Alison McConwell (University of Calgary)
Major transitions are events that occur at the grand evolutionary scale and mark drastic turning points in the history of life. They affect evolutionary processes and have significant downstream consequences. Historically, accounts of such largescale macroevolutionary patterns included progressive directionality, new levels of complexity, and emerging units of selection all toward human existence (Huxley 1942, Stebbins 1969, Buss 1987).
In more recent models, human-centrism is less common, however, it’s not clear all events are of the same kind (Maynard-Smith and Szathmáry 1995, O’Malley 2014, Calcott and Sterelny 2011). The lack of unity is identified as a failure to “get serious about evolution at the macroscale” (McShea and Simpson 2011, 32). Disunity allegedly yields inconsistencies in our explanations, as well as an arbitrary collection of events, or “just one damn thing after another” (ibid, 22, 32). Against this, I argue for a pluralist view of major transitions, which yields a productive disunity.
Epistemically, that all major events have a common property might be explanatorily useful. To unify major events under a single explanatory framework is supposed to reveal something about the robustness and stability of evolutionary processes, and their capacity to produce the same types of events over time. However, this unificatory aim concerning models of transitions is not the only fruitful approach. Setting unification aims aside provides the opportunity for detailed investigations of different transition kinds. Major transitions are diverse across life’s categories, scales, and can vary according to scientific interest. I draw on work from Gould (1989, 2001) who argued for chance’s greater role in life’s history; he denied both directionality and progress in evolution and focused on the prevalence of contingent happenstances. His research on evolutionary contingency facilitated an extensive program, which has primarily focused on the shape or overall pattern of evolutionary history. That pattern includes dependency relations among events and the chance-type processes (e.g. mutation, drift, species sorting, and external disturbances) that influence them. Gould’s evolutionary contingency thesis grounds a contingent plurality of major transitions kinds.
Specifically, I argue that the causal mechanisms of major transitions are contingently diverse outcomes of evolution by focusing on two case studies: fig-wasp mutualisms and cellular cooperation. I also discuss how chance-based processes of contingent evolution, such as mutation, cause that diversity. And finally, I argue that this diversity can be classified into a plurality of transition kinds. Transition plurality is achieved by attention to structural details, which distinguish types of events. Overall, there is not one single property, or a single set of properties, that all and only major transitions share. On this picture, one should expect disunity, which facilitates a rich understanding of major shifts in history. Unity as an epistemic virtue need not be the default position. The lack of a common thread across transition kinds reveals something about the diversity and fragility in evolution, as well as the role of forces besides natural selection driving the evolutionary process. Overall, to accept a disunified model of major transitions does not impoverish our understanding of life’s history.
