What on earth is “mutationism”? Some possible answers
The term “mutationism” appears in the early 20th century literature (e.g., Poulton, 1909 or McCabe 1912) in reference to the views of early geneticists such as de Vries, Bateson, Punnett, and Morgan. These leading thinkers did not use “mutationism” to describe their own views, which were quite diverse. Perhaps they thought of themselves as free thinkers, not tied to any ideology or “-ism”.
For readers of the contemporary literature of evolutionary biology, the meaning of “mutationism”— evolution happens by dramatic mutations alone, without selection— is a strawman developed by Synthesis partisans to promote conformity with their neo-Darwinian view (see the conceptual immune system). The pattern in which “mutationism” is used as a functional pejorative continues today in the Synthesis gatekeeping literature, e.g., Gardner (2012) or Svensson (2023).
Nei (2013) also does not use “mutationism” for his own thinking. However, his book “Mutation-driven Evolution” suggests a broad alternative to the Synthesis tradition, as pointed out in most of the published reviews. Alone among these reviews, Wright rejects Nei’s thinking as mistaken, referring to it as “Mutationism 2.0.” The other 5 reviews are sympathetic. Three of them do not mention the term “mutationism” (Brookfield, Galtier, Weiss). The review by Gunter Wagner entitled “The changing face of evolutionary biology”, like my review, attempts to identify a meaning for “mutationism” that makes sense of Nei’s distinctive project focusing on the importance of mutations in evolution.
One might be tempted to avoid the term “mutationism” (along with “saltationism” and “orthogenesis”) on the grounds that Synthesis partisans have made these terms toxic. Why risk ridicule and invite misunderstanding when one’s main goal is to communicate with readers? I avoided these terms myself for many years, on precisely these grounds. However, I eventually concluded that we cannot acquiesce to rhetorical tactics designed to shame and silence people using strawman arguments, no matter how influential Synthesis gatekeeping has become. On the contrary, promoting good intellectual hygiene obliges us to call out fallacies and to address alternative views fairly and rigorously, without relying on strawmen or tribal mythology.
In the case of the term “mutationism,” if there are distinctive features of the evolutionary views of the early geneticists, nothing prevents us from using the term “mutationism” to denote those distinctive features. If “selectionism” is a legitimate way to designate a focus on the role of selection in evolution, without implying a denial of a role for mutations in evolution, then “mutationism” is a legitimate term for focusing on mutations, without denying that selection is involved in evolution. In my own thinking, I usually associate the term “mutationism” with a non-exclusive form of explanatory mutationism or with the lucky-mutant conception of evolutionary dynamics. But sometimes I think about it as a school of thought with a critique of Darwin’s views (and neo-Darwinism) and a range of views limited only by a reliance on genetics. These forms of mutationism have clearly experienced a resurgence in the past 30 to 50 years.
|Possible meaning of mutationism||Type of meaning|
|evolution happens by dramatic mutations alone, without selection||Strawman featured in Synthesis tribal mythology|
|identifying distinctive mutational-developmental changes is a uniquely powerful way to explain the evolution of form||Explanatory position on what kinds of causal attributions are meaningful|
|reconstructing mutational changes provides uniquely reliable knowledge of past evolution||Methodological position on which causes are most accessible to scientific methods|
|the timing and character of events of mutation determine the timing and character of evolutionary change||Empirical position on evolutionary dynamics|
|diverse evolutionary phenomena arise from mutation and genetics||School of thought associated with Bateson, Punnett and Morgan|
|a preliminary and imperfect expression of (for instance) a future paradigm of dual causation||Transition state mainly of historical interest|
The Mutationism Story in Synthesis tribal mythology
In the version of history that Synthesis partisans tell each other to reaffirm their commitment to neo-Darwinism, mutationism means that evolution happens by dramatic mutations alone, without selection.
