The Kinship theory is founded on work by Bill Hamilton and Bob Trivers, who first introduced the highly influential topics of inclusive fitness and parent-offspring conflict (and sibling-sibling conflict) into evolutionary biology, respectively. In 1964, Hamilton proved mathematically that a gene could increase its frequency in the population by prompting individuals to assist in the survival and reproduction of related individuals (who share a percentage of genes). In other words, inclusive fitness is not only the sum of how many offspring an organism can produce and support but also accounts for the number of equivalent offspring through extended relatedness.
Using this gene-counting approach to kinship, J.B.S Haldane, a geneticist and evolutionary biologist, once remarked that he would be willing to risk his life for the survival of 2 of his brothers or 8 of his cousins. What did he mean by this?
From an evolutionary perspective, kin are worth assisting in direct relationship to their blood relatedness, or the probability that two individuals share genes (r). For example, two siblings share approximately 50% of their genes with one another (as a single gene either came from the shared mother or father). Depending on the cost (c) to the individual and the benefit (b) accrued by the kin with a certain measure of relatedness (r), a gene may increase in frequency if it promotes altruistic behavior.
J.B.S Haldane was remarking on the fact that the combined coefficient of his relationship among 8 first-cousins (0.125 each), 2 brothers (0.5 each), and himself, is identical—1. Assuming identical costs and benefits, J.B.S Haldane correctly deduced that he would increase his inclusive fitness equally in either scenario, aiding in the survival of 8 first-cousins or 2 brothers. Instances of altruism, in which organisms aid in the survival of kin, are easily explained under this premise. (Altruism among non-related individuals (0) is called reciprocal altruism and can be a stable evolutionary practice if certain conditions are met.)
Hamilton’s equation radically transformed how evolutionary biologists perceive family dynamics and led to Bob Trivers’s views on parent-offspring conflict.
For parents, a large relatedness value of 0.5 creates a strong pressure for them to assist in the survival of their offspring. Remember, natural selection will favor genes that promote parents to raise as many offspring as possible over the course of their lives. Though a mother or father could dispense every available resource to one offspring, it must also consider limitations on resources and the potential to conserve its energy for producing future offspring (lifetime reproductive success). A parent, after all, is more related to itself than any of its offspring.
For many mammals, such a dynamic has profound implications for a pregnant mother and a developing fetus and postbirth interactions concerning milk production and weaning. Parent-offspring conflict, under extreme conditions, can even lead to infanticide.
The evolutionary perspective of the offspring complicates the situation. An offspring is more related to itself (1) than any of its siblings (0.5) and will consequently try to secure as many resources from its parents as possible. However, the fact that a parent is equally related to each of its offspring sets up a conflict among the siblings. Parents will allocate resources accordingly, but siblings will compete with one another, especially in scenarios where resources are limited. For example, a chick that cries the loudest and most often has been shown to outcompete its siblings for resources from the parents.
Though there are countless examples of sibling rivalry, perhaps the most striking and extreme example concerns obligate siblicide among brown boobies. The brown booby regularly hatches two eggs, but rarely does more than one chick survive. These chicks hatch, on average, 5 days apart. The senior chick is much larger and generally will force its sibling out of the next, killing the subordinate within 6 days of hatching. Why would any parent bother to produce and invest energy in a second egg that almost certainly will not fledge from the nest?
Researchers believe the second egg acts as an insurance policy for the parent, in case the first one does not survive. With limited resources, the eldest sibling is under evolutionary pressure to murder it sibling because it is more related to itself than its sibling. Guided by natural selection, genes that encourage parents to allow the death will outcompete those who prevent the death –there are not enough resources to promote the survival of both offspring.
Though it can be difficult at first to think in these mathematically terms, inclusive fitness, parent-offspring conflict and sibling-sibling conflict have radically transformed our understanding of animal behavior and evolution.