Isolated areas sometimes have unusual species of plants and animals, and don't
have plants and animals that are common in other places. A theory
called "adaptive radiation" gives an
explanation for this. When a plant or animal species finds its way to the
isolated area, it might "radiate" into several new species as a way of
"adapting" to new ways of life.
For example, the Galapagos Islands are a group of volcanic islands in the Pacific Ocean 600 miles west of Ecuador, South America. There are two kinds of Iguanas on Galapagos: land and marine iguanas. Iguanas are usually desert animals. Galapagos is the only place in the world with marine iguanas. They swim and climb around the rocks just off the beaches and dive as much as ten feet to eat seaweed.
Recent studies of the iguanas of the Galapagos posed a problem for the "adaptive radiation" explanation. According to DNA dating, Galapagos marine iguanas separated from land iguanas about 12 million years ago, but the oldest rock on the Galapagos Islands has been measured by radioisotope dating as being only between 2 and 3 million years old.
How could have the iguana species diverged on the islands 12 million years ago if the islands are only 2-3 million years old?
(Galapagos Marine Iguana photo Courtesy Dr. Stephen Ervin.)