Mendelian-Mutationism (II): The Fluctuation-Mutation Distinction

As discussed in my last post, the mutationist/Mendelians (defined below) have mostly been sidelined in the history of biology. The claims used to justify this argument make up what Arlin Stoltzfus and I call “The Mutationism Story.” While Arlin first discovered this in the scientific literature, we found that scientists were getting many of these mistaken claims from historians and philosophers!

Often, the mutationists/Mendelians are painted as fools who could not think their way through evolutionary theory, creating what David Hull called “an inexplicable embarrassment.” This embarrassment is the delay of the (apparently inevitable and obvious) synthesis between Darwinism and Mendelism. Several historians are guilty of such claims, including Peter Bowler, who wrote in Evolution: History of an Idea (2009), that “it would be several decades before the geneticists realized that mutations cannot create new species and reluctantly called in natural selection to explain why some mutations increased their proportion in the population while others declined.” My previous post showed that this claim is outright false – Punnett was making use of natural selection by 1912!

An important part of the Story is what we call the “Missing Piece.” (Note that Peter Bowler actually rejects this part. No historian recites the entire myth; it is composed of pieces.) This is exemplified by Richard Dawkins, who wrote with regard to this period: “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.” And also by historian William Provine, who wrote, “Mendel’s theory of heredity was the perfect complement to Darwin’s idea of natural selection.” So why this period is inexplicable is because, according to many scientists and several historians, Mendel provided “the Missing Piece” to Darwinism: a theory of heredity. And for some reason, according to the Story, the mutationists could not synthesize the two theories; it had to wait for the population geneticists and the Modern Synthesis. These (and other) claims are a mixture of being misleading or false.

Hugo de Vries

First, to whom do I refer by “mutationists and Mendelians”? These are the early 20th century geneticists who re-discovered and/or elaborated upon Gregor Mendel’s laws of heredity. In this context then, “Mendelian” means their theory of evolution and heredity, not Mendel’s (who had other concerns). Mutationism precedes Mendelism by a few years, centered around Hugo de Vries, the founder of the mutation theory and discoverer of Mendel. His mutationist ideas were adopted and modified by the Mendelians. “The Mutationism Story,” then, frequently points at specific individuals: Hugo de Vries; William Bateson, Mendel’s champion in the English-speaking world; and Thomas Hunt Morgan, the famous fly geneticist. A close associate that is not named, but is essential to our story, is Reginald Punnett, Bateson’s student and the geneticist who did much to put a Mendelian theory of evolution to work (the subject of a third post). Wilhelm Johannsen is another close associate, and is frequently named, but because he wrote primarily in German and Danish, we discuss his work as interpreted by others as we could not undertake the same analysis as we do the others.*

Back to the point: why was there “an inexplicable embarrassment”? What did they get so wrong? Here are two frequently mentioned reasons:

1. The Mendelians thought evolution proceeded through leaps or saltations, i.e., mutations were large.
2. The Mendelians opposed natural selection.

Jean Gayon has argued that addressing (2) is the key to understanding the period. As seen in my previous post, we largely agree – the (forgotten) changes the Mendelians made to natural selection are critical. The Mutationism Story changes to the following diagram when taking Gayon’s revision into account.

 

However, we also argue that this doesn’t capture the full story. Why they changed natural selection makes sense after (1) is addressed. And the diagram becomes a whole lot more complicated!

On Mutations

Arlin and I argue that the key to understanding this period is what we call the Fluctuation vs. Mutation Distinction, almost universally misconstrued by historians as an issue of the sizes of variations (small vs. large, continuous vs. discontinuous), when instead it refers to the causes and consequences of different modes of variation (environmental vs. genetic).

Dogma states that the Mendelians were saltationists who, under the influence of Hugo de Vries, believed evolution proceeded by large jumps. It is certainly true that they thought evolution could work this way, but importantly, Darwin was adamant in arguing that it never worked that way; he was committed to gradualism. Instead, the Mendelians adopted a pluralist stance: mutations could create species, but frequently did not. (Note this is the modern view: see polyploidy in plants.)

Yes, Hugo de Vries’s mutation theory claimed that new species were created by mutations. However, such mutations did not need to be large (but they could be). All that was needed for the production of a new species was a “progressive mutation” that created a new factor with no matching counterpart in another individual. In modern terms, (I speculate) the mutation theory would claim that a new species is created when a new gene is produced that cannot pair up with an existing allele in another individual, i.e., the Mendelian laws of inheritance do not apply. Thus, the size of the mutation’s effect did not matter.

However, the Mendelians (Bateson, Morgan, George Shull, etc.) generalized De Vries’s mutations almost immediately, divorcing it from its species-producing power. As Morgan wrote in 1903, “the term mutation will be used … in a very general way, and it is not intended that the word shall convey only the idea which De Vries attaches to it; it is used rather as synonymous with discontinuous and also definite variation of all kinds.”

So for the rest of the Mendelians (our paper contains a multitude of quotes showing this), mutations did not necessarily create species, as De Vries had argued. Instead, they generalized his mutations to be any definite and discontinuous (not necessarily large) change to the internal constitution of the organism. They then combined it with Mendelism to create their own evolutionary synthesis.

The constant refrain of Mendelians that variations are discontinuous sounds extreme if we see it as a claim about sizes.  Instead, it was a claim about mechanisms and consequences that we all accept today: variations originate not by one factor gradually morphing into another, but by discrete events that generate a new (allelic) type.

