W. D. Hamilton believed in the concept of ______, which includes fitness of relatives.

• A. Inclusive fitness
• B. Kin selection
• C. Group selection
• D. Direct fitness

Answer: (A)

Inclusive fitness is the idea that an individual's genetic success is measured not only by their own offspring but also by the impact they have on the reproductive success of relatives who share genes with them. W. D. Hamilton introduced this framework, showing that helping relatives can increase an individual's genetic contribution to future generations because relatives share many of the same genes. This combines direct fitness (personal reproduction) with indirect fitness (effects on kin), so altruistic acts can be favored by natural selection when the benefits to relatives, weighted by relatedness, outweigh the costs to the actor. Kin selection is the mechanism by which such helping of relatives evolves, while group selection is a different idea about selection at the group level and doesn’t capture the gene-centered focus of Hamilton’s theory. Direct fitness alone misses the kin-related component, so it’s not the concept described here.