690 THE STRUCTURE OF EVOLUTIONARY THEORY
speak of genes using organisms as their vehicles, they commit a deep error by
inverting causality and ascribing to genes (which only record the causal result, and
therefore serve as good units of bookkeeping) the agency in natural selection that
really belongs to the organism—for vehicles (or interactors) operate as units of
selection, or causal agents of Darwinian evolution. But when genes do not use
organisms as vehicles and engage in differential proliferation on their own accord,
then the genes themselves do act as vehicles—and, consequently, can become units
of selection. Gene selection only exists when genes can operate as vehicles
(interactors); thus, these cases illustrate the restricted range of a process that gene
selectionists naively regard as optimal illustrations of a ubiquitous phenomenon.
The resulting irony deserves emphasis. Supposed best cases become only cases,
and therefore disproofs of a generality when properly interpreted. Wilson and
Sober (1994, p. 592) put the point well: "These examples have been received with
great fanfare by gene-centered theorists as some sort of confirmation of their
theory. However, they do not confirm the thesis that genes are replicators—all
genes are replicators by definition and no documentation is needed. These
examples are remarkable because they show that genes can sometimes be vehicles.
They seem bizarre and disorienting because they violate our deeply rooted notion
that individuals are organisms."
Devotees of the genie level may eventually accept the defeat of their theory of
exclusivity with good grace—for the supplanting hierarchical model provides more
than enough room for true (and fascinating) examples of genuine genie selection,
perhaps at quite high relative frequency once we acknowledge and learn to
recognize the synergistic and orthogonal modes, as well as the better-documented
examples of genie selection that harms organisms.
Moreover, when we recognize that many kinds and aggregations of genetic
units can function in selection, the scope of this level becomes even wider.
Selection may operate at the lowest unit of the nucleotide itself, if preferential
substitution arises, for example, by differential production and consequently
greater availability of one nucleotide vs. alternatives (the analog of natural
selection by birth biasing). Selection among entire genes and other DNA segments
of comparable length may also hold great significance in evolution—as in Dover's
important hypothesis of "molecular drive" (Dover, 1982).
In fact, we may be impeding a proper recognition of the substantial frequency
of selection within genomes by naming the phenomenon for only one mode among
many—"gene selection." In the early days of Watson and Crick, biologists tended
to conceptualize genomes as linear arrays of functional units (tightly strung beads
with no spaces between in the usual metaphor). But we now know that most genes
of eukaryotes, with their structure of exons separated by introns, do not maintain
strict spatial continuity. Moreover, the functional genes of most complex
metazoans represent, in any case, just a few percent of the full genome. All other
kinds of genomic elements, forming an overwhelming majority of sites, can also
evolve by processes of drift and selection.