Punctuated Equilibrium and the Validation of Macroevolutionary Theory 993
A third modification of the neo-Darwinian orthodoxy is embraced in the
hypothesis of punctuated equilibrium, proposed in the early 1970's by the
American biologists Niles Eldredge and Stephen Jay Gould. The idea of
punctuated equilibrium is not intended to dispute Darwin's central notion that
the evolutionary destinies of plants and animals are shaped largely or mostly
by natural selection. But it does take issue with two of his subsidiary notions:
the idea that evolutionary change brought about by natural selection is
necessarily gradual; and the idea that natural selection can operate only at the
level of the individual.
No idea in biology has caused more contention and indeed rancor over the
past 15 years. Some opponents of Gould and Eldredge argue that their
observation is just plain wrong—that evolution is gradual. Some argue that
even if it were true it would be trivial. And some suggest that even if it were
true and not trivial, then it is in any case untestable, and therefore not worth
considering.
In truth, the paleontological record sometimes seems to show that one form
of animal may gradually turn into another, in Darwinian fashion, but often it
seems to show precisely the pattern that Gould and Eldredge propose...
It's at this point that some biologists say "So what?" Who ever doubted that
evolution can at times proceed more quickly than at others? Even if true (in
some cases), the observation is trivial. This, however, is a severe
misrepresentation of Gould and Eldredge's idea, for they are not simply
making the banal observation that evolution is sometimes fast and sometimes
slow. They are suggesting that the "jumps" that can be observed in the fossil
record represent the emergence of new species—that is, of groups of
organisms that reproduce sexually with each other but not with other groups...
Indeed, Gould and Eldredge go further than this. They suggest that when a
species divides to form several new species, this is analogous to the birth of
new individuals; and just as natural selection tends to weed out weak
individuals in favor of the strong, so it serves to weed out new experimental
species. Thus, they suggest, natural selection can operate at the level of the
species ("species selection") and not simply at the level of the individual, as
Darwin proposed. This is not a trivial observation....
The attacks on punctuated equilibrium seem powerful. But Gould gives as
good as he gets, and my own betting is that the theory of punctuated
equilibrium, with a bit more buffering from biologists at large, will take its
place as an important modification of Darwin's basic ideas.In an article on Peter Sheldon's claims for extensive gradualism in trilobites, and
therefore generally critical of punctuated equilibrium, James Gleick states that our
theory has provoked "the most passionate debate in evolutionary theory over the last
decade," and then provides a fine summary of our key ideas, and of the intellectual
depth of the resulting debate (New York Times, December 22, 1987):