproductive purposes.

Small organism types like termites, wasps, and ants exhibit this. Since the non-reproductive castes can still boost their inclusive fitness by supporting their closely related reproductive castes, inclusive fitness theory offers an account of how eusociality can develop through kin selection.


Brothers and sisters face distinct adaptation challenges and have done so repeatedly throughout human development. First, a brother or sister might be a valuable social ally; after all, you are linked to your siblings by 50%. Nevertheless, sibs, possibly more than any other relatives, are huge rivals for parental resources.

As the theory of parent-offspring conflict suggests, what is in the parent's best interests is not the same as what is in the best interests of a particular child. As a result, siblings have historically faced the recurrent adaptive problem of competing with each other for access to parental resources. Given this conflict, it is not surprising that sibling relationships are often riddled with ambivalence.

According to an intriguing theory, the adaptation problems imposed by parents on children would produce distinct "niches" for children based on their birth order. Because parents frequently prioritise the eldest kid, the firstborn tends to be more conservative and willing to maintain the status quo. On the other hand, second-borns have nothing to gain by maintaining the current structure and everything to gain by rebelling against it. According to Sulloway, later-borns, particularly middle-borns, acquire a more rebellious disposition since they have the least to gain by sustaining the established system.

On the other hand, the youngest could receive more excellent parental investment than middle children since parents frequently let out all the stops to invest in their final direct reproductive vehicle.

Catherine Salmon and Martin Daly (1998), evolutionary psychologists, found some support for these hypotheses. They discovered that middle-borns scored lower on family solidarity and identity measures than first- and last-borns. Middle-borns, for example, are less likely to name a genetic relative as the person they feel closest to. They are also less likely to assume the role of a family genealogist. Middle-borns have fewer positive opinions about their families and are less inclined to assist a needy family member than first- and last-borns.

Sibs versus Half Sibs

Another critical component of kinship is whether a sib is a full or half-sib. Do you and your sibling share a father, for example, if you share a mother? This differential is potentially significant since complete siblings are genetically linked 50% of the time.

However, half-siblings are genetically related 25% of the time. In a fascinating research of ground squirrels, Warren Holmes and Paul Sherman (1982) revealed that full sisters were considerably more likely than half-sisters to collaborate in the mutual protection of their young.

The difference between full and half siblings was likely a recurring selection pressure throughout human development. Mothers in modern tribal groups frequently have children from many men through extramarital affairs or serial marriage.

Daly, Salmon, and Wilson (1997) speculate that it "could well be the case that in human prehistory it was a virtual toss-up whether successive children of the same woman were full or half-siblings, and the distinction between (r = .5) and (r = .25) is by no means trivial when the decision to cooperate or to compete is a close call". The disputes that develop in stepfamilies having sibs of varied degrees of genetic relatedness would be an appropriate scenario for testing these theories.

Grandparents and Grandchildren

Grandparents are related to their grandchildren by an r of .25. Modern women frequently live well beyond menopause has led to the "grandmother hypothesis," which proposes that menopause evolved as a means of ceasing direct reproduction in order to invest in children and then grandchildren. Across cultures, postmenopausal women contribute substantially to their grandchildren's welfare. If grand parenting has been a recurrent feature of human evolutionary history, adaptations for allocating grandparents' investment might have evolved.

Vigilance over Kin's Romantic Relationships

Even though mating is so close to the engine of the evolutionary process-differential reproductive success-humans has a wide range of mating adaptations. Given the critical importance of success in the mating game, it would be surprising if individuals were unconcerned about their kin's mating relationships. Recent research examined two hypotheses −

  • People will keep a higher watch over the mating connections of their close than distant family

  • Individuals will maintain greater vigilance over the mating of their female than male kin.

The results supported both hypotheses using three dependent measures: awareness of the romantic partner's good and bad qualities, awareness of how the romantic relationship was progressing, and the degree to which they were concerned about how the romantic relationship was progressing. To summarise, the degree of genetic relatedness and the gender of the target both influence the degree to which individuals monitor their kin's behaviour.


Inclusive fitness theory is based on personal fitness and kin selection, which suggests that people prefer to cooperate with those genetically similar. Hamilton's hypothesis explains the role of altruism and sociality in inclusive fitness, that the unconscious goal of reproduction is to propagate one's distinctive alleles.

Inclusive fitness theory suggests that organisms will be more likely to help close genetic relatives, such as siblings and cousins, than distant relatives or non-relatives and engage in reciprocal altruism with genetic relatives.

Updated on: 11-Apr-2023


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