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 evolution.
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 changes.
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 2000 darwins.
"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 rapid."
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.
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