Evolution, the Extended Synthesis

2 - Reconsidering the Importance of Chance Variation

2 Reconsidering the Importance of Chance Variation John Beatty In his book Wonderful Life,Stephen Gould offered the following thought experiment in order to express what he took to be the highly contingent nature of evolutionary outcomes: I call this experiment “replaying life’s tape.” You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past. . . .Then let the tape run again and see if the repetition looks at all like the original. (Gould 1989: 48) His expectation was that “any replay of the tape would lead evolution down a pathway radically different from the road actually taken” (p. 51). I will focus here on one particular source of contingency, namely, chance variation and the order in which it appears. Gould’s emphasis on contingency was part of his case against the all-importance of natural selection. This chapter is also in part about how chance variation—as a source of contingency—undercuts the all-importance of selection. This may sound odd. In discussions of relative importance, natural selection and chance variation generally go hand in hand: natural selection of chance variations, conceived as one process, is usually said to be a more important cause of evolution than any other putative cause, for example, directed variation. To the extent that the importance of chance variation has been distinguished from, and compared with, that of natural selection, the latter usually comes out the overwhelming winner. I will discuss the tradition and line of reasoning, from Darwin throughout the Modern Synthesis, that completely subordinates the importance of chance variation to that of natural selection. This will set the stage for introducing what Gould considered to be the very “essence of Darwinism”: the conviction that selection, not variation—certainly not chance variation—gives direction 22 John Beatty to evolution and is the source of whatever creativity we might attribute to the evolutionary process. And yet, as I will discuss, there are in Darwin’s early work some very nice examples of how the particular pathways and outcomes of evolution may be due—and are perhaps often due—to the chance order in which variation appears. But Darwin subsequently changed his mind about this. The very strong position that he ultimately adopted concerning the sole direction-giving and creative role of natural selection, and the imagery he employed in this regard, were in turn adopted and reinforced by twentieth-century Darwinians. Recently, though, there have been a number of studies that effectively “replay life’s tape,” and suggest that the significance of chance variation and the order of variation, relative to natural selection, may need to be reassessed or even reconceived. Lurking in the background throughout this chapter is the bogey of “directed variation.” There is, I think, a common perception that the generation of variation is important only to the extent that it is predictably ordered (or biased or constrained, etc.). This seems to be assumed by proponents of the importance of natural selection, as well as by proponents of the importance of directed variation. But the post facto order of chance variation can be important as well. Variation (per se) is thus even more important than proponents of directed variation have recognized. Darwin’s Invention and Subordination of Chance Variation That chance variation is a source of evolutionary contingency is hardly a recent discovery. Darwin provided striking illustrations, especially in his work on orchids. But Darwin hardly championed the importance of chance variation relative to natural selection, and in fact accorded it less and less significance in ways and for reasons that are worth recounting. What importance it had for Darwin was always its role in championing natural selection, especially relative to directed variation. I would even say that Darwin came to the notion of chance variation, as we now understand it, in large part through the realization that such a conception of variation would enhance the role of natural selection, again especially relative to directed variation. In the Origin, Darwin explained that he used the term “chance” merely to signify his (and others’) “ignorance” of the causes of variation. He was prompted to give a more positive characterization of chance variation largely in response to the suggestion that he had overemphasized the importance of selec- Reconsidering the Importance of Chance Variation 23 tion, and had failed to consider the possibility that the generation...