Dictyostelium discoideum. Photographed by Usman Bashir (Queller/Strassmann Lab, Washington University in St. Louis). CC-BY-SA-4.0 License. Image obtained from Wikimedia Commons.

In the Major Transitions class, the students keep pointing out that the transitions on Maynard Smith and Szathmáry’s list come in two flavors with very different properties. Sure, there are some important similarities between multicellular organisms and social insects, but they are quite different from cellular slime molds and the conspiracy of prokaryotes that make up eukaryotes.

As it turns out, this distinction has been recognized for a long time; almost as far back as the book itself. In 1997, David Queller wrote a review of The Major Transitions in which he points out that the principles underlying transitions in which the lower-level units are closely related are likely to differ from those involving unrelated individuals. Queller called the former transitions ‘fraternal’ and the latter ‘egalitarian’, taking his inspiration from the rallying cry of the French Revolution, “Liberte, egalite, fraternite.” Liberty, in Queller’s context refers to organisms that go it alone, rather than combine into the kinds of collectives with which Maynard Smith and Szathmáry are concerned. Equality (‘egalite’) refers to transitions involving unlike or unrelated units, and fraternity those involving closely related units.

Since my students kept converging on Queller’s distinction, I thought it would be worth assigning the 1997 paper. This post isn’t about the main point of the paper, though; rather, it’s about something near the end that caught my attention (and that I had previously missed). I’ve written before about philosophers’ efforts to define biological individuality (see for example here, here, here, and here). Queller prefers to use ‘organism’, but I think his meaning is not too different from what I mean by ‘individual’.

Our tendency to think of the organism as one of the levels in the hierarchy of life does not stand up to scrutiny. We find prokaryotic organisms, eukaryotic assemblages, multicellular eukaryotes, and even organismal colonies. We designate something as an organism, not because it is n steps up on the ladder of life, but because it is a consolidated unit of design, the focal point where lines of adaptation converge. It is where history has conspired to make between-unit selection efficacious and within-unit selection impotent. There may be a few active infra-organismal agents, relics of the old order or renegades within the new, but these seem to be remarkably rare. There may also be some supraorganismal cooperation between organisms, but it is haphazard; you cannot pick a level above the organism and expect to see all cooperation and no conflict. If that is what you see, as arguably you do in some social insect colonies, then to maintain consistency I would suggest that you are looking at an organism. All that seems to stand in the way is the lack of a physical connection among the constituent parts, and that strikes me as, well, immaterial. [my emphasis]

Nicely put, and it’s essentially the same view that Queller and Joan Strassmann advocated, in much greater detail, just a few years ago (Queller & Strassmann 2009, Strassmann & Queller 2010). For a book review, this paper has been unusually influential, not so much for the part I’ve quoted here but for the distinction between fraternal and egalitarian transitions. Biologists and especially philosophers of science who think about the major transitions recognize this as an important difference. Queller expanded on it in 2000 (the more recent paper is more often cited, but the book review has priority). Corina Tarnita and colleagues recently modeled both sorts of transitions (oddly, the terms ‘fraternal’ and ‘egalitarian’ don’t show up in this paper).