Species as Individuals in the Hierarchical Theory of Selection 611
then evolution would occur. But we have always labeled such styles of internally
directed change as non-Darwinian, with Lamarckism as a primary and historically
most influential example.
INTERACTION. At each level, the varying individuals of an evolving population
(organisms of a deme, demes of species, species of a clade) must interact with the
environment in such a way that some individuals achieve relatively greater
reproductive success as a causal result of heritable properties manifested by these
fitter, and not manifested (or not as effectively expressed) by less fit individuals.
This causal claim embodies the key feature of natural selection as an active
process. In other words, we must be able to devise a testable causal scenario about
why the differential possession of certain heritable properties yields increased
reproductive success.
These statements inevitably engage the crucial issue of whether we should
define selection by this causal interaction of individuals and environments, or by
the product actually transmitted to future generations (see next section). The logic
of Darwinism dictates that the form of heredity's product—however fascinating in
variety across nature's scales—cannot specify agency of selection. Interaction with
environment defines agency (Lloyd and Gould, 1993; Gould and Lloyd, 1999)—
and agents must be individuals (by both vernacular and evolutionary criteria).
Some interacting individuals (like genes) usually pass faithful copies to the next
generation. Others (like species) pass inevitably modified copies that are still more
like themselves than like any other individual at their level. Still others (like sexual
organisms) disaggregate their personhood and pass hereditary pieces and particles.
All these different strategies for hereditary passage permit us to recognize
interacting individuals as causal agents of Darwinian selection. The special and
unusual tactic of sexual organisms may seem curiously indirect (and we all know
the enormous and confusing literature devoted to this subject), but disaggregation
works as well as relatively faithful passage, so long as the essential Darwinian
imperative remains in force: that is, so long as selectively successful individuals
manage to bias the next generation with relatively more of their own hereditary
material—however that material be passed or packaged. The "goal" of natural
selection cannot be defined by faithful replication, but rather by relative
"plurifaction," or "more-making."* The individual that plurifies by increasing the
percentage of its contribution to the heredity of the next generation (however the
units or items of heredity be constituted) gains in the evolutionary game. And we
call the game Darwinian if plurifaction occurs by a causal interaction between
properties of the successful individual and its environment.
*In the late 1970's and early 1980's, I engaged in long and vociferous arguments
with my graduate students Tony Arnold and Kurt Fristrup about the criteria of species
selection. (As discussed on pp. 656-670,1 now believe that they were right, and I was
wrong.) In the course of these discussions, we developed this idea and name of "more-
making" or plurifaction. (If manufacture means, literally, making it by hand, and
petrifaction means turning it into stone; then plurifaction simply means making more of
it.) I do not now remember who first devised the word, or who contributed most to the
concept's codification.