Précis of Evolution and the Levels of Selection

Abstract

The ‘levels of selection’ question is one of the most fundamental in evolutionary biology, for it arises directly from the logic of Darwinism. As is well-known, the principle of natural selection is entirely abstract; it says that any entities satisfying certain conditions will evolve by natural selection, whatever those entities are. (These conditions are: variability, associated fitness differences, and heritability (cf. Lewontin 1970).) This fact, when combined with the fact that the biological world is hierarchically structured, i.e. smaller biological units are nested within larger ones, gives rise to the levels of selection question. For in principle, entities at many different hierarchical levels, above and below that of the ‘individual organism’, (e.g. gene, chromosome, cell, kin group, colony, lineage, species) can satisfy the requirements for Darwinian evolution. This possibility has long been recognised by biologists, from Darwin himself to contemporary proponents of ‘multi-level selection’; and there exist numerous biological phenomena which suggest that it has actually occurred. Biological altruism, in which one organism performs a behaviour which reduces its own chance of survival ⁄ reproduction but benefits that of others, is an example of a phenomenon that, prima facie, is indicative of selection occurring at a level other than that of the individual organism. For altruism, by definition, is individually disadvantageous, and yet is common in the animal kingdom. Darwin himself suggested that altruism may have evolved by group-level selection, i.e. groups containing many altruists out-performed groups containing fewer, offsetting the individual cost of behaving altruistically. This suggestion, though controversial, is still taken seriously by many contemporary biologists; and the more general link between altruism and levels of selection remains as intimate as in Darwin’s day.