Multilevel Selection

Organism

At the level of the organism, the frequency of a trait in a population is more common in the next generation when the trait increases the reproductive fitness of the organism.

Gene

At the level of the gene/allele, the frequency of a trait in a population is more common in the next generation when the trait increases the reproductive fitness of all the organisms sharing that allele.

Group

At the level of the group, the frequency of a trait in a population is more common in the next generation when the trait increases the reproductive fitness of the organisms within that group over the organisms in another group.