Evolution Basics: At the Frontiers of Evolution, Part 4: Contingency vs. Convergence
Note: This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post we discuss the debate between paleontologists Stephen Jay Gould and Simon Conway Morris over whether evolutionary history is primarily shaped by chance events (i.e. by contingency) or through repeatable events (i.e. by convergence).
In previous posts in this series, we’ve explored features of evolution that are contingent (i.e. what we would call chance events) as well as features that are convergent (i.e. events that are repeatable, and thus very much not chance events). One key example of a contingent feature of evolution is mutation. Mutations, as we have seen, are the source of genetic variation within a population. Other chance events can shape evolutionary history as well – for example large-scale extinction events like the Cretaceous – Paleogene asteroid impact that famously wiped out all dinosaur lineages except birds, among many other groups.
Yet for all these chance-based features, we’ve also seen how evolution is, in some important senses, emphatically not chance-based. Natural selection, for example, is anything but random in its actions. We’ve also seen how evolution of separate groups of animals often arrives at very similar “solutions” to common environmental challenges – the striking similarities between the wings of birds and bats, for example, or the streamlined aquatic shapes of some reptiles (such as ichthyosaurs) and some mammals (such as dolphins and whales). Even cursory examination of these types of pairings indicate that something decidedly non-random is at work here – that evolution, in many cases, can cause separate lineages to converge on similar – but not identical – structures.
Given that both contingency and convergence seem to be significant factors in evolutionary history, it is only natural for scientists to wonder which force has the upper hand. Is evolution primarily contingent, with convergence playing only a minor role? Or is evolution largely a convergent phenomenon, where contingent factors have little overall influence?
Gould and Conway Morris – the Battle over Burgess
It was precisely this question that led to a public debate between well-known champions of alternate views – the late Stephen Jay Gould, and Simon Conway Morris. Gould, a paleontologist and widely-read author of popular science books, was a staunch defender of the role of contingency in evolution. In his book Wonderful Life (1989) he famously asserted that if earth’s evolutionary history were repeated, the results would be markedly different. Gould framed his argument using the diversity and oddity of the Cambrian fossils preserved in the Burgess Shale. In his view, the Cambrian animals represented a large number of only distantly-related major groups (i.e. phyla), of which only a few would persist. As such, he argued that the survival and later diversification of any given lineage (such as our own, the vertebrates) was largely a matter of chance. In keeping with this focus on chance, Gould viewed the production of human-like creatures, or even human-like intelligence, as by no means certain. For Gould, contingency was king – and humans, accordingly, were a biological accident.
For Wonderful Life, Gould drew heavily on the research of leading Cambrian paleontologist Simon Conway Morris. Conway Morris, however, would later revise his views on Cambrian diversity as new data accumulated from other Cambrian deposits. These advances provided evidence that what were once viewed as disparate Cambrian phyla were in fact likely related – and furthermore, that many Cambrian animals were members of existing groups (or alternatively stem-group species of existing groups, as we have discussed previously). As such, Conway Morris objected to Gould’s interpretation of the Cambrian fauna on scientific grounds – in his view the Cambrian was not, as Gould alleged, a case of massive diversification of phyla followed by chance survival of few. Moreover, Conway Morris argued, the reality of evolutionary convergence cut against Gould’s central thesis. Conway Morris would expound these ideas at length in his 1998 book The Crucible of Creation, leading to a public exchange with Gould:
So the Burgess creatures do not form an exception to the orthodox mechanisms and patterns of evolution, as I believe Gould has implied. The new evidence suggests that not only did the sheer number of species increase since the Cambrian (as nearly everyone agrees), but, more significantly, the total number of phyla has been maintained and has not, contrary to, what Gould has written, shown a catastrophic decline. But now we come to the most egregious misinterpretation of the Burgess Shale in Gould's book—a conclusion drawn not from the evidence of paleontology but from Gould's personal credo about the nature of the evolutionary process.
Gould sees contingency evolutionary history based on the luck of the draw—as the major lesson of the Burgess Shale. If you rerun the tape of evolution, he says, the results would surely come out differently. Some creature similar to Pikaia, a small eel-like animal with a rudimentary head, may have survived in Cambrian seas to become the ancestor of all vertebrates. If it hadn't, Gould says, perhaps other—entirely different—major animal groups would have evolved instead from one of the Burgess Shale's other "weird" body plans. Such a view, with its emphasis on chance and accident, obscures the reality of evolutionary convergence. Given certain environmental forces, life will shape itself to adapt. History is constrained, and not all things are possible.
An interesting feature of the exchange between Gould and Conway Morris is that both would imply that the other was influenced not only by the scientific data, but also by their philosophical and/or theological commitments (it being no secret that Conway Morris holds to a Christian viewpoint). As one reviewer of Crucible would comment, this battle was not merely about the science, but also about the implications of the two views on offer:
… this is no coffee-table excursion through the details of an ancient ecosystem. It is a full-scale assault on Gould's interpretation of the Cambrian explosion and on the materialist philosophy of life embodied in that interpretation.
No stranger to mincing words, Gould would reply in kind to Conway Morris:
I am puzzled that Conway Morris apparently, doesn't grasp the equally strong (and inevitable) personal preferences embedded in his own view of life—especially when he ends his commentary with the highly idiosyncratic argument that life might be unique to Earth in the cosmos, but that intelligence at a human level will predictably follow if life has arisen anywhere else. Most people, including me, would make the opposite argument based on usual interpretations of probability: The origin life seems reasonably predictable on planets of earthlike composition, while any particular pathway, including consciousness at our level, seems highly contingent and chancy…
Conway Morris's peculiar and undefended reversal of these usual arguments about probability can arise only from a "personal credo"—and I would value his explicit attention to the sources of his own unexamined beliefs.
And the winner is...?
While the exchange between Gould and Conway Morris makes for interesting reading on several levels, the scientific question undergirding the debate remains an open one, even years later. In the next post in this series, we’ll look at one recent approach to investigating this question – and see that it provides evidence for both contingency and convergence in the evolution of an experimental population.
For further reading
- Bowler, Peter. Cambrian Conflict: Crucible an Assault on Gould's Burgess Shale Interpretation.
- Conway Morris, Simon, and Gould, Stephen Jay. Showdown on the Burgess Shale.