Evolution__3rd_Edition

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Box 21.1
Two Meanings of Gradualism
In the theory of evolution, the words “gradual” and “gradualism”
have both been used in two distinct senses. One refers to the
rate of evolution, and means that evolution has a fairly
constant rate. This is the meaning in the term “phyletic
gradualism.” If evolution proceeds in the mode of phyletic
gradualism, it has a constant rate; if it proceeds in the mode of
punctuated equilibrium, it is slow within a species, and faster as
new species evolve.
A second meaning refers to the evolution of adaptations,
particularly complex adaptations such as the vertebrate eye.
We saw in Section 10.3 (p. 259) that complex adaptations evolve
via many intermediate stages. They do not arise suddenly, fully
formed. It is a deep requirement of Darwinian theory that
adaptations evolve gradually, in many stages. However, it is
not a requirement at all of Darwinian theory that evolution
should have a constant rate.
Darwin, in the Origin of Species (1859) and elsewhere,
repeatedly stressed that evolution is slow and gradual. Gould
has concluded, accordingly, that Darwin was a phyletic gradualist,
and that the theory of punctuated equilibrium contradicts both
Darwin’s own ideas and also those of neo-Darwinism. By contrast,
Dawkins argued that Darwin meant something crucially different
by gradual evolution. Darwin did not make his remarks about
gradualism particularly in the context of evolutionary rates at
and between speciation events. When he did discuss that
subject, he said things that sound quite like punctuated
equilibrium, such as:
But it does not require unorthodox
ideas
CHAPTER 21 / Rates of Evolution 601
Many species once formed never undergo any further
change ...and the periods during which species have
undergone modification, though long as measured by years,
have probably been short in comparison with the periods during
which they retained the same form. (Darwin 1859)
Darwin’s theory and all subsequent versions of Darwinism,
are strongly gradualist about the evolution of adaptation. But
they are not gradualist about the rate of evolution. The only deep
requirement that Darwinian theory has about evolutionary rates is
that fossils should not evolve faster than the fastest rates seen in
selection experiments, using normal genetic variation. If fossils
evolved faster than that, it would suggest macromutations or some
such factor were contributing to fossil evolution. That really would
challenge neo-Darwinism. However, even the fastest rates of fossil
evolution are slower than the rates seen in genetic experiments
(Section 21.1.1). What appears to be fast on a geological timescale
is slow a almost too slow to study genetically a on a genetic timescale.
We saw in Section 21.1 that the main neo-Darwinian authority
on rates of evolution, Simpson, suggested that evolution shows a
range of rates, from slow to fast. Neither Darwin, nor Simpson,
argued that evolution has a constant rate. Thus, the theory of
punctuated equilibrium is interesting to test, but if it turns out right
and phyletic gradualism turns out wrong, no damage will have been
done to any deep Darwinian principle of gradualism. Adaptations
will still have to evolve in many small stages.
Further reading: Dawkins (1986), Gould (2002b).
species occupy different peaks on an adaptive topography (Section 8.12, p. 214).
Simple natural selection cannot then drive evolution from one species to the other.
Some special circumstances, or evolutionary processes, will be required, and evolution
may proceed by a rapid “peak shift.”
When Eldredge and Gould first published their theory in the 1970s, valley crossing
theories of speciation were more popular than they are now. As we saw (Section 14.4.4,
p. 394), the evidence and theoretical trends have moved against valley crossing theories
of speciation. Thus, punctuated equilibrium has been controversial because it has been
associated with a controversial set of theories about speciation. Punctuated equilibrium
has even been associated with the very unorthodox idea that evolution proceeds
by macromutations (Section 10.5, p. 266). However, punctuated equilibrium does not
depend on any of these valley crossing theories. Punctuated equilibrium can be
derived, as we saw, from the well substantiated allopatric theory of speciation. Fossils
can rarely be used to test between theories about the mechanism of speciation. The