Abstract

Nature is replete with borderline cases that fall somewhere between organisms and communities, such as lichens, biofilms, and the Portuguese Man-of-War. At first glance, the existence of such borderline cases might suggest that the concept of what constitutes an organism is too fuzzy to be useful in evolutionary biology. Yet, the notion of organisms is entrenched within central debates in evolution, including discussions over how fitness should be measured, what the bearers of adaptations and fitness are, and the status of holobionts. Finally, accounts of organisms can set the stage for explanations of how multicellularity evolved. In particular, characterizing multicellular organisms in terms of the level of cooperation between their cells suggests that their evolution required the presence of mechanisms that could suppress conflict between cells.