Thus, claim (1) falls. In any case, the claim is inherently contradictory. An associated historical claim is that what changed the Mendelians’ views of mutation were Morgan’s mutating fruit flies. This makes no sense. Morgan’s white-eyed fly is a “large” mutation in that a white eye exists outside the normal range of variation. It could be argued that white-eye showed that mutations do not create new species. But that doesn’t make sense either: the Mendelians did not think the short and tall varieties of Mendel’s peas were different species. Nor the roosters with differnt shaped combs or rabbits with different colored hair. Saying that a core tenet of Mendelism was that mutations are large is false.

On Fluctuations

Why the Mendelian emphasis on “discontinuity” and “definiteness”? Because the articulation of “mutation” ruled out other kinds of variation as important to evolution. The Mendelians called these other sorts of variations “fluctuations,” and controversially, claimed that such unimportant variations had been the core of Darwin’s own theory. We argue they were right in doing so.

Contra the Missing Piece, Darwin’s Theory of Natural Selection was not actually missing theories of heredity and variation. Darwin proposed and incorporated ideas such as pangenesis and Lamarckism to account for these phenomena. Relevant to our discussion, he made three specific commitments**: (1) “Natura non facit saltum,” i.e., gradualism, (2) variations arise from changes in the external conditions of the parents, and (3) blending inheritance.

(1) has already been discussed, (3) is well-known, but (2) has been largely missed, even though it contained one of their fundamental critiques of Darwinism. Darwin believed changes in the external conditions of the parent are what caused variations to appear in the offspring; the Mendelians argued that variations arose solely as a result of internal processes. The Mendelians were the ones to articulate the distinction between the two kinds: what they called Darwin’s fluctuations and Mendelism’s mutations. We call this the Fluctuation-Mutation Distinction. (Again, “fluctuations” are rooted in the work of De Vries, but the Mendelians interpreted his work differently. Punnett was actually not aware of this until E.B. Poulton pointed it out to him in 1912-13.)

On Fluctuations and Mutations

Johannsen’s results, republished in Morgan’s 1916 book. Note that the five pure lines, when combined, create a population showing continuous variation.

The nature of the distinction becomes clear in the context of Wilhelm Johannsen’s oft-discussed pure line experiments. When selecting for larger or smaller size in self-fertilized beans (a “pure line”), Johannsen found no effect. Even though there was variation, the variation was non-selectable, i.e., it was non-inheritable. This is obvious to us now, but why Johannsen (and many others) attempted to demonstrate this makes sense with regard to the Flucation-Mutation Distinction: if a changing environment stimulated inheritable variation, then selection should work! What Johannsen showed is that internal variation, or mutations, were required to make a pure line larger or smaller.

Bateson wrote, it is a “fundamental truth” “that there is a natural distinction between fluctuational variations and actual genetic variations; that the latter are those alone by which permanent evolutionary change of type can be effected…”

Importantly, fluctuations and mutations were not distinguished by size. They were distinguished by their inheritability. In 1905, Punnett made this clear: “Doubtless some of the so-called fluctuations are in reality small mutations, whilst others are due to environmental influence . . . The simultaneous existence of small mutations and large fluctuations leads to the disguising of the former by the latter.” Another geneticist, George Shull, wrote in 1907: “They [mutations] are not to be distinguished from fluctuations by being large and striking variations but by the fact that they represent a fundamental change in the internal composition [of the hereditary material]…”

While such a distinction is obvious now, it was not so at the time. Although its effects were masked by later developments, this distinction is so fundamental that it breaks Darwinism. (The particulate nature of genetic factors also breaks Darwinism (blending inheritance), but this is more widely acknowledged.) Natural selection no longer has free reign to act upon an endless wealth of minute variations supplied by a constantly changing environment. Selection instead has to wait for discrete mutations. This is a key difference between Mendelism and Darwinism: the Mendelians argued that mutation was the source of evolution’s creativity; not natural selection. These differing points of views were the subject of William Castle’s debate with the Mendelians (sadly, I couldn’t find any good online resources on this).

As discussed in my last post, the Mendelians did not leave the problem there. Recognizing what they had done, they also changed how selection worked. Contrary to the Mutationism Story, the Mendelians did not oppose selection. They did oppose Darwinism, meaning, there are actually multiple ways to formulate natural selection. When the Mutationism Story states that the Mendelians delayed the synthesis of Mendelism and Darwinism, they are partly right; what it misses, however, is that Darwinism was dead. In the meantime, the Mendelians salvaged natural selection and helped create the basis of modern evolutionary thought.

I began this post with a diagram showing the “classic view,” or “Mutationism Story.” Below is the diagram that results when you take into account what I have written in these two posts (and some additional information in the paper):

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These two short posts on our paper get at the substance of our paper but in a somewhat superficial way. I had to skip over a lot of our reasoning, as well as some nuance, to concisely articulate our argument (in ~3500-4000 words!). Take a look at our paper, it’s open access, and let us know what you think! If you have any questions or comments, please post them below.

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* Also, there are major differences between De Vries and the others (discussed briefly), as well as amongst each other (not discussed), but here, those differences will only be mentioned where needed.

** By commitments, we mean that Darwin explicitly rejected alternatives when they were proposed to him. This matters: Darwin’s Theory of Natural Selection is not a theory outside of history; it is instead contingent upon the choices and commitments Darwin made.