Abstract
A popular account of evolutionary transitions in individuality (ETIs) postulates a crucial change in the nature of fitness during an ETI. Fitness at the collective level is supposedly “transferred” or “decoupled” during the process (Michod, 2005; Okasha, 2006). Recently, this view of ETIs has been challenged on the grounds that it may be better to focus directly on traits as opposed to fitness (Bourrat et al., 2021). In this paper, I will attempt to reconcile these two views by attending to their distinct conceptions of fitness. Following one account, fitness is considered a complex trait whose measurement involves summing over the totality of an entity’s phenotypic traits (Brandon, 1990, Bouchard & Rosenberg, 2004). Following the alternative account, fitness is the long-term reproductive output of an entity (Sober, 2001). If one adopts the former approach to fitness, I will argue that it becomes possible to understand how fitness changes during an ETI. When fitness is a complex trait composed of all the other traits of an entity, it is unsurprising that its nature changes during a transition. This follows from the fact that the selectively relevant traits of individuals often change during a transition. For instance, during the transitions from unicellular to multicellular organisms, trade-offs between different traits of the unicellular organism no longer apply when they become part of a collective. However, this does not correspond to a “transfer” of fitness. It is simply a change in the relationship between traits and their environment(s). Further, it seems that this change in the nature of fitness is not reconcilable with the competing view of fitness as the long-term number of descendants.
