The Essence of Darwinism and the Basis of Modern Orthodoxy 145
In a sense, the specter of directed variability threatens Darwinism even more
seriously than any putative failure of the other two postulates. Insufficient variation
stalls natural selection; saltation deprives selection of a creative role but still calls
upon Darwin's mechanism as a negative force. With directed variation, however,
natural selection can be bypassed entirely. If adaptive pressures automatically
trigger heritable variation in favored directions, then trends can proceed under
regimes of random mortality; natural selection, acting as a negative force, can, at
most, accelerate the change.
Lamarckism (defined in the modern sense of "soft" heredity) represents the
quintessential theory of directed variability. Variation arises with intrinsic bias in
adaptive directions either because organisms respond creatively to "felt needs" and
pass acquired features directly to their offspring, or because environments induce
heritable variation along favored pathways. Other directional theories differ in
viewing intrinsic variation as unrelated to adaptation, but still capable of
overwhelming any counteracting selection, and therefore setting the path of
evolutionary change. Historically important theories in this mode include various
notions of orthogenesis that postulate the inevitable origin of hypertrophied and
inadaptive structures; and theories of "racial life cycles" that envision an
ineluctably aging protoplasm doomed to extinction despite any effort at
"rejuvenation" by natural selection. (I shall discuss such ideas in Chapter 5.)
Darwin clearly understood the threat of directed variability to his cardinal
postulate of creativity for natural selection. He explicitly restricted the sources of
variation to auxiliary roles as providers of raw material, and granted all power over
the direction of evolutionary change to natural selection. Drawing his customary
analogy to artificial selection, Darwin writes (p. 30): "The key is man's power of
accumulative selection: nature gives successive variations; man adds them up in
certain directions useful to him. In this sense he may be said to make for himself
useful breeds."
Darwin also understood that variation could not be construed as truly random
in the mathematical sense—and that history did not imply or require this strict
form of randomness. He recognized biased tendencies to certain states of variation,
particularly reversions toward ancestral features. But he viewed such tendencies as
weak and easily overcome by selection. Thus, by the proper criterion of relative
power and frequency, selection controls the direction of change: "When under
nature the conditions of life do change, variations and reversions of character
probably do occur; but natural selection, as will hereafter be explained, will
determine how far the new characters thus arising shall be preserved" (p. 15).
We may summarize Darwin's third requirement for variation under the rubric of
isotropy, a common term in mineralogy (and other sciences) for the concept of a
structure or system that exhibits no preferred pathway as a consequence of
construction with equal properties in all directions. Darwinian variation must be
copious in amount, small in extent, and effectively isotropic. (Think again of a
dynamic sphere, with all radii accessible. The modal form lies at the center and
may move by selection along any radius. At any new location, a sphere of
comparable size may be reconstituted about the altered