Final answer:
Altruism increases the altruist's fitness by enhancing their inclusive fitness.
Step-by-step explanation:
Altruism and Individual Fitness
Altruism is a behavior in which an individual sacrifices its own fitness to benefit another individual.
In terms of evolutionary fitness, altruism can enhance the fitness of the individual performing the altruistic act if it is directed towards closely related individuals who share genes with the altruistic individual.
This is known as inclusive fitness. By helping close relatives survive and reproduce, the altruistic individual indirectly passes on its own genes, thus increasing its own fitness.
For example, in social insects like worker bees, the sterile workers maintain the queen and help in raising her offspring.
Although the workers themselves cannot reproduce, they share genes with the queen and her offspring.
By assisting in the survival and reproduction of closely related individuals, the workers increase their own inclusive fitness.