Versions of this Mutationism Story are given in this blog (e.g., Dawkins 1987, p. 305 of The Blind Watchmaker; Cronin 1991, p. 47 of The Ant and the Peacock; Ayala and Fitch 1997; Futuyma, 2017; Segerstråle 2002, Oxford Encyclopedia of Evolution 2, pp. 807 to 810; Charlesworth and Charlesworth 2009). Here is Dawkins’s version:
“It is hard for us to comprehend but, in the early years of this century when the phenomenon of mutation was first named, it was regarded not as a necessary part of Darwinian theory but as an alternative theory of evolution! There was a school of geneticists called the mutationists, which included such famous names as Hugo de Vries and William Bateson among the early rediscoverers of Mendel’s principles of heredity, Wilhelm Johannsen the inventor of the word gene, and Thomas Hunt Morgan the father of the chromosome theory of heredity. . . Mendelian genetics was thought of, not as the central plank of Darwinism that it is today, but as antithetical to Darwinism. . . It is extremely hard for the modern mind to respond to this idea with anything but mirth”Dawkins, 1987, p. 305
Dawkins’s account fits well with the self-serving tone of Synthesis Historiography, in which critics of neo-Darwinism behave irrationally and hold laughable views with obvious flaws. In the Mutationism Story, the early geneticists are too stupid to understand population thinking and selection, which they reject.
For people who are not part of the Synthesis identity-group, the Mutationism Story is a fairy tale wildly inconsistent with the facts of history. Far from being unable to think about the implications of Mendelism in populations, Bateson and Saunders (1902) immediately recognized the Hardy-Weinberg paradigm, the first rigorous paradigm of population thinking:
“It will be of great interest to study the statistics of such a population in nature. If the degree of dominance can be experimentally determined, or the heterozygote recognised, and we can suppose that all forms mate together with equal freedom and fertility, and that there is no natural selection in respect of the allelomorphs, it should be possible to predict the proportions of the several components of the population with some accuracy. Conversely, departures from the calculated result would then throw no little light on the influence of disturbing factors, selection, and the like.”Bateson and Saunders, 1902, p. 130
Thomas Hunt Morgan won a Nobel prize in genetics. He was not too stupid to understand populations and selection. His tendency to refer to “survival” of “definite variations” and to avoid “natural selection” reflected, not some kind of mental block, but a belief that shifting the goal-posts to avoid accountability is bad for science. For Morgan, the term “natural selection” refers to Darwin’s non-Mendelian theory based on the blending of environmentally stimulated fluctuations (“indefinite variability”), a theory correctly rejected by the scientific community when it was experimentally refuted by Johannsen.
By contrast, the early geneticists did not reject what we would call “selection” today, e.g., Morgan (1916), in his closing summary, writes:
“Evolution has taken place by the incorporation into the race of those mutations that are beneficial to the life and reproduction of the organism” (p. 194)
Bateson, Punnett, de Vries and Johannsen were the other early geneticists most well known for their views on evolution. Johannsen and de Vries both carried out successful selection experiments. de Vries begins his major 1905 English treatise by writing that …
“Darwin discovered the great principle which rules the evolution of organisms. It is the principle of natural selection. It is the sifting out of all organisms of minor worth through the struggle for life. It is only a sieve, and not a force of nature” (p. 6)
In Materials for the Study of Variation, Bateson (1894) refers to natural selection as “obviously” a “true cause” (p. 5). Punnett (1905) explains that mutations are heritable while environmental fluctuations are not, concluding that “Evolution takes place through the action of selection on these mutations” (p. 53).
The views of these influential scientists were not a secret: they were published, cited, and discussed. Bateson, Punnett, Morgan and de Vries all were awarded the Royal Society Darwin medal in the period from 1900 to 1930. That is, the Mutationism Story is a fabrication contradicted by readily available historical sources: its ongoing success is a testament to the power of propaganda and the insularity of the Synthesis tribal culture (again, see the conceptual immune system).
Explanatory or methodological mutationism
Explanatory and methodological versions of mutationism are useful to contemplate, and relate clearly to approaches in evo-devo and structuralism. The way to understand this kind of distinction is to consider how Godfrey Smith (2001) identifies 3 flavors of adaptationism: empirical, methodological, and explanatory. The position of empirical adaptationists is ontological, based on a belief about how the world actually is: living things are pervasively adapted, down to the finest detail, and therefore, we will see adaptation everywhere we look. Methodological adaptationists believe that, even though adaptation might not be pervasive, it is the thing we are uniquely equipped to study using the methods of science. This view tends to travel together with the belief that evolution is a combination of selection and “chance”, with the latter being hard to study systematically. Explanatory adaptationists believe that, although selection might not be everything, and although we might be able to study other kinds of causes in evolution, a focus on selection and adaptation is justified because adaptation is the distinctive problem in evolution— as Fisher said, evolution is adaptation and consists of nothing else— and selection is the necessary and distinctive principle behind adaptation.
