text: Under natural selection , a gene encoding a trait that enhances the fitness of each individual carrying it should increase in frequency within the population ; and conversely , a gene that lowers the individual fitness of its carriers should be eliminated . However , a hypothetical gene that prompts behavior 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 , behavior which enhances the fitness of relatives but lowers that of the actor , may nonetheless increase in frequency .