Why flying dogs are rare: A general theory of luck in evolutionary transitions

Highlights

• Evolutionary transitions involve a discontinuity in fitness.

• First steps in evolutionary transitions involve a nonselectionist chance process.

• We provide a systematic account of evolutionary transitions that is non-selective.

• Our account is based on a second-order regularity of chance events.

Abstract

There is a worry that the ‘major transitions in evolution’ represent an arbitrary group of events. This worry is warranted, and we show why. We argue that the transition to a new level of hierarchy necessarily involves a nonselectionist chance process. Thus any unified theory of evolutionary transitions must be more like a general theory of fortuitous luck, rather than a rigid formulation of expected events. We provide a systematic account of evolutionary transitions based on a second-order regularity of chance events, as stipulated by the ZFEL (Zero Force Evolutionary Law). And in doing so, we make evolutionary transitions explainable and predictable, and so not entirely contingent after all.

Introduction

It is a historical fact that hierarchical complexity has increased over the history of life on Earth. That is, early life began as small simple individuals, perhaps like modern cyanobacteria, and not only did individuals become more complex at a given level, but new levels of hierarchy were formed. Some cells formed colonies, some formed multicellular organisms, then some multicellular organisms joined together and became societies, and some of these societies joined together to form super-colonies (although this only rarely) (cf. Giraud et al., 2002, McShea, 2001, McShea and Changizi, 2003). This rise in hierarchical complexity is the subject of what has come to be known as the major transitions in evolution. This name comes from the highly influential book by John Maynard Smith and Eros Szathmáry (1995). They quite correctly pointed out that explanations of the major transitions in evolution face a special problem: “One feature is common to many of the transitions: entities that were capable of independent replication before the transition can replicate only as part of a larger whole after it” (1995, p. 6). But this raises the question: “Why did not natural selection, acting on entities at the lower level (replicating molecules, free-living prokaryotes, asexual protists, single cells, individual organisms), disrupt integration at the higher level (chromosomes, eukaryotic cells, sexual species, multicellular organisms, societies)?” (1995, p. 7). We agree with Maynard Smith and Szathmáry on the problem raised by hierarchical transitions in evolution, but we think they, and the large literature that has grown up around this issue, fail to fully appreciate what is required for a solution.

In this paper we will make explicit two points imbedded in the above quotes. These points, we will argue, are both correct and widely accepted as such. From these two points we will then draw a logical conclusion that, as far as we know, has never been noted before. Some will see this new point as bad news. The bad news is this: selection cannot possibly explain the crucial early steps in an evolutionary transition; that step necessarily involves a nonselectionist chance process. But if it is just chance, then can anything systematic be said about evolutionary transitions? Is there theoretical unity, or just a “series of miscellaneous transitions”? (McShea & Simpson, 2011). The latter part of this paper shows that we can say something systematic about (at least some such) evolutionary transitions by harnessing a second-order generalization governing certain probabilistic evolutionary processes. This generalization has been termed the ZFEL (Zero Force Evolutionary Law) by Dan McShea and Robert Brandon (2010). The ZFEL pulls order out of chaos and in this application shows how a necessary early stage in evolutionary transitions is made more probable, and thus explainable and predictable, and so not totally contingent.