Bad takes #5. It’s just contingency
(This post is part of a series focusing on bad takes on the topic of biases in the introduction of variation, covering both the theory and the evidence. For more bad takes, see the links at the bottom.)
A common “stages of truth” meme holds that successful disruptive ideas are first (1) dismissed as absurd, then (2) resisted— the idea is declared unlikely and the evidence is strenuously disputed—, and finally (3) regarded as trivial and attributed to long tradition. Haldane’s version is that “The process of acceptance will pass through the usual four stages: (i) this is worthless nonsense; (ii) this is an interesting, but perverse, point of view; (iii) this is true, but quite unimportant; (iv) I always said so.” The QuoteInvestigator piece on the stages-of-truth meme has this version:
For it is ever so with any great truth. It must first be opposed, then ridiculed, after a while accepted, and then comes the time to prove that it is not new, and that the credit of it belongs to some one else
In their ruthless parody of Bad Synthesis Apologetics a la Futuyma, Svensson and Berger (2019) model all the stages of truth in the same paper: first they misrepresent the notion of mutation-biased adaptation as an absurdity contrary to basic principles (see Bad Takes #3 and Bad Takes #4), then they present the actual theory but insist that the evidence is inconclusive and that the phenomenon is unlikely for technical reasons of population genetics, and finally, implicitly admitting that the phenomenon is real and that the theory is correct, they describe it as trivial and familiar:
These studies therefore only exemplify how historical contingency and mutational history interact with selection during adaptation to novel environments [31, 38, 52], entirely in line with standard evolutionary theory and the uncontroversial insight that different genomic regions contribute differentially to adaptation driven by selection, with mutations merely providing the genetic input .
In this way, the reader is guided through the stages of truth from heresy to textbook wisdom.
However, our focus here is only on the end-point of this progression, in which Svensson and Berger (2019) give the impression that the new work on mutation-biased adaptation represents familiar textbook wisdom, so that the results induce no changes in evolutionary reasoning, raise no new questions, and suggest no new priorities for research. The specific implication of the passage above is that these findings are merely a matter of “contingency” and present nothing original or new relative to the contents of references 31, 38 and 52.
Yet contingency is not a mechanistic theory: it is an explanatory concept indicating that a system is non-equilibrium, so that the state of the system cannot be predicted without knowing the initial conditions and detailed dynamics. The notion of contingency, by itself, does not provide a theory of the dynamics. If we try to answer the odd question, “what does contingency predict about how the mutation spectrum shapes the spectrum of adaptive substitutions?” then we will get nowhere without a theory for the dynamics, and this theory will not be about contingency (an empty explanatory concept), but about the dynamical issue of how the details of mutation rates influence the spectrum of adaptive substitutions.
References 31 and 38 are from the field of quantitative genetics, and simply do not provide any such dynamical theory, e.g., here is the abstract to reference 31:
The introduction and rapid spread of Drosophila subobscura in the New World two decades ago provide an opportunity to determine the predictability and rate of evolution of a geographic cline. In ancestral Old World populations, wing length increases clinally with latitude. In North American populations, no wing length cline was detected one decade after the introduction. After two decades, however, a cline has evolved and largely converged on the ancestral cline. The rate of morphological evolution on a continental scale is very fast, relative even to rates measured within local populations. Nevertheless, different wing sections dominate the New versus Old World clines. Thus, the evolution of geographic variation in wing length has been predictable, but the means by which the cline is achieved is contingent.
Reference 52 is Good, et al (2017), a deep sequencing study of samples from Lenski’s LTEE (long-term evolution experiment). This is mainly an empirical analysis of allele trajectories and clonal interference and so on. There are no explicit claims for an effect of mutation bias on the spectrum of adaptive substitutions (mutation bias is mentioned only in relation to mutators, but they generate a lot of hitch-hikers so this does not establish an effect of mutation bias on adaptation). Indeed, the presentation of results indicates in various places (e.g., the comments on parallelism) that the authors are not paying attention to the issue of how mutation bias influences probabilities of beneficial changes.
