Johannes Martens

Darwinian indivisibility, egalitarian transitions and the 3-layered modely

In this presentation, I will address an important contrast between two (broad) construals of the notion of an evolution transition in individuality (ETI), itself rooted in a more fundamental opposition between two conceptions of a Darwinian individual. As is well known, an ETI is usually represented as a process during which most of the selective forces happen to be “transferred” from the lower to the higher level, so that, at the end, the bulk of the adaptations end up being concentrated at the level of the collectives, and the entities at the lower level turn out to be largely “de-Darwinized” (Michod 1999; Godfrey-Smith 2009, 2013; Clarke 2016). On this account, an ETI is thus equivalent (at least at a first approximation) to the emergence of a distinctive population of “Darwinian indivisible units”—that is, a population of Darwinian individuals that are no longer made of (actualized) Darwinian individuals.

So far, this theoretical definition—together with the notion of “Darwinian indivisibility”—has often been employed (more or less explicitly) as a general and fruitful heuristic to think about the nature and variety of ETIs. But the very idea of a “Darwinian indivisible unit” (which I borrow here, and with some modifications, from Godfrey-Smith’s 2013 paper) is also quite equivocal; for two different “measures” can actually account for the presence of an “indivisible core” at the collective level. The first, as I will show, emerges from a classical trend in the unit-of-selection debates (Williams 1966; Okasha 2006; Sober and Wilson 2011), and stipulates—very roughly—that a Darwinian indivisible unit should be the ultimate beneficiary of the adaptations that it possesses—with the crucial requirement that rb > 0 (Birch 2018). The second, by contrast, derives from the assimilation of the class of Darwinian individuals to a particular subclass of reproducers, and emphasizes the transfer of the capacity of reproductive autonomy from the lower level to the organismal level (Griesemer 2000; Godfrey-Smith 2009, 2013, 2015).

Prima facie, these two measures stand out as legitimate (and desirable) criteria that any Darwinian indivisible unit should satisfy. But both, in practice, disagree upon the things which fall under the extension of the “ETI” concept, leading to an apparent inconsistency in the very characterization of its domain. Thus, the “beneficiary requirement”—properly understood—doesn’t allow for the recognition of any symbiotic (i.e. “egalitarian”) instances of this concept (since r = 0), whereas the “reproductive autonomy requirement” is more general, and admits of a non-empty set of egalitarian transitions, such as the “classic” mitochondria/eukaryotic cells association—though at a parsimony cost.

In this talk, my goal will be to propose a kind of “best-compromise” solution to this puzzle (parsimony vs. generality), by developing a theoretical model which not only accommodates the two aforementioned measures, but also reconciles the need to include some (uncontroversial) egalitarian transitions with the kind of parsimony considerations relative to the beneficiary requirement. To this end, I shall argue that, contrary to appearances, these two “measures” do not embody mutually exclusive interpretations of the concept of a Darwinian indivisible unit, but represent rather different “kinds” of such units, which both need to be integrated into a specific sort of 3-layered compositional structure (the units of the first kind being the proper parts of the units of the second kind) to fully account for the case of egalitarian ETIs.