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Symbiotic associations (i.e. cooperative associations of heterospecific individuals) are often highly integrated and well-defined units. Yet, because of their intrinsic heterogeneity, it is unclear whether they should be granted with any sort of ontological status in evolutionary theory. In this paper, I address the related issue of their status as Darwinian individuals. To this end, I first distinguish the notion of a Darwinian individual from the notion of a unit of selection. I then envisage whether some symbiotic associations may fall under one or both of these categories. Finally, I conclude that instances of both types can be found in nature, and provide some criteria for this category-belonging.

Le problème des unités de sélection porte sur la question de savoir quelles entités biologiques sont susceptibles de faire l'objet d'une sélection naturelle. Ce problème trouve ses racines dans l'oeuvre de Darwin (1859, 1871), lequel, en s'interrogeant sur l'existence de caractères apparemment contre-adaptatifs au sein de certaines espèces (tels que les comportements de sacrifices chez les insectes sociaux ou chez les humains), avait conclu que la sélection naturelle ne devait pas seulement agir sur les organismes, mais aussi sur des organisations biologiques plus larges (colonies, clans, etc.).

International audience

International audience

International audience

International audience; Organisms constitute one of the most remarkable features of our living world. However, they have not yet received any accepted characterization within the framework of the evolutionary theory. The reasons for this contrast between the saliency of organisms in the biological landscape and their theoretical status are multiple and they are analyzed in the first part of this paper. Starting from this contrast, I argue for a theoretically grounded concept of organism within the framework of evolutionary theory itself. To this effect I argue that the theory of major transitions in evolution (Maynard Smith and Szathmáry 1995; Michod 1999) provides us with the theoretical basis for an understanding of the individuality of organisms and I propose a first characterization of organisms as evolutionary units structured by a division of reproductive labor among their parts. I also discuss one of the most important implications of this definition, namely that some colonial entities are to be counted as superorganisms. Finally, I show that though theoretically satisfying, this definition does not suffice in order fully to individuate the organisms and superorganisms in practice. To this end, physiology is needed, because it offers us some criteria for their individuation in ecological space. These criteria, however, are not immune to errors through misidentification and their shortcomings are discussed in the last section. In conclusion, I emphasize the positive implications of these criteria concerning the ecological significance of organisms.