[This is one more post in my growing "X for dummies/idiots/morons/etc." series.]

I've been enjoying Provine's The origins of theoretical population genetics for the last couple of days, but I must confess that I find his description (on pp 127-8) of the logic behind Sewall Wright's (and W. Castle's) selection experiments with "hooded rats" about as clear as if it had been chiseled in Linear A...

Which is really bumming me out, because I'd love to understand these experiments.

Does anyone know where I can find a description of these experiments aimed at those with only a very limited grasp of classical genetics? (I took a graduate-level genetics class a billion years ago, and I did not find it easy.)


I think there might be several places to read a description of these experiments, but they are discussed extensively in a book by historian-philosopher Lindley Darden, entitled "Theory Change in Science: Strategies from Mendelian Genetics", parts of which are available online. See p. 112 of Darden's book for references to other accounts of these experiments by historians E.A. Carlson (1966) and W. Provine (1988).

Everyone knows what Castle did but his aims have been obscured by "Synthesis Historiography" (i.e. making things turn out right for Ernst Mayr and his homies).

What Castle did was to breed rats in the laboratory and implement selection on coat patterns, with such success that he has able to get a large seemingly continuous range of forms. Imagine breeding oreos, with one line of selection of darker forms, and one of lighter, and achieving the success of getting both pure-black or pure-white oreos, and then lining up all the oreos from black to white to show the continuous range of forms.

The story that we hear from people like Wright and Provine is that this proved that selection can work on continuous variation, an idea that the "mutationists" allegedly rejected, paving the way for acceptance of Darwinism.

Yet, while the experiments may have helped one version of Darwinism, they also represented the dying gasps of another form of Darwinism-- the form that Darwin proposed and Castle was (partially) defending.

You can get an inkling of this from p. 143 of Provine's book, where you will find a curious confession, made in passing. Provine is explaining why Pearson did not want to publish Fisher's famous 1918 paper. He writes "Pearson claimed, and Darwin would probably have agreed, that the continuous variations in a pure line were heritable and that continued selection in a pure line should be effective."

Why would Pearson believe-- 17 years after the rediscovery of Mendelian inheritance-- that the environmental variations in Johannsen's pure lines were suitable material for selection? The reason is that this is what Darwin believed, and Pearson was a follower of Darwin. Darwin's "indefinite variability", the fuel for modification in his theory of "natural selection", described in Ch. 1 of the OOS, is clearly a description of environmental variation-- it is always present and emerges anew every generation in response to "conditions of life" (see Winther, 2000). Darwin knew about "definite" single variations or "sports", which could be inherited perfectly (whereas indefinite variations were inherited by blending), but he didn't think they were important for evolution. Darwin bet on the wrong horse, as Johannsen showed in 1903.

Pearson, Castle, and others defended Darwin's original view well into the 20th century. They were not satisfied with the idea that selection merely sorted out stable Mendelian factors that undergo rare mutations. Instead, they held out hope that there was some other form of heredity, or that the Mendelian units were squishy and underwent continuous shifts in potency under conditions of selection. Thus Castle and Phillips in 1914 argued that "the unit character for hooded pattern is itself variable"

Muller, Sturtevant and others from Morgan's lab disagreed, arguing instead that there were simply various Mendelian modifying factors in the background affecting coat color. There was a long-running dispute over the proper genetic interpretation of Castle's results. If you want a very clear statement of the dispute, go to a 1916 article by Jennings, p. 287:

"Castle finds that in rats he can, by selection, gradually increase of decrease the amount of colour in the coat, passing by continuous stages from one extreme to the other. As to this, he holds two main points:

  1. The change is an actual change in the hereditary characteristics of the stock; not a mere result of the recombination of Mendelian factors. This is the general and fundamental point at issue.

  2. More specifically, he holds it to be an actual change in a single unit factor; this single factor changes its grade in a continuous and quantitative manner.

On the other side, the critics of these views maintain that the changes shown are not actual alterations in the hereditary constitution at all, but are mere results of recombinations of Mendelian factors. And specifically, they find a complete explanation of such results as those of Castle in the hypothesis of multiple modifying factors."

A few years later in 1919, Castle recanted his earlier view and accepted the multifactorial theory (the development of which is described by Kyung-Man Kim, cited below).

