Consider a world in which everyone adored everyone other. There would be no preferential treatment. You are just as likely to feed a passing stranger as you are to feed your children. Your parents are just as likely to pay for a neighbour's college degree as yours. However, if you were obliged by fate to rescue only one person's life while two others were drowning, you would just as likely save a stranger as your brother or sister.
It is not easy to fathom such a world. Why is it so difficult to imagine? The evolutionary theory of inclusive fitness explains why. In general, we are linked to our parents, children, and siblings by 50%. We are connected to our grandparents and grandkids by 25%, half-brothers and half-sisters, uncles, aunts, nieces, and nephews. On average, we are 12.5 per cent related to our first cousins.
Children have different values than their parents; in this chapter, we will look at the hypothesis that relatives have different values than ourselves. If all else is equal, selection will prefer adaptations for assisting kin in proportion to their genetic relatedness. Selection will prefer methods that benefit us twice as much as aid our brothers.
In the early 1960s, William D. Hamilton, a young graduate student at University College, London, began working on his PhD dissertation. Hamilton presented a radical new revision of the evolutionary theory, which he dubbed "inclusive fitness theory." Legend has it that his teachers misunderstood the dissertation or its relevance (perhaps due to its high mathematical content), and his work was first rejected.
When Hamilton's idea was ultimately accepted and published in the Journal of Theoretical Biology in 1964, it ignited a revolution that transformed the whole discipline of biology. Hamilton reasoned that classical fitness, defined as an individual's immediate reproductive success in passing on genes through child production, was too limited to capture the process of evolution by selection.
He proposed that natural selection promotes traits that permit an organism's genes to be handed along, regardless of whether the creature directly produces children. Parental care, or investing in your offspring, was reinterpreted as a subset of caring for relatives who carry copies of your genes in their bodies. An organism can also boost gene reproduction by assisting brothers, sisters, nieces, and nephews in surviving and reproducing.
All of these relatives have a chance. William D. Hamilton's idea of inclusive fitness, published in 1964, revolutionised evolutionary biology. He proceeded to make significant theoretical contributions on themes ranging from the development of vengeance to the origins of sexual reproduction of harbouring copies of the organism's DNA. Hamilton's brilliance was in his realisation that classical fitness was too restrictive and needed to be widened to include inclusive fitness.
Technically, inclusive fitness is a quality of actions or outcomes rather than a trait of individuals or organisms. Inclusive fitness may be defined as the total of a person's reproductive success (classical fitness) plus the consequences of that person's activities on the reproductive success of his or her genetic relations. The effects on relatives must be weighted by the appropriate degree of genetic relatedness to the target organism for this second component—for example, 0.50 for brothers and sisters, 0.25 for grandparents and grandchildren (25 per cent genetic relatedness), 0.125 for first cousins (12.5 per cent genetic relatedness), and so on.
According to inclusive fitness theory, selection will favour mechanisms that result in kin investment based on genetic relatedness. Expected relatedness depends on two factors: (1) genealogical connection (for example, sisters are more closely related than uncles and nephews) and (2) paternal uncertainty generated by extra-pair copulations. Evidence suggests that as paternal uncertainty increases down the paternal line, so makes the investments in grandchildren. Is this benefit confined to the investment of grandparents, or does the rationale apply to other kin ties, such as aunts and uncles?
Maternal aunts (mother's sisters) should invest more than paternal aunts, according to this argument (sisters of the father). Likewise, maternal uncles (the mother's brothers) should spend more than paternal uncles (the father's brothers). Paternity certainty, and genetic relatedness, should be more lavish through the maternal line on average.
In contrast, on average, genetic relatedness should be lowest through the paternal line. To investigate this topic, researchers investigated 285 college students in the United States, all indicating that both of their biological parents were living. Using a seven-point scale, each participant was asked to score a series of questions −
"How concerned is the maternal (paternal) uncle (aunt) about your well-being?"
Which of your maternal and paternal uncles (aunts) is more concerned about your well-being?"
The term "care for your wellbeing" was used by the researchers so that participants would think widely about the numerous sorts of advantages they may get.
Likewise, maternal uncles (the mother's brothers) should spend more than paternal uncles (the father's brothers). Paternity certainty, and genetic relatedness, should be more lavish through the maternal line on average. In contrast, on average, genetic relatedness should be lowest through the paternal line.
To investigate this topic, researchers investigated 285 college students in the United States, all indicating that both of their biological parents were living. Using a seven-point scale, each participant was asked to score a series of questions −
"How concerned is the maternal (paternal) uncle (aunt) about your well-being?"
Which of your maternal and paternal uncles (aunts) is more concerned about your well-being?"
According to inclusive fitness theory, the distribution of wealth in a person's will can be predicted based on the degree of genetic relatedness and reproductive value of potential beneficiaries. A study by psychologists Smith, Kish, and Crawford (1987) tested three hypotheses based on this theory.
Firstly, the study predicted that individuals would leave a more significant portion of their estates to genetically related kin and spouses than to unrelated individuals. Spouses were included in this prediction because they can distribute resources to their shared offspring and grandchildren.
Secondly, the study predicted that people would leave more to close kin than distant relatives.
Thirdly, the study hypothesized that people would leave more to their offspring than their siblings, despite the same average genetic relatedness. This prediction was based on the idea that younger offspring have higher reproductive value and are more likely to convert resources into future offspring. In contrast, siblings are typically past their childbearing years when wills are typically written or go into effect.
Descent refers to the social affiliation of a child with the parents' group. There are two kinds of descent −
In some societies, the child is considered equally descended from the father and mother, with titles and surnames usually passed down along the male line. This system is called Bilateral or Cognatic. An individual can belong to multiple descent groups, including those of both parents, grandparents, and great-grandparents, limited only by memory or a conventionally established cutoff point. Membership can overlap in small communities with intermarriage, causing divided loyalties in disputes or feuds. In some cognatic systems, individuals have the right to belong to multiple groups, but only if they live in the group's territory. This requirement is often seen in modern nationality laws.
In certain societies, the descent is reckoned through a single line only: the fathers or the mothers. This is called unilineal descent. On the other hand, in some societies, the descent is reckoned through the mother and father, a system called bilateral or cognatic descent. In double unilineal descent, the child is affiliated with the group of either parent depending on the purpose, such as inheritance of property or ritual roles. Unilineal descent provides individuals with a clear identification with a social group that exists before birth and continues after death. Members of descent groups share a sense of identity, often referring to each other as "brother" or "sister" even when there is no genealogical relationship. Exogamy, or marriage outside the group, is often a characteristic of descent groups.
William D. Hamilton's "inclusive fitness theory" revolutionised evolutionary biology in 1964, proposing that natural selection promotes traits that permit an organism's genes to be handed along, regardless of whether the creature directly produces children. Inclusive fitness is a quality of actions or outcomes rather than a trait of individuals or organisms.
It is defined as the total of a person's reproductive success plus the consequences of their activities on the reproductive success of their genetic relations. According to inclusive fitness theory, selection will favour mechanisms that result in kin investment based on genetic relatedness. The researchers used the term care for your well-being so that participants would think widely about the numerous advantages they may get.
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