Evolution on the level of communities.

According to traditional models, natural selection is largely insensitive to an organism's effect on its community. Effects on the community at large cannot feed back differentially to the organisms that cause them, and, hence, cannot lead to the differential fitness of the organisms. However, if a spatial variation exists in community composition, organisms do differentially feel their own effects on the community, and this leads to a form of evolution on the community level. Without violating the principle of individual selection, the concept of an organism that exists for the "function" its performs in its community may be valid in some cases.