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2. Alternative Patterns of Explanation for Major Transitions
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2Alternative Patterns of Explanation for Major Transitions Brett Calcott Two central claims in Maynard-Smith and Száthmary’s book are that (a) some events in evolutionary history are special because they changed what was subsequently possible to evolve, and (b) these events share many similar properties. The goal in this chapter is to explore this second claim, to assess what these claims of similarity amount to, and how and why we can make them. My goal is different from that of McShea and Simpson (this volume), who look for similarity in the outputs of these transitions; they wonder, for example, whether all the transitions increase hierarchical complexity. Instead, I shall focus on similarities in the mechanisms responsible for bringing about the transitions identified by Maynard-Smith and Száthmary. Much of the subsequent literature has identified the common mechanism as an especially potent form of multilevel selection; potent enough to suppress the perennial threat of defection. The main point of this chapter is to highlight additional ways that these transitions might be unified by the causal mechanisms that explain them. Though not widely acknowledged, Maynard-Smith and Száthmary identified a number of similarities across the major transitions in addition to the problem of differing levels of selection (Maynard Smith and Szathmáry 1995, 12). The arguments I give here suggest that exploring these—and other possibilities—would provide a richer understanding of the major transitions. To make good on the project, I need to outline a general model of explanation and show how, in biology, distinct explanatory projects coexist and can complement one another. I first show that claims of similarity across the transitions are best understood as broad explanatory generalizations. The properties that are similar pick out key factors that enabled particular transitions to occur, yet they do so in a way that remains abstract enough to apply across diverse events. In doing so, they both explain and unify the recurring pattern of transitions. I then join this observation with a familiar idea: that there are different kinds, or patterns, of explanation in biology. Some well-known ways of carving these up are Mayr’s proximate and ultimate explanations,1 and Tinbergen’s four questions (Mayr 1961; Tinbergen 1963). These distinctions capture an important fact: Even when we identify a particular 36 Brett Calcott biological phenomenon, we can still go about explaining it in a number of ways. I show that different patterns of explanation are applicable to the kinds of evolutionary change identified as major transitions, too. I do this by examining three explanations for a single transition: the evolution of multicellularity in Volvox carteri. Together, these two ideas show that there are multiple ways of identifying similarities that unify the various major transitions. Once alternative explanatory options are laid out, it is clear that the work on the major transitions concerning cooperation and the levels of selection has focused on a single pattern of explanation, while others have largely been ignored. I suggest that a richer account of major transitions is possible if we deploy multiple patterns of explanation, as these different explanatory hypotheses can interact and mutually constrain one another. I begin the chapter with a brief summary of a manipulationist account of explanation, which is well suited to capturing how many explanations work in biology (Hitchcock and Woodward 2003; Woodward 2003; Woodward and Hitchcock 2003).2 This preamble provides a unified foundation for thinking about explanation, for, although there are different patterns of explanation, they all share a core structure. Explanatory generalizations: A Short Plausible Account In this section, I use a simple biomechanical example to examine the structure of explanatory generalizations. Once the basic structure is laid out, it will be clear how it fits other kinds of biological explanation. In the next section I show how it fits explanations for major transitions, too. Let’s say we want to explain how a kangaroo jumps. A typical response might mention a number of factors: how the tail is used as a counterbalance, how the alignment of the enlarged fourth toe with the leg bone serves to drive the jump, and how the elastic tendons in the ankle store and release the energy to aid the jump.3 Given a target of explanation (how the kangaroo jumps), we respond with a set of difference-makers (the tail, the toe, and the tendons), and some generalizations about how these difference-makers affect the target of explanation.4 Why do these factors explain how a...