326 Margaret Morrison
but their relative importance was unknown.
Wright was able to extend his method to include cases where causes were cor-
related instead of independent. He used it to establish the relative importance of
heredity and environment in determining piebald patterns in guinea pigs, and also
applied it to systems of mating. By looking at the effects of various systems of
inbreeding on the genetic composition of the population, the effects of assortive
mating, and the effects of selection, Wright was able to calculate the percentages
of homozygosity in successive generations. Like those of Fisher, Wright’s math-
ematical methods allowed for precise quantitative determination of the effects of
natural selection as the agent of evolutionary change.
Around the same time Fisher [1922] presented an extended discussion of the
conditions under which variance could be maintained and a way to determine the
overall effects of selection and mutation in achieving this. He wanted to show how
gene frequencies would change under selection pressures and particular environ-
mental conditions. Necessary for that approach was the reduction of a population
to their composite genes and an isolation of selection pressures from other types
of conditions. One again the analogy with gas theory was a crucial piece of the
methodological puzzle. By using the model of an ideal gas, Fisher was able to
“create” a population in which he could measure effects not measurable experi-
mentally. The mathematical technique used for characterizing a population in the
1918 paper provided the instrument for investigating the role of selection in hu-
man populations by replacing actual populations with idealized ones. In the 1922
paper “On the Dominance Ratio”, Fisher argued that an equation representing
the stochastic distribution of Mendelian determinants in a population over time
was the key to an accurate and quantitative understanding of evolution in that
population. He needed only general statistical laws about the behaviour among
individuals, rather than specific knowledge of the individuals themselves, in order
to determine the effects of evolutionary mechanisms.
The paper began with a discussion of equilibrium under selection. Fisher first
demonstrated that the frequency ratio for the alleles of a Mendelian factor was a
stable equilibrium only if selection favoured the heterozygotes. He then showed
that the survival of an individual mutant gene depended on chance rather than
selection. Only when large numbers of individuals were affected would the effect
of selection override random survival, and even then only a small minority of the
population would be affected. Fisher also examined the distribution of factors not
acted on by selection, cases of gene extinction counterbalanced by mutation, and
extinction in the absence of mutation and selection, where one saw a steady decline
in variation due to the effects of random survival (Hagedoorn effect). On the basis
of his calculations of the number of genes exterminated in any one generation
and the distribution of factors in successive generations he was able to show that
even in a population of roughly 10,000 random-breeding individuals without new
mutations the rate of gene extinction was extremely small. Hence, the chance
elimination of genes could not be considered more important than elimination by
selection.