And finally, the idea that the "mutationists" rejected selection on quantitative variation is mistaken. What the mutationists rejected was selection on non-heritable environmental variations. Johannsen's 1903 results were revolutionary because scientists at the time understood that they refuted Darwin's theory. We don't "get" this today because "Darwinism" has been redefined to disentangle "natural selection" from Darwin's errant views of heredity-- see Jean Gayon's "Darwinism's Struggle for Survival" for more explanation.


Kim K-M. 1994. Explaining scientific consensus : the case of Mendelian genetics New York: Guilford Press. xxiv, 239 p. p.

Gayon J. 1998. Darwinism's Struggle for Survival: Heredity andthe Hypothesis of Natural Selection Cambridge, UK: Cambridge University Press.

Winther RG. 2000. Darwin on Variation and Heredity. Journal of the History of Biology 33:425-55.


I don't have access to Provine's book, and I can't describe the details of the hooded rat experiments, but here is an attempt to explain the importance of the work.

Darwin published “Origin of Species” in 1859. He proposed that modern species were all descended from ancestral species, and that evolution proceeded by natural selection. He believed that evolution proceeded gradually by an accretion of small changes (gradualism). The origins of hereditary change were, of course, unknown to Darwin.

Mendel’s work (published in 1865) was rediscovered in 1900, and gave rise to a school of thought referred to as mutationism. Mutationist theories emphasised the importance of single factor changes with large effects, and placed these “leaps forward” at the centre of the mechanism for the generation of new species. This was in contrast to the biometricians who argued, in the Darwinian tradition, that natural selection acted on populations in which there was continuous variation. Extreme mutationists believed that continuous variation within a species had no relevance to the evolution of new species. Their view was that within a species most genes were fixed (100% wild-type allele), and that only mutations could create new species (‘hopeful monsters’). T.H. Morgan, for example, was a strong proponent of the mutationist theory, so this wasn't some minor controversy.

Sewall Wright joined the laboratory of W.E.Castle as an assistant in 1912, when he was 23. Starting with black and white hooded rats (white rats with black ‘hoods’) they instigated breeding programmes and showed that they could progress gradually, through a number of generations, towards all white or all black rats. The importance of this work was that it demonstrated that there was underlying genetic variation within this strain of rats that could be acted upon by selection (selective breeding) resulting in gradual changes in coat colour. This provided strong evidence for the gradualist school.

By the 1930s the modern synthesis began to emerge, which combined mutationist and gradualist ideas: natural selection acts upon populations (gene frequencies etc.) but mutation provides the raw material that creates new variations in the population. Key to the acceptance of the modern synthesis was the development of a mathematical treatment of population genetics. This led to the current view of phenotypic traits influenced by multiple loci, with natural populations harbouring genetic variability which can be acted upon by selective forces. We have all accepted this and so of course it is difficult to think ourselves into the controversies that existed in genetics at the beginning of the twentieth century.

Here is an article in which Sewall Wright refers briefly to his work with Castle, placing it in historical context. It also describes subsequent work with guinea pigs. This is a quotation from the article which underscores what I have written:

From assisting Prof. Castle, I learned at firsthand the efficacy of mass selection in changing permanently a character subject merely to quantitative variability. Because of this and a distaste for miracles in science, I started with full acceptance of Darwin's contention that evolution depends mainly on quantitative variability rather than on favorable major mutations. Thus, I have assumed that species are typically heteroallelic in tens of thousands of loci in which the leading alleles differ only slightly in effect, a situation that is maintained in a continually shifting state of near-equilibrium by the opposing pressures of recurrent mutation, diffusion and weak selection. Only a few loci at any time can show fairly rapid changes in allelic frequencies from strong selection.

  • $\begingroup$ Thanks for this summary (BTW, the first half of Provine's book is a chronicle of the biometrician/mutationist battle), the link, and the quote by Wright. His is a clarity of understanding that we seem to have lost somehow. $\endgroup$ – kjo Jun 12 '13 at 10:43
  • $\begingroup$ @kjo So, you could have probably written this better yourself! Hopefully it will help other readers of your question and maybe you'll get the answer that you are looking for. I agree about clarity - it reminds me of the writing of AD Hershey, a "founding father" in a later generation of geneticists. $\endgroup$ – Alan Boyd Jun 12 '13 at 11:05

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