This is the final of three reviews of MIT Press books in The Vienna Series in Theoretical Biology dealing with convergent evolution, something I consider to be one of evolutionary biology’s most exciting topics. Are evolutionary changes happy accidents, i.e. contingencies? Or is there a law-like repeatability underneath, explaining why some traits evolve time and again, i.e. convergence? Is it even a matter of either-or? And what lessons does this hold for life elsewhere in the universe? Philosopher Russell Powell wrestles with these questions in a manner that is as rigorous as it is intellectually rewarding. Evolutionary biologists will want to give this excellent book a very close read.
Powell starts by examining the legacy of Stephen Jay Gould, the famous evolutionary biologist and science populariser who tragically passed away in 2002. In his book Wonderful Life, Gould proposed the thought experiment of rewinding the tape of life to see how it would evolve on repeat plays. Based on the exceptional Cambrian fossils found in the Burgess Shale in British Columbia, he argued that lifeforms would be radically different, contingent on small variations that amplified with time. Others criticised this idea by pointing to the many examples of convergent evolution, the ubiquitous pattern of evolution repeatedly hitting on the same or similar solutions to a problem in different organisms.
Before proceeding, however, Powell offers a surprising little detour into astrobiology. Since we cannot rewind the tape of life, the next best thing would be to see how life evolved elsewhere. Just some of the interesting topics considered are the anomaly of life’s single origin, the timing of the emergence of intelligent, multicellular life compared to the lifespan of our Sun, and the problem of the observer selection effect. Where I found the previously reviewed Convergent Evolution on Earth lacking in its lessons for astrobiology, Powell engages with this topic in a far more thorough manner. Despite decades of effort, the stars remain eerily silent. Which brings us back to contingency and convergence as the third best option to answer Gould’s questions.
But what, exactly, did Gould have in mind with evolutionary contingency? And does the criticism from convergence really engage with his argument? These are the kinds of deeper questions that Powell explores in the first half of his book.
“Some, […] have interpreted contingency as meaning non-repeatability, others […] have equated it with unpredictability. Neither is correct, writes Powell.”
See, the problem is that Gould never clearly defined contingency, nor used it consistently. His idea remains, as philosophers put it, underspecified. Like others before him, part of Powell’s mission here is to piece together what Gould intended, based on a close reading of especially Wonderful Life and the magisterial The Structure of Evolutionary Theory. Laying bare the theoretical underpinnings of, as he dubs it, Gould’s “radical contingency thesis” is an incredibly instructive exercise. Also because it explains how Gould has been misinterpreted and why the critique from convergence often descends into “dismantling “straw man” versions of Gould’s thesis” (p. 101).
Some, prominently Simon Conway Morris and George McGhee, have interpreted contingency as meaning non-repeatability, others (myself included) have equated it with unpredictability. Neither is correct, writes Powell. Gould’s argument of contingency applied specifically to conditions in the early Cambrian: “small changes [here] would have led to a very different initial occupation of morphospace” (p. 43). But he acknowledged that after this many body plans became “developmentally entrenched”, something that has been confirmed by research in the field of evolutionary developmental biology, or evo-devo. Evolution prefers the easier option of reusing and repurposing existing structures. So, with time, the layers upon layers of interacting genes and regulatory networks laying out gross body morphology became too hot to touch for evolution, with mutations often being lethal.
Powell here makes a distinction between deep and shallow replays of the tape of life. The problem is that most examples of convergent evolution that are being wielded as overturning Gould’s contingency are of the shallow kind. Seen this way they are not that surprising, a point Jonathan Losos also raised in Improbable Destinies. According to Gould, convergent evolution cannot overcome developmentally entrenched body plans, their existence remains a matter of contingency – or so argues Powell. This points the way to the kinds of evidence required for a more convincing critique of contingency.
“Powell here makes a distinction between deep and shallow replays of the tape of life. […] most examples of convergent evolution […] are of the shallow kind.”
In my opinion, the first half of this book is obligatory reading for evolutionary biologists. The text will require your close attention, but it is engagingly written and incredibly rewarding. Do not be intimidated by the table of contents, but do expect to have to look up some words: I doubt that many outside of philosophy circles regularly speak of exegetical shortcomings or the “nomological vacuum of biology”.
The second part, which is more Powell’s personal challenge to Gouldian contingency, argues that, next to bodies, minds and cognition are highly convergent evolutionary outcomes. Partially for the sake of brevity, and partially because the first half is such a bombshell, I will be glossing over this second half. It is no less interesting, though it is the tougher part of the book, dealing as it does with slippery concepts such as cognition and consciousness.
Briefly, the starting point is the evolution of “image-forming sensory modalities”; a broad definition of vision that includes echo- and electrolocation. The ability of organisms to access real-time, detailed representations of their environment has been hailed by some as the kick-start for evolution, heralding the Cambrian explosion. It also led to the evolution of – borrowing and adapting the German term Umwelt from 19th-century biologist Jakob von Uexküll – Umweltian cognition and consciousness. Without going into the many subtleties Powell discusses here, this can be very coarsely defined as your “first-person portal on the world” (p. 212). This also entails fascinating discussions of the evolutionary history of brains and neurons (more on this in my upcoming review of Metazoa), and behavioural evidence of sophisticated perception and cognition in invertebrates, specifically in cephalopods and insects.
Overall, I was very impressed with the depth and rigour of Contingency and Convergence. It provides the intellectual challenge and reward you would associate with this book series. Powell not only provides a valuable analysis of this debate but also challenges readers to engage better, both theoretically and empirically, with contingency and convergence in evolution. Without a doubt, this is a must-read for evolutionary biologists.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
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