Final answer:
Kin selection is the term that describes how an individual's inclusive fitness is increased through the reproductive success of relatives, which can explain seemingly altruistic behavior in animals.
Step-by-step explanation:
The phrase that fills in the blank for this question is 'kin selection.' Kin selection refers to the evolutionary strategy that favors the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction.
This concept explains the seemingly altruistic behaviors observed in animals such as social insects, where individuals support the reproductive efforts of their relatives. These behaviors, although not directly beneficial to the individual's personal reproduction, increase their inclusive fitness by ensuring that the genes shared with their relatives are passed on to future generations.