Levels, Time and Fitness in Evolutionary Transitions in Individuality

Abstract

In this paper, I identify two major problems with the model of evolutionary transitions in individuality (ETIs) developed by Michod and colleagues, and extended by Okasha, commonly referred to as the "export-of-fitness view". First, it applies the concepts of viability and fertility inconsistently across levels of selection. This leads Michod to claim that once an ETI is complete, lower-level entities composing higher-level individuals have nil fitness. I argue that this claim is mistaken, propose a correct way to translate the concepts of viability and fertility from one level to the other and show that once an ETI is complete, neither viability nor fertility of the lower level entities is nil. Second, the exportof-fitness view does not sufficiently take the parameter of time into account when estimating fitness across levels of selection. As a result fitness is measured over different periods of time at each level. This ultimately means that fitness is measured in different environmental conditions at each level and misleads Okasha into making the claim that the two levels are ontologically distinct levels of selection. I show that once fitness is measured over the same period of time across levels, the claim about two levels of selection can only be an epistemic one.

Meta-population structure and the evolutionary transition to multicellularity

“…Selection over the long timescale caused the fitness of mats to 109 increase (as determined by the relative ability of mats to give rise to offspring mats), 110 while fitness of the individual cells comprising mats declined (when measured relative to 111 ancestral types). This can be understood in terms of selection over the longer timescale 112 trumping the effects of individual cell selection: over the long-term, successful cells are 113 those whose fitness aligns with the longer timescale defined by the longevity of the 114 nascent multicellular organism (Bourrat, 2015;Black et al 2020). Such an alignment of 115 reproductive fates during the transition from cells to multicellular organisms has been 116 referred to as "fitness decoupling" (Michod and Roze 1999) -a term that captures the 117 sense that when selection comes to act over the longer timescale, fitness of the lower 118 level particles "decouples" from that of the higher level collective.…”

Group and individual selection during evolutionary transitions in individuality: meanings and partitions

“…The meaning of 'cell fitness' in these models has been the subject of some commentary. Godfrey-Smith [56] and Bourrat [57] argue against the idea that actual cell fitness within a colony goes to zero at the end of the transition, a position that they attribute incorrectly to Michod [58]. 7 In the modelling paper being described in Michod [58], Michod et al [55, p.…”

The evolution of individuality revisited

“…Studying this so‐called ‘export of fitness’ is generally challenging, and there are difficulties applying the concept of individual fitness consistently across levels of organisation and different timescales, e.g. multicellular organism generations or single cell generations, as argued by Bourrat (). In addition, while important to our understanding of germ–soma evolution, mechanisms of selfish cell policing, and the evolution of development, these models are less insightful about the mechanisms driving the initial stages of the transition starting with independent solitary cells.…”

“…This can happen, for instance, when groups evolve germ–soma differentiation or developmental regimes with reproductive bottlenecks. Again, there is some discussion regarding the measurement of fitness and the timescales involved (Bourrat, ), as well as the general utility of Okasha's framework. Waters (), for instance, argues that the distinction of two types of multilevel selection is purely conceptual and merely offers a convenient model, but does not reveal anything fundamental about the nature of evolutionary transitions.…”