510 THE STRUCTURE OF EVOLUTIONARY THEORY
amount (saltations occur only rarely, but minor variants are ubiquitous); and (2)
greater potential for utility (small changes can often be advantageous, but large
excursions will almost always be disruptive—see Fig. 7-1, from Fisher). When De
Vries and other biologists had disparaged small-scale variation as ineffective and
different in kind from Mendelian effects, saltations had prevailed faute de mieux.
But once we can assert a continuum in effect and a uniformity in genetic
mechanism for variation at all scales, then we must prefer an omnipresent and
potentially advantageous mode to an exceedingly rare and almost always
deleterious extreme in the spectrum. Fisher writes: "The chance of improvement,
for very small displacements, tends to the limiting value one-half, while it falls off
rapidly for increasing displacements (p. 39). In any highly adapted organism the
probability of advantage through any considerable evolutionary step (saltation)
rapidly becomes infinitesimal as the step is increased in magnitude" (p. 114).
Orthogenesis, the third and last of Kellogg's alternatives to Darwinism,
expires with Fisher's initial argument on the virtues of particulate inheritance. The
rates of mutation required by Darwinism become much lower under particulate
models (and observation confirms that such a workably modest rate of mutation
exists in nature). Since orthogenesis can only operate when mutation pressure
becomes high enough to act as an agent of evolutionary change, empirical data on
low mutation rates sound the death-knell of internalism. Fisher writes: "For
mutations to dominate the trend of evolution it is thus necessary to postulate
mutation rates immensely greater than those which are known to occur, and of an
order of magnitude which, in general, would be incompatible with particulate
inheritance" (1930, p. 20).
Fisher, with his mathematical training, reigned as the master of abstract
7 - 1. R. A. Fisher's classic illustration of the negative relationship between magnitude of change
and probability of evolutionary utility. The vast majority of large mutations are deleterious;
small mutations are both far more frequent and more likely to be useful. From Fisher, 1930.