622 THE STRUCTURE OF EVOLUTIONARY THEORY
The surviving particles need to reproduce in genealogical systems of
evolutionary individuals. They may do so by fissioning (faithful passage) or by
disaggregation and reconstitution of new individuals as mixtures of hereditary
parts of previous individuals. The individuals of the old generation eventually die
and evaporate. The individuals of the new generation now live on the sieve,
waiting for the next shake.
But this specification of the varied modes for constituting new individuals
does not represent what we mean by selection. An entity must be able to reproduce
to be defined as an evolutionary individual, but this entity need not replicate
faithful copies of itself. Rather, it needs to be able to plurify—that is, to increase,
relative to other individuals, the representation of its hereditary contribution to the
next generation. Integral "you" may be disaggregated in the process, but so long as
the next generation contains a relative increase in your contributions, and so long
as you operated as an active causal agent of the Darwinian struggle while you
lived, then you qualify as a unit of selection (and a winning unit in this case).
An interesting episode in the history of Darwinism clarifies this concept in a
striking manner. We all know that Darwin accepted the idea of "blending
inheritance," or the averaging of parental characteristics in the offspring of sexual
reproduction. Now blending inheritance marks an ultimate denial of half in
breeding degrades faithful replication—for the hereditary basis of any selected
character with an average individual. A paradox therefore arises. If units of
selection must be faithful replicators, and if Darwin both understood natural
selection and believed in blending inheritance, then why did he ever imagine that
selection could work as a mechanism?
We can only resolve this conundrum by recognizing that faithful replication is
not—and never was—the defining characteristic (or even a necessary property) of
a unit of selection. Darwin, even given his belief in blending inheritance, could
view sexual organisms as primary units of selection because he understood agency
in a different way that remains valid today: units of selection are evolutionary
individuals that interact with the environment and plurify as a causal result. We
may return to the metaphor of sieving. Natural selection can work under blending
inheritance because shaking the sieve favors the possessors of advantageous traits
in each generation—for any individual with a phenotype biased in the favored
direction gains a better chance of remaining on the sieve. The offspring of the most
favored individuals will blend substantially back to the mean, but this style of
inheritance only slows the process of selection—for, as a result of differential
survival and reproduction in each generation, the mean itself still gradually moves
in the favored direction.
Interaction as the proper criterion for identifying units of selection
The aforementioned arguments about sieves, plurifaction, and the
inappropriateness of faithful replication for designating units of selection lead to a
simple conclusion: we can only understand the causal nature of selection when we
recognize that units of selection must be defined as interactors, not as replicators.
Hull's distinction has great merit, but he fell into an overgenerous