Analogously, we can imagine empirical, explanatory, and methodological versions of mutationism. The lucky mutant view mentioned below is one possible ontological or empirical flavor of mutationism. In methodological mutationism, we focus on identifying the mutational-developmental changes involved in evolution on the grounds that this is the uniquely reliable and productive way to study evolution. In explanatory mutationism, our focus is on identifying the detailed mutational-developmental changes underlying changes in form, because explaining changes in form over time is the distinctive challenge of evolutionary biology.
Consistent with a methodological mutationist program, Bateson believed that, in order to understand how evolution happens, the first step was to study variations. Accordingly, his Materials for the Study of Variation is a catalog of 886 numbered cases of discontinuous variations. Bateson planned a second volume on continuous variation but subsequent work on quantitative trait distributions made this unnecessary.
Bateson’s approach was observational, but today we see various experimentally oriented mutationist projects in evolutionary biology:
- attempts to reconstruct mutational changes involved in key changes in development, in the context of evo-devo
- systematic experimental characterization of M in quantitative genetics, e.g., Houle, et al (2017)
- the work of biochemically-oriented molecular evolutionists (from early work of Daniel Dean to recent work of Weinreich or Thornton and colleagues) to reconstruct specific protein molecules and their mutants in order to reconstruct the path of history and test hypotheses
- the recent focus on using deep sequencing methods to characterize the mutation spectrum in quantitative detail in a variety of organisms, and in the context of cancer biogenesis
Note that these projects are generally situated in paradigms that are not focused solely on mutation, but also reflect functionalist concerns. This is clearly true of evo-devo, for instance, as the analysis of Novick (2023) makes clear. The evo-devoists are not merely concerned with understanding why certain types of transformations are mutationally and developmentally likely, they are also concerned with selection and function. The same is obviously true of the line of work from Thornton and colleagues, which combines the reconstruction of mutants with functional assays and even selection experiments. In the study of cancer drivers and clonal haematopoesis mutants, contemporary research on mutation spectra, mutation rates, and repair mutants is premised on the understanding that clinical prevalence reflects both the rate of mutational origin and the selection intensity (figure; see Cannataro, et al. 2019; Watson and Blundell, 2022).
If we look at methodological mutationism as an extreme or exclusive position, it is difficult to separate from an extreme form of skepticism about selection that seems unwarranted today, when we can test hypotheses of selection in rigorous ways and assign some non-negligible proportion of the variance in outcomes to selection. Apropos, Nei (2013) does not reject selection as a causal principle in evolution, yet in practice, he seems to reject every attempt to attribute something concrete to positive selection. His approach recalls the attitude of Bateson, who (a century earlier) disparaged adaptationist story-telling by appealing to Voltaire’s Dr. Pangloss, a trope made famous in the “Panglossian paradigm” of Gould and Lewontin (1979). A scientist in Bateson’s time might find it easy to dismiss the vast majority of claims about selection as armchair speculation, not science. Punnett was so deeply skeptical of adaptive explanations that he rejected adaptive mimicry as an explanation for apparently mimetic morphs in butterflies!
Likewise, it’s hard to think of explanatory mutationism as an exclusive position. Clearly we can study the evolution of form from a structuralist viewpoint as a series of transformations based on genetic encodings and the dynamics of material systems, but we also can study the evolution of form from an adaptationist perspective.
So, rather than supposing that mutationism is uniquely explanatory for evolution in general, perhaps we can suppose instead that it is distinctively explanatory in some limited but important context. What is the limited but important context in which selective explanations are the least informative or trustworthy, and in which mutational explanations have more power to explain what we wish to understand? I think the best answer here is that there are some aspects of deep divergence, such as the formation of new body plans, major taxa, or key innovations, in which the power of selective explanations is at its lowest—because there are too many degrees of freedom— and the power of mutational explanations are at their highest, e.g., when key innovations can be associated with specific changes in developmental genetics, against a background of conserved features that do not change.
