Mendelian-Mutationism (I): The Forgotten Synthesis

tl;dr: I am published!

What did early geneticists such as William Bateson, Hugo de Vries, Thomas Hunt Morgan and R.C. Punnett contribute to evolutionary thought? Nothing, according to many scientific sources. They aren’t included in various timelines of the history of evolutionary biology and most are not included in the Oxford Encyclopedia of Evolution. When they are mentioned with regard to evolution, they are depicted as fools who missed the big picture, rejected natural selection, and developed a dead-end “mutationist” alternative to Darwinism.  The standard story is that clear and reasonable thinking about evolution vanished for a generation— the so-called “eclipse of Darwinism”—, returning with Fisher, Haldane, Wright, et al., who showed that genetics is the missing piece of Darwin’s theory, resulting in the Modern Synthesis.

This is wrong.

The first geneticists made substantial contributions to the theory of evolution, even though history has generally not recognized their achievements. Following their critique of Darwinism (next post), the Mendelians (Bateson, Punnett, Morgan, etc.) synthesized genetics with natural selection, laying down the foundations for later evolutionary theorizing. This post illustrates some of these developments.

In The Journal of the History of Biology, biologist Arlin Stoltzfus and I (co-authors) published an article called “Mendelian-Mutationism: The Forgotten Synthesis.” (It is open access so feel free to read it yourself!) The goal of our paper is to place the mutationists/Mendelians (like Bateson and Morgan) firmly where they belong within the history of evolutionary thought: as main characters, and more especially, as protagonists, rather than antagonists or as not important at all, as seen in this UC Berkeley timeline.

Timeline from UC Berkeley’s evolution website. Notice the only figure of early genetics shown is Mendel.

Historian Jean Gayon has argued that Darwin’s original theory was stuck in the 19^th century because his concept of the mechanism of “natural selection” was incompatible with genetics. He credits early geneticists with re-working the concept of selection for a Mendelian world, resulting in a different theory than Darwin imagined. We elaborate upon his thesis to argue that the genetical view of evolution, called “neo-Darwinism” and credited to Fisher, et al., could be just as rightfully called neo-Mendelism and credited to Bateson, Punnett, Morgan, and Johannsen.

Before getting into the nitty-gritty of why early geneticists criticized Darwin and why they have been sidelined (next post), I provide brief examples of some of their key accomplishments. (For the full version with citations, see our open-access paper.)

William Bateson and Edith Saunders

In 1902, William Bateson and Edith Rebecca Saunders published the first major work on genetics and it was rather forward-thinking. While most of the report is focused upon illustrating the Mendelian laws of inheritance, they also made some crucial insights as to how evolution would work when genetics was incorporated. The most striking example is their clear verbal articulation of what would become known as Hardy-Weinberg Equilibrium:

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.

Not only do they describe the “law,” they foresee its utility as a method of detecting evolution within a population. After bringing it to Hardy, who mathematized this description, Reginald Punnett, Bateson’s student, was the scientist who first put Hardy-Weinberg to use, albeit primitively. (Because Punnett’s work is so cool,  how he made use of Hardy-Weinberg will be the subject of another post.)

In the same report, Bateson and Saunders also described how Mendelism can explain a continuous distribution of a given trait. The importance of this insight is that histories of biology typically make the debate between the Mendelians and the biometricians as one of discontinuous vs. continuous variation. However, the Mendelians were perfectly fine with apparently continuous variation:

In the case of a population presenting continuous variation in regard to say, stature, … There are doubtless more than two pure gametic forms of this character, but there may quite conceivably be six or eight. When it is remembered that each heterozygous combination of any two may have its own appropriate stature, and that such a character is distinctly dependent on external conditions, the mere fact that the observed curves of stature give ‘chance distributions’ is not surprising and may still be compatible with purity of gametes in respect of certain pure types.

i.e., the various combinations of homozygosity and heterozygosity at multiple loci affecting height, interacting with environmental effects like nutrition, can produce a continuous distribution of variation. This argument was empirically verified by the famous experiments of Wilhelm Johannsen the following year (discussed in the next post) and a few years afterward by Herman Nilsson-Ehle and Edward Murray East.

Thus, the Mendelians recognized how multiple genes could produce continuous distributions long before R. A. Fisher’s 1918 publication. What bothered them was not the idea that natural traits show apparently smooth variation that appears to blend, but that hereditary variation is actually smooth and actually blends.

That is not all the Mendelians did: they actually synthesized Mendelism and selection! Contrary to what we call the Mutationism Myth (next post), figures like De Vries, Bateson, Morgan, etc. did not reject natural selection. Instead, rejecting Darwin’s ideas of blending inheritance and the environmental stimulation of variation, they created the model upon which modern selection models are based: selection upon types within a population. Jean Gayon calls this reformulation of selection “the most important event in the history of Darwinism.”

