On the Trails of Macroevolution (excerpt)
by Carl Zimmer
n
1972 Stephen Jay Gould attacked the puzzle of varying evolutionary rates with
one of his most controversial theories, which he proposed with Niles Eldredge
of the American Museum of Natural History. In the fossil record, species often
appear suddenly, hang on relatively unchanged for millions of years, and then
vanish. Darwin had pointed out that when it comes to the past life of this
planet, fossils are pebbles from a mountain, and he was sure that a full record
would always show evolution obeying natural selection's gradual pace.
Gould and Eldredge suggested instead that the fossils could
often be taken for their face value: new species often did branch suddenly
away from older species, lingering for millions of years relatively unchanged
until they became extinct—during which time newer species might abruptly
branch away from them. Animals didn't go to sleep one night and in the morning
find a new species running across their savanna. An isolated fragment of a
population may be able to evolve in only fifty thousand years or so into a
new species—too quickly for paleontologists to witness. If they find even
a handful of fossils of a single species in so short a span of time they
count themselves lucky. Chances would be overwhelming that those fossils would
belong to the big, unchanging section of the population rather than from the
small coterie that was actually evolving. If the new species thrived, it would
eventually spread from its small birthplace and mingle with the ancestral
species, and leave its own fossils which would seem to have appeared out of
nowhere.
According to punctuated equilibrium (the name Gould and
Eldredge put to their hypothesis), most changes happen as species originate,
not during their lifetime. In other words, species are born from other species
with certain traits which they carry to their extinction—just as an
individual animal does. And just as the variation of individuals is the raw
material that natural selection uses during microevolution, the variation
from species to species may be the raw material for macroevolution. Species
may compete, and they may give rise to new species at different rates. A
lineage in which species don't speciate much might go extinct or linger as
a living fossil, while others may be transformed—species by
species—into unimaginable new forms.
When I talk to evolutionary biologists about punctuated
equilibrium, I'm often surprised at the sting in their off-the-record remarks
pro and con, twenty-five years after the theory was first hatched. In that
time, some paleontologists have searched cliffs and mountainsides for
unbroken sketches of fossil-rich rocks where they can test this idea. In
many cases new lineages do seem to branch suddenly from one species to
another, while in some others they drift apart more gently. Meanwhile, some
researchers who have been trying to measure evolution's natural pace in
living animals have been surprised at how quickly it can move. Evolution
can change an animal's body rapidly, as Gould and Eldredge argued, but the
change doesn't have to happen in conjunction with the origin of a new
species. The sting comes from the fact that testing punctuated equilibrium
is an unfinished business. Yet no matter how it survives, it has already had
one clear effect: it prodded paleontologists to invent new ways to test the
patterns of macroevolution. It has become apparent to all sides of the
debate, for example, that many of the long evolutionary coasts suggested by
the fossil record are real, and deserve an explanation.
To some gradual-minded scientists, a record of fossils
with a 100,000-year resolution that looks like stasis may actually be hiding
a riot of generation-by-generation change that ends up not going very far in
any one direction. To others stasis means that an animal's surroundings
simply make no demands on it to change for a long time. Those who prefer a
punctuated view point out that the climate—probably the most important
part of an animal's surroundings—can dramatically swing many times over
the lifetime of a species, and yet the species will often seem unaffected.
Drastic change is rare. In the face of a stampeding glacier, it's easier for
a species of beetle to head south than stay and adapt to the new climate.
[See habitat
tracking.]
[ Carl Zimmer,
At the Water's
Edge, New York: Touchstone, 1998, pp. 230-232. ]
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