Species as Individuals in the Hierarchical Theory of Selection 683
by a principle of "disparate thirds," say with equal mixtures of dugong, aardvark,
and howler monkey. (We all know, of course, though we rarely discuss the subject
in polite company, that the Linnaean logic, which presupposes a topology of
branching without amalgamation, cannot apply to groups that do show massive
mixture, as in some families of plants with extensive hybridization, or especially in
prokaryotes evolving with frequent lateral transfer—a phenomenon that, on
accumulating evidence, may be common enough to truly discombobulate the
Linnaean version for the pre-multicellular majority of life's tree (see Doolittle,
1999), with practical and theoretical consequences as broad as any revolutionary
discoveries in the recent history of evolutionary biology.) Similarly, we all
appreciate the conceptual difficulties imposed by some prominent cases in
evolution, mostly at the genie or cellular level, that do violate the hierarchy of
inclusion—most notably, the origin of some organelles as symbiotic prokaryotes.
Since units of selection operate as interactors with the environment, and since
entities obeying the criteria of "personhood" (see pages 602-613) do occasionally
cohere by distant genealogical amalgamation, nature does present some exceptions
to the principle of a fully nested hierarchy for evolutionary individuals. But these
exceptions truly function as the "rule provers" of our mottoes (in the sense of
probing, or testing, our generalities), and not as falsifiers. The most widespread
cases, including the origin of cellular organelles by endosymbiosis, represent
"frozen" phenomena of history, not active amalgamations presently building
evolutionary individuals by junction of disparate genealogical lines. (However,
genie exceptions, as noted above, may be rife if lateral transport occurs as
frequently as current theory and data are now beginning to suggest, especially for
prokaryotes.) The most common, active cases involve symbiotic and
coevolutionary unions tight enough to obey the Biblical rule of Naomi and Ruth:
"whither thou goest, I will go." Wilson and Sober (1989), for example, present a
fascinating discussion of "phoretic associations," or obligate carriage, by wingless
insects as they move among resource patches, of various mites, nematodes, fungi,
and microorganisms. In some cases, the load of these "hangers on" can disable or
even kill the insect, and conventional Darwinism will then work in its usual,
competitive, and organismic mode. But the phoretic associates may be limited to
densities that do not affect the insect, and may also provide resources indispensable
for successful colonization of new patches—in which case, the entire association
may be evolving as a "superorganism."
With these caveats in mind—the somewhat arbitrary division of the
evolutionary hierarchy into six levels, and the acknowledgment of interesting
exceptions to full nesting among nature's various individuals—I shall try to specify
some distinctive "allometric" properties of the levels and their interactions:
THE GENE-INDIVIDUAL As we enter this first unfamiliar world of such great,
and literally basic, importance to evolution, we encounter an initial rung of strong
difference from the organism-individuals that, if only for psychological