What term describes an action that negatively impacts the actor's fitness while benefiting another individual?

The correct term that describes an action negatively affecting the actor's fitness while benefiting another individual is altruism. In evolutionary biology, altruism refers to behaviors in which one organism acts to benefit another at a cost to itself. This concept is often observed in social animals, where individuals may sacrifice their own well-being or reproductive success to help others, such as in the case of cooperative breeding, alarm calling, or food sharing. The apparent contradiction of self-sacrifice contributing to the overall success of the species raises interesting questions in evolutionary theory, particularly how such traits can evolve under natural selection.

In contrast, mutualism involves interactions where both parties benefit, selfishness denotes behaviors that benefit the actor at the expense of others, and competition refers to individuals vying for the same resources, which typically does not include self-sacrifice. Therefore, altruism stands out as the term that precisely captures the essence of benefiting another while incurring a personal cost.