Inclusive Fitness

Inclusive fitness is an individual's total ability to pass down their genes both directly through offspring and indirectly by helping close relatives reproduce. Worker honeybees exemplify this: though they don't reproduce, their altruistic behaviors help their queen survive and produce offspring carrying shared genes. This indirect gene transmission increases the worker's inclusive fitness despite their own reproductive inactivity.

Kin selection is the natural selection of behaviors that increase the reproductive success of relatives. Most altruistic behavior occurs between relatives because helping them reproduce increases the helper's inclusive fitness. Hamilton's rule explains this by weighing altruism's cost against benefits proportional to genetic relatedness, making animals more likely to help close relatives who share more genes.

In eusocial colonies like bees and naked mole rats, non-reproductive members are closely related to the queen, typically her daughters, siblings, or nieces. By helping the queen reproduce, these workers increase their own inclusive fitness since they share a similar genetic makeup. Their altruistic care and protection of the queen and her offspring preserves their shared genes in the gene pool.

Genetic relatedness directly determines whether altruistic behavior will evolve between animals. Hamilton's rule shows that animals sharing more genes are more likely to engage in altruism toward one another because their inclusive fitness increases to a greater extent. The degree of relatedness acts as a proportional weight, making close relatives more valuable targets for altruistic investment than distant ones.

Worker honeybees increase their inclusive fitness indirectly by engaging in altruistic behaviors such as foraging for food that sustains their queen. Since workers are closely related to the queen and share many of her genes, the queen's reproductive success directly benefits the workers' genetic legacy. Their altruism toward the queen effectively passes down their shared genes through her offspring.

Hamilton's rule is a mathematical principle that weighs the cost of altruism against its benefits, adjusted for the degree of genetic relatedness between individuals. It predicts that altruistic behavior evolves when the benefit to relatives, multiplied by their relatedness coefficient, exceeds the cost to the altruist. This rule explains why animals are more likely to help close relatives than distant ones.

Direct reproductive success measures only genes passed through an individual's own offspring, while inclusive fitness includes genes passed indirectly through relatives. An animal can have zero direct reproductive success but high inclusive fitness by helping relatives reproduce. This broader measure explains why altruistic behaviors toward kin, despite personal reproductive costs, persist and evolve in populations.