The Modern Synthesis as a Limited Consensus 511
generality among the early synthesists. He favored infinitesimal changes in large
panmictic populations, as illustrated by physical metaphors and analogies; and he
maintained little interest in the historical quirks, vagaries and complex structuring
of actual populations. When subject to such constant and unconstrained natural
selection in large panmictic populations, living creatures become exquisitely fitted
to their environments: "Organisms in general are, in fact, marvelously and
intricately adapted, both in their internal mechanisms, and in their relations to
external nature" (p. 41).
In Fisher's world of panadaptationism and pure Darwinian generality,
neutralism can only maintain an insignificant relative frequency, and natural
selection must reign. "Within such an unrelenting context of slow changes and
large populations, Darwinian functionalism must triumph: "The very small range
of selective intensity in which a factor may be regarded as effectively -neutral
suggests that such a condition must in general be extremely transient. The slow
changes which must always be in progress, altering the genetic constitution and
environmental conditions of each species, must also alter the selective advantage
of each gene contrast" (1930, p. 95).
Many examples could be cited from Fisher's Genetical Theory, but two
features strike me as particularly revealing in illustrating Fisher's maximally
exclusive and general Darwinism.
THE ANALOGY OF FISHER'S "FUNDAMENTAL THEOREM" WITH THE SECOND LAW
OF THERMODYNAMICS. After explicating and justifying his "fundamental theorem"
of natural selection—"the rate of increase in fitness of any organism at any time is
equal to its genetic variance in fitness at that time" (p. 35, Fisher's italics)—Fisher,
enamored as ever with physical analogies, compares this central principle of his
own construction with the second law of thermodynamics:
It will be noticed that the fundamental theorem proved above bears some
remarkable resemblances to the second law of thermodynamics. Both are
properties of populations, or aggregates, true irrespective of the nature of
the units which compose them; both are statistical laws; each requires the
constant increase of a measurable quantity, in the one case the entropy of a
physical system and in the other the fitness ... of a biological population ...
Professor Eddington has recently remarked that "The law that entropy
always increases—the second law of thermodynamics— holds, I think, the
supreme position among the laws of nature." It is not a little instructive that
so similar a law should hold the supreme position among the biological
sciences (1930, p. 36).In a curious and even ironic sense, the most striking feature of this analogy
lies in its imprecision and inaptness (especially as devised and presented by such
an exacting thinker)—as Fisher himself admits directly after the statement quoted
above, when he lists, as exceptions, several "profound differences" (p. 37). (The
claim for such generality in physical terms must have struck Fisher as vitally
important if he chose to make the comparison, declare its status as fundamental,
and then immediately proceed to hedge or retract