Abstract

The idea that natural selection acts on many levels—and not only at the level of organisms or individual genes—is increasingly accepted among biologists. However, it is not easy to reconcile this idea with the strictly “individualistic” conception of the evolutionary process that has always characterized Darwinian thought. Moreover, the individuality of some forms of life is a vague concept and therefore controversial. This is the case of Candidatus Magnetoglobus multicellularis, whose discovery immediately inspired the following question: Does the concept of individuality have degrees? Alternatively, how far is this structure of prokaryotic cells from deserving to be called an organism? In this article, we propose a new conceptual scheme based on an idea of individuality that is not limited to organisms and that makes sense in terms of Darwinian evolution. In this conceptual scheme, selection at levels above that of the individual organism is interpreted as the evolutionary emergence of higher-level individuality. This proposal may serve as a basis on which to construct a promising hierarchical evolutionary theory.