Sewall Wright for dunces
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.