Seeds of Hierarchy 245
prevail over other variants in more central environments? After all, the more
central ancestor continues to survive after the extreme variant buds off. The
ancestor should therefore be favored in its own central environment, and the
descendant in its new peripheral station. Why, then, should the descendant ever
replace the ancestor—so long as central environments persist along with marginal
places? In all his writing on divergence, Darwin recognized that trends to
specialization could not occur unless extreme descendants tended to wipe out more
central ancestors in competition: trends, in other words, required a pattern of
differential extinction as well, for the number of species in a region cannot increase
indefinitely.
And here, after so much effort and careful development, Darwin bogged
down. For this most resolutely higher level phenomenon of the supposed
differential success of extreme vs. central species, Darwin could not provide a
tenable argument based upon natural selection. With evident discomfort, Darwin
resorted to an ad hoc assumption: he argued that while extreme variants adapt to
their marginal stations, they also retain all the adaptations of their parents for the
original central habitats. Thus, the descendant extremes remain as good as their
parents in the ancestral environment, while adding a capacity for survival in
marginal habitats.
But how can such a proposition be defended? Why should a species that has
left one environment, and explicitly adapted to another, still retain all its prowess
in an environment no longer inhabited (and from which it has actively diverged)?
Not only does this proposition make no sense prima facie; such a claim also
contradicts the canonical argument (often embraced by Darwin and his
contemporaries) that specialization leads to "locking in" and decreased flexibility.
In short, Darwin knows that he has run into a severe logical problem in trying to
justify a central implication of his general argument: the differential survival of
extreme taxa with a consequently preferential extinction of central species. How
can such a pattern be explained—for central and marginal species should not, after
all, be in overt competition, and central environments cannot be regarded as
generally more evanescent than extremes? Darwin therefore invokes his ad hoc
argument (described just above) for an expanded range of adaptation in extreme
species, in order to place the organisms from these extremes into competition with
their parents, thus generating a hypothetical explanation for differential parental
death in terms of natural selection.
Darwin begins by stating his ad hoc assumption: "As m^1 tends to inherit all
the advantages of its parent M [see right side of Fig. 3-6], with the additional
advantage of enduring somewhat more drought, it will have an advantage over it,
and will probably first be a thriving local variety, which will spread and become
extremely common and ultimately, supplant its own parent" (in Stauffer, ed., 1975,
p. 239).
But Darwin immediately senses a problem and recognizes that descendants
might not retain the parental range, and that ancestors might survive the onslaught
of their phyletic children by living in a different station, thereby avoiding
competition. "If m^1 "^10 had been produced, capable of enduring more