Introduction: Multilevel Selection Theory Comes of Age

The concept of adaptation is a powerful tool for understanding the properties of organisms. If organisms have become adapted to their environment by natural selection, then principles of functional design can be used to predict how they are structured and how they behave, often in minute detail. Organisms are not invariably well adapted to their environment, and the analysis of adaptations is often complicated by historical factors and other constraints, but we have undoubtedly learned a tremendous amount about nature by reasoning on the basis of adaptation and natural selection. Using the concept of adaptation requires a clear definition of the terms organism and environment. Superficially, it seems that we can easily define an organism as a single creature, bounded by its skin, and the environment as everything outside the organism. However, organisms are composed of subunits, such as genes, and are themselves subunits of larger entities such as social groups, species, and multispecies communities. Do these other units qualify as organisms, and if not, why do single creatures enjoy such a unique status in the hierarchy of life? These questions, which are surely among the most fundamental that can be asked in evolutionary biology, have a long and turbulent history. Before Darwin, all of nature was often portrayed as being like an organism in the harmony and coordination of its parts. The explanation, of course, was that nature was the creation of God. Darwin’s theory explained functional organization as the product of natural selection, which he envisioned as a process that operates among individuals. Individuals vary in their properties, some of the differences have consequences for survival and reproduction, and offspring resemble parents, resulting in adaptation to the environment over a number of generations. Unfortunately, adaptation at the individual level has a number of disturbing consequences. Individuals who exploit their neighbors would seem to be adaptive according to Darwin’s criteria, whereas individuals who help their neighbors should quickly go extinct. Adaptation at the individual level can create highly dysfunctional groups. The seemingly immoral outcome of the struggle for exis-