38 Robert A. Skipper, Jr.
2 AT ROTHAMSTED: A PERIOD OF INTENSE ACTIVITYIn 1919, Fisher accepted a statistician’s post at Rothamsted Experimental Station,
one of the oldest agricultural research institutions in England. There, between
1919 and 1933, Fisher made substantial contributions to statistics and genetics.
And by 1929, he was elected to the Royal Society. In statistics, Fisher introduced
the concept of “likelihood” in 1921. The likelihood of a parameter is proportional
to the probability of the data and it gives a function which usually has a single
maximum value, i.e., the maximum likelihood. The next year Fisher introduced a
new conception of statistics the aim of which was the reduction of data and the
problematic of which was the specification of the kind of population from which the
data came, estimation, and distribution. Soon thereafter, in 1925, Fisher published
Statistical Methods for Research Workers, in which he articulated methods for the
design and evaluation of experiments. The 1925 edition was the first of many.
In genetics, Fisher published the first paper, in 1922, adumbrating a mathemat-
ical synthesis of Darwinian natural selection with the recently rediscovered laws
of Mendelian heredity. In that paper, entitled “On the Dominance Ratio,” Fisher
discusses, as he says, “the distribution of the frequency ratio of the allelomorphs
of dimorphic factors, and the conditions under which the variance of the popula-
tion may be maintained” [1922, p. 322]. He sees this paper as following on the
heels of his 1918 paper, “The Correlation Between Relatives on the Supposition of
Mendelian Inheritance.” In broad brush strokes, what I take this to mean is that
where the 1918 paper defended the principles of Mendelian heredity against the
criticisms of the biometricians (and in fact showed the two schemes to be com-
patible), the 1922 paper continues by carrying through its mathematical methods
and concepts as well as defending Darwinismusingthe principles of Mendelian
heredity. Specific to “On the Dominance Ratio,” Fisher’s aim was to respond to
a set of criticisms that Darwinian natural selection cannot be the correct expla-
nation of the modulation of genetic variation in populations because the genetics
of populations are such that there is not enough variation available for selection
to act on. In his response, Fisher considered the interaction of natural selection,
random survival (genetic drift), assortative mating, and dominance. During the
course of the paper, Fisher eliminated from consideration what he took to be in-
significant evolutionary factors, such as epistatic gene interaction and genetic drift,
and argued that natural selection acted very slowly on mutations of small effect
and in the context of large populations (10^4 )maintaining a large amount of genetic
variation.
Consider drift, or what Fisher referred to variously as random survival, steady
decay, or the “Hagedoorn effect.” The phrase “random drift” comes from Wright’s
1931 landmark paper, “Evolution in Mendelian Populations.” Notwithstanding
Wright’s obvious contributions to the development of the concept and mathemat-
ical modeling of drift, it was Fisher who, in this 1922 paper, was the first among
the architects of population genetics to mathematically explore the evolutionary
consequences of drift in a Mendelian population.