1) Allelic Selection: When a mutation arises (a new allele), natural selection, depending upon its strength, increases its frequency or eliminates it from the population with a given rate. This was first thoroughly articulated by Punnett in his 1915 work on butterflies, but was published as early as 1912. (The table showing this model is well-known (amongst historians of biology), probably because William Provine included it in his 1971 book, The Origins of Theoretical Population Genetics, in which Punnett’s role is minimized.)

Figure 1. From Punnett, 1911.

2) Selection on a Quantitative Character: In a population with multiple existing types (A, B, C in Figure 1), i.e., a continuous distribution of variation, natural selection increases or decreases the frequencies of those types. Selection sorts from among the types. (Increasing C while decreasing A would create a heavier population of seeds.)

(According to Ernst Mayr, the early geneticists were “typologists” who thought only in terms of mutational transformations from one type to another, and were unable to understand selection and populations because they had not acquired “population thinking.” Actually, Mayr is completely mistaken. “Typologist” evokes strong differences, but as one can see in Punnett’s diagram, a “type” can simply be a genetically and slightly heavier seed. This means Punnett thought mutations could be small and this is the focus of my next post. Also, clearly, Punnett is thinking about a population!)

To get at the difference between these models and Darwin’s, I quote from our paper:

Both the Mendelian and the Darwinian view allow the complete transition from an ancestor population with one state, to a descendant population with another state. The difference emerges when one imagines the half-way point: in a Mendelian world, 50% of the population has the new state, and there is no intermediate in which 100% of the population has changed halfway. By contrast, in the Darwinian world, blending makes the converse true: at the halfway point, 100% of the population has changed halfway, but there is no point at which 50% has changed all the way. If we allow a bit of random noise, these two views might look the same when the change involves a few extra hairs on a fly’s abdomen, but they cannot look the same if the change involves an extra pair of wings. In the Darwinian view, these two states must be separated by thousands of phenotypic intermediates.

The early geneticists usually aren’t given credit for this. Instead, it is credited to Darwin, as though the meaning of “natural selection” was always clear and always Mendelian. The UC Berkeley evolution website provides these pictures as an illustration of “Darwin’s grand idea of evolution by natural selection.” However, this is not Darwin’s natural selection; it’s a Mendelian-mutationist one! (The Wikipedia article on natural selection makes a similar mistake.) For Darwin, the population would have began as green and gradually shifted to brown – at the half-way mark, the population would have been half way between that dark green and light brown.

Source: University of California Museum of Paleontology’s Understanding Evolution (http://evolution.berkeley.edu).

The non-Mendelian nature of Darwin’s view explains why the early geneticists rejected it. But what about the alleged restoration of Darwinism in the Modern Synthesis? Did Fisher, et al. simply steal the ideas of Bateson, et al. and re-brand them as Darwinian? Is the Modern Synthesis just another name for Mendelian-mutationism?

With regard to much of the scientific content (ignoring the social elements detailed by Betty Smocovitis), yes. Fisher, et al. took the conceptual foundation built by early geneticists, claimed it on behalf of Darwin— or rather, a re-invented version of Darwin as a closeted Mendelian-, and simultaneously denigrated the geneticists’ roles in these developments.

There were real scientific differences, however. A form of Darwinism was eventually restored via the Modern Synthesis, based on a very specific claim (the gene pool) about how population genetics works, a claim that the early Mendelians did not accept.

This story is told in Provine’s seminal history, The Origins of Theoretical Population Genetics. On the surface, Provine seems to be telling the standard story of bumbling geneticists unable to combine selection and genetics, but if one reads closely, you’ll find that the early geneticists didn’t just accept selection, they carried out selection experiments! (Unfortunately for them, historically speaking, their conclusions usually limited the power of selection.) Thus, they combined genetics and selection (their version), not genetics and Darwinism. The synthesis with Darwinism came with the demonstration that selection could shift the distribution for some quantitative trait (e.g., height) of a population well past its initial limits without mutations, based only on recombination of standing variation. This gave selection the creative power Darwin had originally attributed to it: selection can create new forms without variation playing a directive or creative role. The Modern Synthesis was built on this premise. Of course, it eventually “came unraveled” according to Provine’s later writing, but for several decades, it was widely held that population genetics validates Darwin’s view and refutes all alternatives, including “mutationism.”

Now that I (hopefully) convinced you that the Mendelians were not clueless or a dead-end in the history of evolutionary thought, my next post will counter the common claim that the Mendelians assumed mutations were large, as well as explain why they criticized Darwin to such a strong degree in the first place.