This kind of contrast is evident in Nei (2013). If one reads “Mutation-driven Evolution” with Nei’s career in mind, it appears that Nei himself has abandoned the abstract mathematical laws of population genetics that occupied him for decades, and which he finds increasingly useless, in favor of the concrete molecular-biological details of mutational effects. We can see this as an explanatory shift. Looking back over changes in developmental-genetic organization over deep time, perhaps selection and drift are not as useful as knowing which mutations happened to be fixed (and again, this position is hard to separate from Nei’s extreme skepticism about claims of selection).
Mendelo-mutationism as a school of thought
The “school of thought” version of Mendelian mutationism is not a unified theory, but a loose collection of beliefs and ideas, overlapping substantially with how the “Modern Synthesis” is misconstrued today as a loose collection of beliefs consistent with genetics and selection (see this blog or Stoltzfus and Cable, 2014 for a review).
The early geneticists were the first scientists to accept particulate inheritance and mutation as the foundation of their understanding of evolution, in the sense that they viewed with suspicion any idea that could not be reconciled with particulate inheritance and mutation. This position sounds very familiar today, but in 1910 this was a disruptive view that pissed off everyone who was not a geneticist. Imagine these upstarts telling leading thinkers— paleontologists, systematics, embryologists— that the foundation for all future thinking in evolution would be the new discoveries of particulate inheritance and mutation!
As noted above, Bateson and Saunders (1902) clearly articulated the research program of looking for deviations from Hardy-Weinberg expectations as a way of detecting causes other than inheritance.
In the same 1902 paper, they explain what became known as “the multiple factor theory” in which a smooth distribution of trait-values reflects, not blending inheritance and fluctuation, but the joint effect of Mendelian variation at many loci, combined with environmental noise.
But of course they also considered non-gradual changes via distinctive mutations, i.e., saltations. To the extent that non-gradual changes reflecting distinctive mutations are important in evolution, understanding evolution requires knowing how and when these distinctive mutations arise, based on relevant theories and systematic data. This is why Bateson (1894) catalogued distinctive variations as a way of understanding evolution. Morgan later made a systematic search for mutations in fruit-flies. It was Morgan who first clearly depicted evolution as a series of mutations that are accepted by virtue of being beneficial to the survival of the species. He articulated the concept of a probability of fixation in 1916, distinguishing the case of beneficial, neutral and deleterious mutations (the mathematical problem was later solved partially by Haldane, 1927 and more thoroughly by Kimura, 1962).
Interestingly, it was also Morgan (1909) who first suggested the randomness of mutation as a kind of metaphysical gambit, a working assumption that, so long as the origins of mutations remain a mystery, we will treat them as random and not entertain any ideas in which they have special properties.
Whether definite variations are by chance useful, or whether they are purposeful are the contrasting views of modern speculation. The philosophic zoologist of to-day has made his choice. He has chosen undirected variations as furnishing the materials for natural selection. It gives him a working hypothesis that calls in no unknown agencies; it accords with what he observes in nature; it promises the largest rewards. He does not deny, if he is cautious, the possibility that there may be a purposefulness in the sense that organisms may respond adaptively at times to external conditions; for the very basis of his theory rests on the assumption that such variations do occur. But he is inclined to question the assumption that adaptive variations arise because of their adaptiveness. In his experience he finds little evidence for this belief, and he finds much that is opposed to it. He can foresee that to admit it for that all important group of facts, where adjustments arise through the adaptation of individuals to each other—of host to parasite, of hunter to hunted—will land him in a mire of unverifiable speculation.
Note again the stark contrast between the facts of history and the gatekeeping literature in which an association of “mutationism” with directed mutation has been repeatedly fabricated in the attempt to discredit both (Gardner, 2013; Svensson, 2023).
However, Morgan frequently noted that mutations happen at different rates. He and Punnett both believed that this was important for evolution, and might play a role in parallel evolution, citing cases like albino or melanic forms. Under a neo-Darwinian view, melanic forms are expected to emerge gradually, like the all-black rats in Castle’s experiments, from the gradual accumulation of many small differences; and the repeated appearance of melanism in different taxa would indicate that it is some kind of adaptive optimum. For the mutationists, the repeated occurrence of melanic forms suggested that such forms were readily mutationally accessible.
