At the Frontiers of Evolution: Contingency vs. 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 ofexisting 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.

As we saw, Simon Conway Morris favors a view that evolution is largely repeatable (i.e. convergent) – and thus not dominated by chance. The late Stephen J. Gould, in contrast, famously argued that “replaying the tape of life” would lead to dramatically different results, given his view that chance (i.e. contingency) has the upper hand in evolution.

Of course, the most rigorous way to test Gould’s conjecture (and Conway Morris’ objection to it) would be to do the experiment: rewind the “tape” to the Cambrian period, allow it to go forward multiple times, and note the pattern that emerges over numerous trials. As fascinating as such an experiment would be, this is (of course) not even remotely feasible. Scientists have to settle for far more humble approaches in hopes of addressing these issues.

The LTEE: biosphere in a bottle

One of the more ambitious experiments to date that sheds light on the roles of contingency and convergence in evolution is the “Long Term Evolution Experiment” (LTEE). This experiment, which began in 1988 and continues to this day, is remarkably simple – it tracks the evolution of twelve bacteria populations and compares them over time. Housed in the laboratory of microbiologist Richard Lenski, the LTEE began with twelve cultures of the bacterium E. coli derived from a single cell. From this identical starting point, the twelve cultures have been grown separately, effectively playing twelve “tapes” from an identical starting point.

The mechanics of the experiment are simple: each day, the cultures are provided with new nutrient broth to grow in. The next day, a fraction of the previous day’s culture is transferred to new broth – and so on. Every so often, samples of the cultures are frozen in a way that puts the bacteria into long-term stasis and stored – providing something analogous to a fossil record for each strain, but with the advantage that every “fossil” can be revived and studied. The twelve strains are given the exact same conditions (the same broth, the same temperature, et cetera) and the experiment has gone on, day in and day out, since 1988.

Lenski has described his early expectations for the LTEE (PDF):

When I began this experiment, I thought big differences among the 12 lines would soon be apparent. The random occurrence of mutations meant that some populations would get lucky by generating a beneficial mutation (and one that survived the daily dilutions) sooner than others. And just as in a game, different early moves—mutations—might open some doors while closing others. Some populations might get stuck with beneficial mutations that ultimately led nowhere, while others would follow paths that had long-term potential.

In other words, Lenski expected contingency to be the main player, shaping the trajectories of the lines early on in ways that would lead to their significant divergence. Much like Stephen J. Gould’s famous thought experiment, Lenski expected that these twelve simultaneous “tapes of life” would play out in markedly different ways, with chance as the predominant force shaping their evolution.

The experimental results, however, did not match Lenski’s initial prediction. Though many of the precise mutations were unique to a given bacterial line (and thus contingent, as one would expect for random mutations), the overall evolution of the twelve lines was strikingly similar. As Lenski would note, it was this overall pattern ofconvergence that stood out against the backdrop of contingency:

To my surprise, evolution was pretty repeatable. All 12 populations improved quickly early on, then more slowly as the generations ticked by. Despite substantial fitness gains compared to the common ancestor, the performance of the evolved lines relative to each other hardly diverged. As we looked for other changes—and the “we” grew as outstanding students and collaborators put their brains and hands to work on this experiment—the generations flew by. We observed changes in the size and shape of the bacterial cells, in their food preferences, and in their genes. Although the lineages certainly diverged in many details, I was struck by the parallel trajectories of their evolution, with similar changes in so many phenotypic traits and even gene sequences that we examined.

Contingency strikes back

At this point in the experiment, it certainly seemed like convergence was the order of the day – the twelve lines were divergent in the details of their mutations, yes, but the overall pattern, driven by selection, was that of boringly repetitive convergence. In their identical environments, the lines were shaped to very similar outcomes despite the effects of chance. The tapes had been replayed, and the outcomes were predominantly convergent. Case closed.

Or was it?

It was at this point in the LTEE that something dramatic would happen within just one of the twelve cultures – something so shocking that Lenski and his colleagues thought one culture had been accidently contaminated. Suddenly, one culture was using a food source it had never been able to use before: citrate.

Now E. coli cannot, in the presence of oxygen, normally use the compound citrate as a food source. The nutrient broth used in the LTEE has a significant amount of citrate in it, interestingly enough – but it is there solely as an inexpensive means to buffer the pH of the solution. The broth recipe is an old one, hailing back to a time when microbiologists did things more simply and cheaply. Since the E. coli lines of the LTEE are grown with oxygen present, the citrate in the broth was useless to them – until, by chance, one of the twelve cultures hit upon a way to use it. To return to Lenski’s description,

For 15 years, billions of mutations were tested in every population, but none produced a cell that could exploit this opening. It was as though the bacteria ate dinner and went straight to bed, without realizing a dessert was there waiting for them. But in 2003, a mutant tasted the forbidden fruit. And it was good, very good. The descendants of that mutant rose to dominance owing to their access to that second course. At first, I thought this flask had been contaminated by some other species that consumed citrate. However, DNA tests showed the citrate-eating cells were descendants of the E. coli ancestor used to start the experiment.

Later work would reveal that this striking change was highly contingent in nature, requiring numerous mutations to assemble over tens of thousands of generations. After years of convergence, the most significant change observed in the LTEE would be shown to be a chance, one-off event unlikely to be easily repeated in the other lines – at least in the near future.

Of course, “in the near future” is the key caveat. Will some of the other 11 lines someday find a way to exploit this resource in their environment? Only time will tell. Until then, both Conway Morris and Gould can find support for their views in the LTEE, and the larger question of convergence and contingency remains open – as one would expect for a frontier area.

In the next (and final) section of this series, we will return to the larger question raised by the Gould / Conway Morris debate: does evolution, as a science, preclude holding a purposeful view of life on earth?