Philosophy of Biology

Haldane and the Emergence of Modern Evolutionary Theory 71

context that could be effectively modeled. What was surprising — and reassuring
to Haldane — was that natural selection was important in so many of them. But
natural selection was subtle: it worked in a bewildering variety of ways. Evolution
was complex: for Haldane, there was no single cause of evolution.
Five years later, in the altered conceptual landscape Dobzhansky [1937] was
the first to apply the new genetics to the problem of speciation and his treatment
was a significant advance over Haldane — particularly in the incorporation of
the conceptual apparatus being developed by the naturalists. He coined the term
“isolating mechanism” to denote the factors that maintain reproductive barriers
between populations. He included both genetic factors and geographical isolation
among possible isolating mechanisms. Huxley [1940] collected together major con-
tributions to systematics that led to the breakdown of the concept of the species as
a type. Mayr [1942] soon followed with hisSystematics and the Origin of Species
in which he collected an extraordinary body of data showing how geographical
isolation was critical for speciation. Mayr called this mechanism “allopatric spe-
ciation” which he distinguished from “sympatric speciation”, which would not
require geographic isolation. The latter was supposed to be rare enough to be
unimportant in the course of evolution. Mayr ignored the fact that Wright had
been emphasizing the importance of isolation since 1931. What Mayr, in effect,
had actually achieved was to provide data from the field in support of Wright’s
emphasis. This is how Dobzhansky read Mayr in 1955 (see below). Mayr, however,
came to think rather differently about what he had achieved.

6 HISTORICAL RECONSTRUCTIONS

To return to the two reconstructions of the history of evolutionary theory in the
early twentieth century, in the early 1970’s, Provine’s original views reflected what,
with very few exceptions, had been the scientific consensus for a generation. Hux-
ley put it this way in 1942:

Darwinism to-day... still contains an element of deduction, and is

none the worse for that as a scientific theory. But the facts available

in relation to it are both more precise and more numerous [than in

Darwin’s time], with the result that we are able to check our deductions

and to make quantitative prophecies with much greater fullness than

was possible to Darwin. This has been especially notable as regards

the mathematical treatment of the problem, which we owe to R. A.

Fisher, J. B. S. Haldane, Sewall Wright, and others. [1942, 21]

In 1954, the naturalist, P. M. Sheppard, was more blunt about the role of
Fisher, Haldane, and Wright: “The great advances in understanding the process
of evolution, made during the last thirty years, have been a direct result of the
mathematical approach to the problem adopted by R. A. Fisher, J. B. S. Haldane,
S. Wright, and others” [1954, 201]. However, the “mathematical theories give no