Vavilov (1922) took this idea of parallel evolution by parallel variations to extreme lengths. From his extensive observations of plants, especially crop species, he developed a theory that each major group of organisms has a set of characteristic variants that eventually manifest as distinct species, e.g., if family F has a tendency to produce long-eared forms, this tendency would manifest in genera G1, G2, … each having both long-eared and short-eared species within the genus. He also proposed a kind of mimicry— now called Vavilovian mimicry— that turns out to be quite important among domesticated crop species. In Vavilovian mimicry, the model is a cultivated species actively harvested and propagated by humans, and the mimic starts out as a weed that is eventually propagated by humans by virtue of mimicking the model in terms of the time of maturation, and similar responses to threshing and winnowing techniques. For instance, rye and oats are believed to be Vavilovian mimics that emerged in the context of wheat cultivation (see the wikipedia article on Vavilovian mimicry).
With regard to species and speciation, the early geneticists tended to believe that reproductive incompatibilities were “the true criterion of what constitutes a species” (Punnett, 1911, p. 151). With the Modern Synthesis, this “biological species concept” became the prevailing view (Mallet 2013). They allowed for different kinds of speciation, including speciation by non-Mendelian mutations like de Vriesian macromutations, but also by the accumulation of what we now call “Bateson-Dobzhansky-Muller” incompatibilities.
To summarize, the early geneticists opened up and explored a new field, considering a wide range of possibilities (excluding only Lamarckism) and contributing a number of key concepts to evolutionary genetics. Few people know of their accomplishments today because, in Synthesis Historiography, scientific progress only comes from people with the right Darwinian lineage, and not from critics of neo-Darwinism, who are treated as aliens or un-persons. For instance, in Synthesis Historiography, the credit for rejecting 19th-century views of heredity and introducing modern notions of hard inheritance is awarded, not to the geneticists responsible for this innovation, but to 19th-century physiologist and infamous mouse-torturer August Weismann. The Oxford Encyclopedia of Evolution does not have biographic entries for Bateson, de Vries, Punnett, or other early geneticists except Morgan, whose entry says nothing of his views of evolution, although he wrote 4 books on the topic. For a graphical example of how the early geneticists are erased from history, read this.
The lucky mutant view or “mutation-driven” evolution
The lucky mutant version of mutationism is a focus on the regime of population genetics in which origination events are important, so that the timing and character of evolutionary change depend on the timing and character of events of mutation that introduce new alleles (or phenotypes). This is sometimes called “mutation-driven” or “mutation-limited” evolution. For me, “mutation-driven” evokes evolution by mutation pressure, so I don’t like the term, but I feel obliged to use it occasionally because this is what some readers recognize. I also don’t like “mutation-limited” because it is either wrong or ambiguous. Finally, I also dislike “lucky mutant view”.
As noted above, this is not the only view of the early geneticists. However, it is how we might choose to see mutationism in retrospective contrast to the neo-Darwinian view of the Modern Synthesis. Darwin’s followers, in their dialectical encounter with the early geneticists, were most concerned to defend the power and creativity of selection, to defend gradualism, and to reject a lucky mutant view of dynamics.
Allow me to clarify this point using the contrast posed previously in regard to melanism. The repeated occurrence of melanic morphs in related species might suggest to us the possibility of a common mutation to blackness that has occurred repeatedly. Under neo-Darwinism, by contrast, this would only happen by the accumulation of many small effects, as in the Castle experiment with hooded rats. Actually, the early geneticists accepted both types of change as being possible. Indeed, in a famous dispute with Castle and colleagues, members of Morgan’s group insisted that the gradual emergence of all black and all white rats was entirely consistent with incremental Mendelian change under the multiple factor theory, and did not require any kind of blending effects in heterozygotes.
Again, the defining position of the Mendelian school of thought was not so much any doctrine about the behavior of evolution per se (other than, perhaps, denying the creativity of selection), but only that Mendelian inheritance and mutation are foundational, so that everything has to be built on top of that. However, what is distinctive about their view, relative to the subsequent Modern Synthesis, is their tendency to think of evolution in terms of the acceptance or rejection of individual mutations having small or large effects. The acme of this view, theoretically, is the case of origin-fixation dynamics.
