Sir Ronald Aylmer Fisher 45populations. Haldane is closer to Fisher than he is to Wright. But Haldane’s sup-
port of Fisher is not in the form of an uncritical summary. Haldane carried the
mathematical exploration of selection in finite population sizes, changing envi-
ronments, and in multiple dimensions further than Fisher had, and in part as a
critical consideration of Wright’s work on the topic. Moreover, and importantly,
Haldane emphasized the problem of the units of selection, recognizing that selec-
tive forces acting on gametes, organisms, or populations may come into conflict.
Today, the book is known primarily for its critical comparison of Fisher’s, Wright’s,
and Haldane’s own mathematical explorations of this issue. But the book is much
more than that. Haldane accomplishes a synthesis not merely of Darwinism and
Mendelian heredity, but also, under the rubric of population genetics, of chromo-
somal mechanics, cytology, and biochemistry. (See, e.g., [Sarkar, 2004].)
Wright’s 1931 paper, “Evolution in Mendelian Populations,” is his main early
paper demonstrating the mathematical unification of Darwinian natural selection
and the principles of Mendelian heredity. Wright communicated this synthesis in
the form of his Shifting Balance Theory of evolution. Wright would use his famous
“adaptive landscape” diagram to communicate his theory a year later, in 1932.
Flowing from his work as a physiological geneticist, Wright emphasized genetic
interaction in evolution by way of the following assumption: Because the field of
gene combinations in a population are vast, genes adaptive in one combination
may not be adaptive in another; consequently, the field of joint gene frequencies
graded for adaptive value will produce a “hilly landscape.” Populations thus face
the problem of shifting from one peak to another. Wright solved the problem of
“peak shifts” with his three-phase shifting balance process. In the first phase,
subdivided populations would be subject to genetic drift, pulling them into a
“valley.” In the second phase, those populations would be dragged up the next
peak by intrademe selection, or within-group selection. And in the third phase, the
global population would find the highest peak by interdeme selection, or selection
driven by differential dispersal.
Wright’s mathematical analysis of the reconciliation between Darwinism and
Mendelism agreed with Fisher’s and Haldane’s. However, Fisher and Wright dis-
agreed about the extent to which epistatic gene interaction, genetic drift, and
population structure were important in accounting for cumulative evolutionary
change. Where Wright imagined a “hilly” adaptive landscape, requiring a con-
stellation of evolutionary factors to traverse it, Fisher saw a landscape with a
single peak, requiring only selection and mutation to traverse it. And beginning
in 1929, Wright and Fisher became mired in controversy over their alternative
understandings of the evolutionary process that would last until Fisher’s death in
1962 [Provine, 1986]. But Fisher’s death did not end the controversy. Fisher’s
and Wright’s acolytes, as well as Wright himself (until his death in 1988), would
continue exploring the differences between the apparently alternative evolutionary
theories. Between 1997 and 2000, in fact, a dispute erupted reminiscent of the de-
bates between Fisher and Wright between the team of Jerry Coyne, Nicholas Bar-
ton, and Michael Turelli [1997; 2000] and the team of Michael Wade and Charles