The Modern Synthesis as a Limited Consensus 553
works only on organisms. But nature may (and does) work in this hierachical
manner.
- We should recognize Williams's claim as a statement about reductionism,
not (as he thought) as an invocation of parsimony. Organismic selection is not
intrinsically "simpler" than group or species selection. (One could only call
organismic selection "simpler" in the obviously invalid psychological sense of
affirming our habits and legacies as Darwin's intellectual children.) Consider
Williams's argument: "Various levels of adaptive organization, from the
subcellular to the biospheric, might conceivably be recognized, but the principle of
parsimony demands that we recognize adaptation at the level necessitated by the
facts and no higher. It is my position that adaptation need almost never be
recognized at any level above that of a pair of parents and associated offspring"
(1966, p. 1 9).
Lower levels in a hierarchy cannot be deemed inherently simpler, either to
conceive or to operationalize, than higher levels. If we had been brought up in an
intellectual world that emphasized populations, rather than organisms, as primary
entities, we would probably regard interdemic selection as maximally simple, and
organismic selection as an unwelcome complication. A priori preference for lower
levels represents a claim for reductionism, not parsimony. I do not say that such a
preference therefore becomes invalid; I simply ask evolutionists to recognize the
proper status of Williams's claim as an argument about reductionism—and also to
acknowledge that reductionism, as a cultural prejudice, may be far harder to defend
than true parsimony, when properly invoked as a logical principle (though aspects
of our preferences for parsimony may rank as cultural prejudice as well). *
In Western science, which developed with such strong traditions for
explanation by analytic division into constituent parts, claims for reduction have
often been mistakenly advanced in the name of parsimony—most notably in
biophilosopher C. Lloyd Morgan's early 20th century dictum that no human
activity should be explained by a higher psychological faculty when a lower
faculty suffices.
This inappropriate invocation of parsimony did not disable Williams's
argument because he usually proceeded beyond this theoretical point. That is,
Williams extended his argument further by presenting direct evidence favoring the
organismic mode in particular cases. He wrote: "This conclusion seldom has to rest
on appeals to parsimony alone, but is usually supported by specific evidence"
(1966, p. 19). But subsequent developments force us to
*Footnote added in proof stage: Just as I submitted this completed book to the
publisher, the press conference on Darwin's birthday (Feb. 12, 2001), announcing the
very low number of genes in the human genome, struck the deepest blow of our lifetimes
against the conventions of reductionism, and for the irreducibility of proteomic (and full
phenotypic) explanation to simple properties of codes at lower levels. Combinations,
replete with emergent properties, and the specifics of contingent phyletic histories, must
become a key partner, if not a primary locus, for biological explanation (see Gould,
2001).