If this is what “mutationism” means, then we have seen a broad resurgence of mutationism in evolutionary biology, starting with the molecular evolutionists in the 1960s. See The shift to mutationism is documented in our language.
A transition to …
Finally we can think of mutationism not as a resting point or destination, but as an unstable transition-state on the path to something else. The most productive line of thought, perhaps, is that it points the way toward a paradigm of dual causation that combines functionalism and structuralism, with a major goal of partitioning variance in outcomes to variational and selective causes. This kind of program has been expressed (without sufficient detail) in some evo-devo works and in Vrba and Eldredge (1984):
“Developmental biologists variously stress: (1) how indirect any genetic control is during certain stages of epigenesis; (2) that the system determines by downward causation which genomic constituents are stored in unexpressed form versus those which are expressed in the phenoytpe; (3) that bias in the introduction of phenotypic variation may be more important to directional phenotypic evolution than sorting by selection. This is in contrast to the synthesis, which stresses more or less direct upward causation from random mutations to phenotypic variants, with selection among the latter as the prime determinant of directional evolution.”
The general idea here is that, instead of casting evolution as shifting gene frequencies, we can depict it more broadly as a process of the introduction and reproductive sorting of variation in a hierarchy of reproducing entities. To the extent that evolution has any predictable behavior, it arises from biases in introduction and biases in sorting. The classical functionalist position of neo-Darwinism and the Modern Synthesis focuses on biases in reproductive sorting (i.e., selection) as the cause of everything interesting. The success of this research program is proof that effects of biases in reproductive sorting are profoundly important in evolution. However, the reason for the resurgence of mutationist thinking is that selection does not actually govern evolution in the way that neo-Darwinians imagined. It is a directional force, but not the directional force. We can also pursue a research program based on the role of generative biases in evolution and, even more broadly, a research program that focuses on both biases in the introduction of variation and biases in the reproduction of variation, with the goal of quantifying their relative influence on the predictability of evolution.
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 This is based on my familiarity with published works of Bateson, Punnett, Morgan and de Vries. They just do not rely on this term. I’m not going to say they never used it, but I haven’t found a case. I found one instance where Davenport (1909) refers to the view of the “the mutationist”. De Vries literally proposed a MutationsTheorie so it is natural to call him a mutationist. But de Vries’s thinking was extremely complex and mainly non-Mendelian, and the other early geneticists developed their own views, not relying on de Vries’s thinking.
 As a technical description of dynamics, “mutation-limited” behavior could mean either (1) behavior responsive to changes in u or (2) the limiting behavior as u approaches 0, which is origin-fixation dynamics. When people like Dawkins (2007) invoke the idea that “evolution is not mutation-limited” as a way of discounting a focus on mutation, this only makes sense if it means that evolutionary behavior is not responsive to changes in u, which is what Dobzhansky and others stated explicitly. However, the best label for mutation-responsive dynamics would be something like “mutation-responsive dynamics.” The term “mutation-limited” is negative and sounds deterministic; for the vast majority of readers, it suggests some kind of limit to the outcomes that selection can access, which is not the same thing. For instance, consider evolution under mutation bias on a smooth landscape with one peak. Ultimately the system goes to the peak: mutation places no limits in this sense. However, the rate and trajectory of the approach to the peak will reflect the rate and bias of mutations. So, the dynamics are mutation-responsive but the ultimate outcome and the ultimate level of fitness or adaptation is not limited by mutation.
 And note that this really excludes de Vries as a Mendelian, because his views were very much based on non-Mendelian ideas of heredity. For a historical analysis of de Vries’s extremely complex and self-contradictory views, see Stamhuis, 2015.
 Technically the list should be something more like “introduction, hereditary transmission and reproductive sorting” with biases possible in each process. Biased gene conversion is a transmission bias. So is meiotic drive. Effects of mutational hazard in the thinking of Lynch can be understood as biases in transmission, i.e., longer sequences have lower transmission due to mutational damage (mutational hazard is not an effect of introduction, and although it is possible to cast it as a form of selection, that is weird IMHO).