Original: However , a gene that prompts behaviour which enhances the fitness of relatives but lowers that of the individual displaying the behavior , may nonetheless increase in frequency , because relatives often carry the same gene ; this is the fundamental principle behind the theory of kin selection .
Simplified: However , a gene that prompts behaviour which enhances the fitness of relatives but lowers that of the individual displaying the behavior , may nonetheless increase in frequency - relatives often carry the same gene , after all .