636 THE STRUCTURE OF EVOLUTIONARY THEORY
Because bookkeeping is not the same enterprise as causality, and because we
are not, in simply counting, trying to establish the causes of differential success,
we want to make sure, above all, that we choose a unit better suited than any other
to record all evolutionary changes, whatever their causal basis. No single unit of
bookkeeping can monitor every conceivable change, but the gene becomes our unit
of choice because the nature of hierarchies dictates that genes inevitably provide
the most comprehensive record of changes at all levels. (Even so, gene records will
miss certain kinds of changes that we generally call evolutionary. For example, as
Wilson and Sober (1994) point out, assortative mating of organisms within a
population may greatly increase the ratio of homozygotes to heterozygotes at many
loci, but need not change gene frequencies in the population.)
Hierarchies are allometric, not fractal (see Gould and Lloyd, 1999), and
various levels translate a common set of causes to strikingly different results and
frequencies. Moreover, hierarchies are directional, and therefore not indifferent to
the nature of the flow of influence. As the most important of all such asymmetries,
change at a low level may or may not produce an effect at higher levels—"upward
causation" in the standard terminology (see Campbell, 1974; Vrba, 1989). But
change at a higher level must always sort the included units of all lower levels—by
the analogous process of "downward causation."
If a gene increases in copy number within a genome by duplication and lateral
spread (gene selection in the genuine sense), phenotypes of organisms may or may
not be affected. But selection on higher-level individuals always sorts the lower-
level individuals included within. If ugly organisms out compete beautiful
conspecifics, then genes for ugliness increase in the population. If stenotypic
species prevail over eurytopes in species selection, then genes associated with
stenotypy increase within the lineage. If species of polychaetes eliminate species of
priapulids in competition over geological time, then polychaete genes increase in
the marine biota.
Given this intrinsic asymmetry, what single unit would a good bookkeeper
choose? Obviously not the organism, or any high-level individual, because we
would then miss many changes at lower levels—and a good bookkeeper wants, as
the chief desideratum of his profession, to record all changes. As noted above, low-
level selection need not impose any effect upon higher levels at all. Equally
obviously, our optimal bookkeeper will choose genes— not because genes are
intrinsically more basic (the reductionist fallacy); not because genes are primary
causal agents, or causal agents at all (the gene selectionist fallacy); and not because
genes replicate faithfully (for other kinds of individuals do so as well); but, rather,
because genes, as the lowest-level individuals in a hierarchy, manifest the unique
property of recording all changes. Thus, the intrinsic nature of hierarchies sets our
preference for genes as units of bookkeeping—for only genes act as nearly
ubiquitous recorders of all evolutionary alterations, whatever their level or cause of
occurrence.
Finally, we must note one other property that, while strongly favoring genes
as units of bookkeeping, shows even more clearly why genes cannot be