What is going on here? For the naive reader, the richness of the satire by Svensson and Berger (2019) may obscure their aims. The reader will surely lose track of all the different ways that they avoid the real issues, and that is the point: they are illustrating all the different ways of not addressing the novelty of (1) a formal pop-gen theory that focuses on the introduction process, and which makes novel predictions about evolution based on tendencies of variation (addressing aspects of trends, GP maps, findability, etc), in a way that directly contradicts the classic Haldane-Fisher “mutation pressure” argument, and (2) empirical results confirming a distinctive prediction of this theory, namely effects of mutation biases on adaptation (not requiring neutrality or high mutation rates), contradicting a long neo-Darwinian tradition of dismissing internal biases in evolution.
One way to avoid these key issues is to engage in whataboutery, i.e., responding to an issue by demanding attention to a second issue. What about other research? What about selection? Whataboutery provides the writer an opportunity to engage the reader on some related topic, e.g., for purposes of name-dropping. Apropos, rather than expanding the reader’s knowledge by offering insightful and detailed explanations of new and poorly known studies on the topic of mutation bias and molecular adaptation, Svensson and Berger instead lavish their attention on older and much better known work on related topics by eminent scientists, e.g., the LTEE from Lenski and colleagues, lizard stuff from Jonathan Losos, the famous stickleback Pitx1 example, or David Houle’s work on fly wings.
More generally, the approach is to identify new work that is significant for specific reasons, and then, rather than mapping the new work onto the relevant issues (i.e., the ones that make it significant), it is mapped to other issues that make it seem ordinary. Clearly Erik and David had a lot of fun playing this game!
To understand how the game works, consider a completely unrelated claim of novelty, e.g., the invention of a telephone 150 years ago. The critic of novelty may object as follows: You say there is something new here? No, nothing new at all! This is merely a device, and humans have been making devices for centuries! I could show you 15 devices from just the past few years that are more impressive than this one, with more parts. I could build an identical device in an afternoon for $20. There is no new fundamental technology here, merely pieces of wood and metal and wire! There are no new basic principles at work, merely electrical currents and vibrations controlled by magnets. It looks like other devices I have seen. I could break it easily with a hammer. I doubt that it can fly like an airplane.
The problem is not that these objections are false statements when considered in isolation. The problem is that they fail to address the crucial issue: the telephone prototype instantiates a generalizable technology to support remote voice communication through electrical wires.
Likewise, when Svensson and Berger (2019) argue that the new line of work on mutation-biased adaptation is just another example of contingency, this represents a deliberate choice — all in jest, of course — to describe the work in an irrelevant way, like objecting that the telephone is not new because it looks like other devices, or because it has no fundamentally new parts.
What is the true significance, in a nutshell? The essence of neo-Darwinism is a dichotomy of variation and selection, in which variation merely provides raw materials (substance, not form), and selection is the source of order, shape, and direction. Theories of internal biases directly contradict neo-Darwinism. The argument of Haldane and Fisher that such theories are incompatible with population genetics (see Bad takes #2) was eagerly adopted by the architects of modern neo-Darwinism, yet (1) this classic conclusion is unwarranted theoretically and (2) its implications are refuted empirically. These two provocative claims are established by the line of work on mutation-biased adaptation; they are not part of textbook knowledge; they are not established in well known studies cited by Svensson and Berger to illustrate scientific name-dropping.
For more bad takes on this topic
- We have long known (Bad Takes #1)
- Mutation pressure (Bad Takes #2)
- Independent cause of adaptation (Bad Takes #3)
- Mutation-driven (Bad Takes #4)
- Contingency (Bad Takes #5)
Good BH, McDonald MJ, Barrick JE, Lenski RE, Desai MM. 2017. The dynamics of molecular evolution over 60,000 generations. Nature 551:45-50.