Inclusive fitness explains altruistic behavior in animal communities by taking into account genetic relatives. It allows for behaviors that pass on common genes, rather than just an organism’s own genes. The costs and benefits of altruism are measured by the degree of relationship, cost, and benefit.
Inclusive fitness provides an evolutionary explanation for altruistic behavior among animal communities. When we think of “survival of the fittest,” we associate selfish behavior with organisms that are always trying to live longer and reproduce more successfully. But inclusive fitness takes into account close genetic relatives in the larger mission of passing on common genes, rather than just one organism’s genes. The principles of Darwinian evolution are not violated.
When WD Hamilton conceived of inclusive fitness in 1964, he was thinking about the reasons why some animals seem to do things that benefit someone above themselves. Perhaps he envisioned ant colonies where sterile workers work endlessly for the benefit of the queen and the colony in general, with no hope of reproducing. Wasn’t this in contradiction with the fundamental principle of evolution that the goal of an organism is to live long enough to reproduce, and that the reproducers are the strongest of the group?
Instead of limiting an organism’s drive to reproduce only its own exact genetic code, Hamilton broadened his desire to include behaviors that allowed the passage of genes closely related to it. This means that the animals act primarily to protect themselves, but they will also act to protect their immediate family members, such as parents and siblings, because they share some of its genes. In rarer circumstances his behavior will protect other relatives, such as cousins and siblings of the parents. When the costs do not outweigh the benefits, we observe so-called altruism in many animal communities which is best explained by inclusive fitness.
For example, prairie dogs will work cooperatively to seek out predators. If one spots a snake, a warning will sound so that others can take cover. However, by making a noise, the prairie dog actually calls attention to itself and the snake is more likely to attack it. Inclusive fitness makes sense of this counterintuitive behavior by pointing out that the prairie dog’s relatives, those who share many of its genes, will be at an advantage to survive and reproduce. In this sense, “fitness” means the likelihood that a genome, a set of genes, will be passed on to the next generation.
Those who study inclusive fitness have developed a handy equation that shows when the costs of altruism outweigh the benefits or not. R stands for the degree of relationship between two individuals. R is highest when they are close relatives, such as full siblings, and lowest when they are distant relatives. C is the cost to the individual if he risks the behavior, represented as the probability that he will fail to reproduce. Finally, B is a number that measures the benefit the altruistic act provides to the recipient. Therefore, if R times B minus C is greater than zero, the animal will act “altruistically” in the name of inclusive fitness.
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