American Scientist

To the Editors:

Zachary D. Blount’s article “Replaying Evolution” (May–June) is most interesting. I think the anoles that demonstrate the repeatability of evolution all came from the same stock and the differences are easy adaptations, much like Darwin’s finches on the Galapagos Islands.

Ernest McCray
Tucson, AZ

Dr. Blount responds:

I think Mr. McCray is largely correct. The adaptations displayed by the different Anolis ecomorphs involved quantitative changes of traits that existed in their common ancestor. As I note in the article: “Adaptation is more likely to recur if it requires only quantitative changes in existing traits—such as toe pad and body size, or leg and tail length—and the anoles had the right existing traits.” In the case of Darwin’s finches, we see closely related birds that radiated from a common ancestor to fill a variety of food-based niches on the Galapagos Islands. During this radiation, the different species evolved different beak sizes and shapes as they adapted to different food resources. As with the anoles, the adaptation was based on quantitative changes in an existing trait. Consequently, I think it quite likely that we would see the same radiation occur again and again if we could replay the finches’ evolution in the Galapagos. Quantitative changes in a trait are likely to be quite easy. After all, once a trait exists, there will almost certainly be heritable mutations that have quantitative effects upon it. This means that the potential for adaptation on the basis of quantitative variation will also exist, making repeated instances of evolution in the face of similar selective pressures more likely.