Lizard experiment suggests rapid evolution
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An experiment with lizards in the Caribbean has demonstrated that evolution
moves in predictable ways and can occur so rapidly that changes emerge in as
little as a decade.
The experiment bears on two theories of evolution, that of punctuated
equilibrium and that of gradualism. Gradualism states that evolution is a
relatively slow, constant process, producing changes over millions of years.
Punctuated equilibrium states that environmental constraints hold species
remain unchanged for millions of years, which then undergo rapid evolution
when environmental changes demand it. The results of the experiment suggest
that there are no constraints, and no difference between gradual and rapid
The experiment involved the introduction of one species of lizard to fourteen
small, lizard-free Caribbean island near the Exumas in the Bahamas. The
lizards were left for fourteen years.
The original intent of the experiment
was to study extinction. The experiment, started by Thomas Schoener of the
University of California at Davis, would have provided scientists with
important information as they observed the extinction of the introduced
lizards. Unfortunately, the lizards adapted to their new environments, and the
focus of the experiment changed to study this rapid evolution.
Lizards on Caribbean islands have been carefully studied by biologists for
their adaptation to different conditions on different islands with
corresponding changes in body shape.
Birds, most notably the Finches of the
Galapagos islands, also show such specializations when favoring a certain
island. One of the specializations of lizards noted by scientists over the
years has been that lizards that inhabit large trees tend to have long legs,
whereas those lizards that live on twig-like plants have short legs.
Jonathon Losos of Washington University in St. Louis stated that such
adaptations allowed scientists to predict what would happen to the lizards
placed on the islands, some of which are smaller than a football field. The
more the vegetation differed from that of their original home, Staniel Cay,
the more the lizards should evolve.
The scientists predicted that evolutionary
pressure would cause the long-legged lizards to produce short-legged forms as
the Caribbean islands are almost treeless.
Losos and his colleagues report in the journal Nature that the lizards evolved
in the direction as predicted. Those with the shortest legs are found on
islands with the scrubbiest vegetation.
Douglas Futuyama of the State University of New York at Stony Brook, states
that while there are many known instances of rapid evolution in biochemistry,
such as evolving resistance to pesticide, there are fewer examples of bodily
A long-standing issue in biology is whether small evolutionary changes are the
same as the large evolutionary changes seen over millions of years. In biology
terms, the question is whether microevolution is the same as macroevolution.
One well known macroevolutionary event is the specialization of lizards on
Caribbean islands. Lizards have evolved into 150 different species spread
across these islands. Losos and his colleagues write that their lizard
experiment suggests that macroevolution is simply microevolution observed over
a much larger time period.
Punctuated equilibrium proponents suggest that a given species may remain
unchanged for millions of years until some event shakes up the ecosystem,
causing rapid evolution. Since the lizards on all 14 islands evolved as
expected, Futuyma states that "it means you don't need to invoke a complicated
hypothesis of this type."
The rate of evolutionary change is measured in units called darwins. Darwins
provide a measure of the proportional change in a given organ over time.
Changes typically seen over millions of years in the fossil record usually
amount to 1 darwin or less. The transplanted lizards evolved at rates of up to
"Darwin thought that natural selection had to be slow and gradual," Losos
said. "I think it is clear he was mistaken. In some cases change can be very
The results of the experiment echo observations made in the 1980s of one
species of Finch on one Galapagos island. Over a ten year period there were
three major swings in the ecosystem of the island. At the start of the
observation period, there were two morphs of the Finch, a large beaked morph
and a small beaked morph. One change in the ecosystem favored the large beaked
morph over the small beaked morph, with the latter nearly becoming extinct. A
second change in the ecosystem favored the now nearly extinct small beaked
morph, and within a short time it was the dominant morph, with the large
beaked morph on its way to extinction. Finally, the ecosystem shifted again,
and populations stabilized.
Today, a third morph has appeared, with a beak
intermediate between the large and small beaked morphs. Over a ten year
period, three natural selection events occurred, suggesting that evolutionary
change might be more rapid than ever before suspected.
Wade, Nicholas. 1997. Exiled lizards clearly show evolution.
Dispatch, 4 May, sec. D, 6.
Copyright © 1997 by Jeff Poling.
Revised: May 5, 1997; New: May 5, 1997
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