Species as Individuals in the Hierarchical Theory of Selection 707
- Although I recognize that some notion of a common environment must be
invoked when we wish to define allopatric species as competing inter-actors, I do
not view such a requirement as either rarely met or particularly difficult to specify.
(I only mean, by the last phrase, "any more difficult to specify than for sympatric
interactors." We cannot know, in fully adequate detail, how individuals at any level
react to all nuances of the environment, in all their horrendously complex and
nonlinear interactions. Who can say whether two sympatric organisms, given their
inevitable differences, perceive the same local change of environment—even such
a linear effect as a falling temperature—in the same way? I am only arguing that
we face the same difficulty for sympatric, as for allopatric, interactors in this
respect.)
At least two strong arguments support the notion of adequate environmental
similarity in allopatry:
(i) Environments cannot be conceptualized (or even operationalized) as
objective places or circumstances in a world fully external to the organisms
involved. First of all, environments include all interactions with other organisms,
both conspecific and belonging to different taxa, and not just the climates,
substrates, and other more measurable properties of a surrounding physical world.
Second, and more important, as Lewontin has emphasized so forcefully (1978,
2000), environments are intrinsically referential, and actively constructed by the
organisms in question. Environments, in short, are made, not found. Thus,
important properties of the environment must be sufficiently comparable in a set of
closely related and partly allopatric species engaged in a process of species
selection. These species share key traits as autapomorphies of their clade—and
since these traits help to construct the relevant environment, sufficient similarity
becomes, in part, an active construction of related organisms, not only a
happenstance of common externalities.
(ii) Organisms needn't occupy the same turf in order to be impacted in similar
ways by the kinds of broad environmental changes that seem so important at
geological scales. To choose an extreme example, when, 65 million years ago, a
large bolide struck the earth in the region now occupied by the Yucatan peninsula,
I suspect that Tyrannosaurus rex in the western United States, and its recently
discovered sister taxon in Africa, experienced consequences sufficiently common
and negative to influence their extinction (while some small-bodied mammals,
living there and elsewhere, survived as a consequence of organismal or higher-
level characters that also do not require sympatry with dinosaurs for meaningful
comparison). Again, Williams (1992, p. 25) explains the issue succinctly and at
more immediate scales: "Suppose a climatic change causes the brown trout of the
upper Rhine to die out but lets the brown trout of the upper Danube survive.
Suppose further that the difference in fate is attributable to some difference in gene
frequency that causes a difference in vulnerability to the change. That is surely
clade selection. The ultimate prize for which all clades are in competition is
representation in the biota." - In many cases of species selection, the success of one species over an-