70 Sahotra Sarkar
Though Wright did not write a book during this period, at the end of his long
paper from 1931, he presented his theory of evolution in detail:
Evolution as a process of cumulative change depends on a proper bal-
ance of the conditions, which, at every level of organization — gene,
chromosome, cell, individual, local race — make up for genetic ho-
mogeneity or genetic heterogeneity of the species.... The type and
rate of evolution... depend upon the balance among the evolutionary
pressures considered here. In too small a population... there is nearly
complete fixation [of an allele at each locus], little variation, little effect
of selection and thus a static condition modified occasionally by chance
fixation of new mutations leading inevitably to degeneration and ex-
tinction. In too large a freely interbreeding population... there is
great variability but such a close approach to complete equilibrium of
all gene frequencies that there is no evolution under static conditions.... In a population of intermediate size... there is continual ran-
dom shifting of gene frequencies and a consequent shifting of selection
coefficients which leads to a relatively rapid, continuing, irreversible,
and largely fortuitous, but not degenerative series of changes, even un-
der static conditions. The rate is rapid only in comparison with the
preceding cases... being limited by mutation pressure. Finally in a
large population, divided and subdivided into partially isolated local
races of small size, there is a continually shifting differentiation among
the latter... which inevitably brings about an indefinitely continuing,
irreversible, adaptive, and much more rapid evolution of the species.
Complete isolation in this case, and more slowly in the preceding, orig-
inates new species differing for the most part in nonadaptive respects
but is capable of initiating an adaptive radiation. [Wright, 1931, 158–
159]
Wright clearly preferred the last possibility, and there, in outline, were the
rudiments of what Wright eventually refined as his three-phase “shifting balance
theory of evolution”.
In contrast to Fisher and Wright, Haldane consistently refused to endorse any
single factor asthe cause of evolution. In this sense, Haldane did not have a
unified theoryof evolution. This attitude stems from Haldane’s catholic interest
in all details of evolutionary change, as revealed through genetic and biochemical
work. Evolution was too diverse for Haldane to endorse a particular theory. Fisher
conceived of the theory of natural selection as an analog of the most universal
of physical theories, the kinetic theory of matter. Wright came to believe that
all important evolutionary change was dominated by population structure, size,
patterns of isolation and, perhaps only to a lesser extent, selection. Haldane
refused to endorse either of these alternatives as beingthe cause of evolution.
Though he did not present any alternative he did something which, in retrospect,
was perhaps even more important. He systematically explored every evolutionary