Species as Individuals in the Hierarchical Theory of Selection 715
and Gould, 1972; Gould and Eldredge, 1977), we had only the germ of an insight
that its tenets could lend support to a generalized theory of macro-evolution, then
entirely undeveloped. We did, however, dimly grasp the key notion that punctuated
equilibrium might help to grant species a sufficient stability and coherence for
status as what we would now call an evolutionary individual, or unit of selection.
We developed this insight by groping towards an analogy that, when generalized
and fully fleshed out (with apologies for another parochial organismic metaphor of
common language!), sets a foundation for macroevolutionary theory. We dimly
recognized, in short, that if species act as stable units of geological scales, then
evolutionary trends—the fundamental phenomenon of macroevolution—could be
conceptualized as results of a "higher order" selection upon a pool of speciational
events that might occur at random with respect to the direction of a trend. In such a
case, the role of species in a trend would become directly comparable with the
classical status of organisms as units of change within a population under natural
selection. We wrote (1972, p. 112):
A reconciliation of allopatric speciation with long-term trends can be
formulated ... We envision multiple... invasions, on a stochastic basis, of
new environments by peripheral isolates. There is nothing inherently
directional about these invasions. However, a subset of these new
environments might... lead to new and improved efficiency ... The overall
effect would then be one of net, apparently directional change: but, as with
the case of selection upon mutations, the initial variations [species] would
be stochastic with respect to the change [trend].Several paleontologists groped towards a generalization during the next few
years, but Stanley (1975, 1979) made the greatest headway in appreciating the full
generality of such an analogistic procedure for macroevolutionary theory: "In this
higher-level process species become analogous to individuals, and speciation
replaces reproduction. The random aspects of speciation take the place of mutation.
Whereas, natural selection operates upon individuals within populations, a process
that can be termed species selection operates upon species within higher taxa,
determining statistical trends" (Stanley, 1975, p. 648).
Stanley preceded this statement with a claim that I regard as fully justified
and prescient, but that became a lightning rod for unfair criticism: "Macroevolution
is decoupled from microevolution, and we must envision the process governing its
course as being analogous to natural selection but operating at a higher level of
organization" (1975, p. 648). Largely on the basis of this claim about "decoupling,"
Stanley, Eldredge and I, and others, were often accused of trying to scuttle
Darwinism, and to invent an entirely new (and fatuously speculative) causal
apparatus for evolutionary change (meaning, and explicitly so stated in this
reductionistic critique, a new genetics).
We made no such claim, and the words quoted above speak for themselves.
We were trying to explore the different workings of selection on individuals at
levels of the evolutionary hierarchy higher than the conventional Darwinian focus
upon organisms. Not only do I continue to regard this procedure as