Indexed on: 01 Sep '85Published on: 01 Sep '85Published in: Evolution
The unit that directly evolves under the action of higher-level natural selection "among species" must be the higher-level analogue of the population. Contrary to present formulations of "species selection," clades (or other higher taxa) do not fulfill the basic structural and dynamic criteria to be so considered. Clades are not localized, their members do not share an environment, and they cannot be said to respond to local selective regimes. Traditional species selection does not provide a causal mechanism for evolutionary change in terms of the interaction of the units of selection with a shared environment in the way that conventional organismic selection does; as used by some authors, species selection is a purely descriptive term. Communities do fulfill the criteria required by a theory of natural selection. Within communities, selection is among the populations of different species that make up the community, here termed "avatars" of those species. Avatars are the closest analogues of individual organisms in traditional selection theory. Just as populations evolve by organismic selection, communities evolve by avatar selection, and more inclusive units, the higher-level analogues of the species, evolve as their component communities do. This formulation of higher-level selection reveals a congruence with processes at the lower, organism-based level and suggests the most profitable direction to be taken in attempts at formal extension of selection theory.