Douglas Futuyma has done the best job of marshalling the supporting evidence, and here are the examples he gives of observations that confirm the creative effectiveness of natural selection:
1. Bacteria naturally develop resistance to antibiotics, and insect pests become resistant to insecticides, because of the differential survival of mutant forms possessing the advantage of resistance.
2. In 1898 a severe storm in Massachusetts left hundreds of dead and dying birds in its wake.
Someone brought 136 exhausted sparrows to a scientist named Bumpus, I imagine so they could be cared for, but Bumpus was made of sterner stuff and killed the survivors to measure their skeletons. He found that among male sparrows the larger birds had survived more frequently than the smaller ones, even though the size diffferential was relatively slight.
3. A drought in the Galapagos Islands in 1977 caused a shortage of the small seeds on which finches feed. As a consequence these birds had to eat larger seeds, which they usually ignore. After one generation there had been so much mortality among the smaller finches, who could not easily eat the larger seeds, that the average size of the birds (and especially their beaks) went up appreciably. Futuyma comments: "Very possibly the birds will evolve back to their previous state if the environment goes back to normal, [In fact this is exactly what happened. The article "Oscillating Selection on Darwin's Finches" by Gibbs and Grant (Nature, vol. 32, p. 511, 1987) reports that small adults survived much better than large one folowig the we year 1982-83, completely reversing the trend of 1977-82] but we can see in this example what would happen if the birds were forced to live in a consistently dry environment: they would evolve a permanent adaptation to whatever kinds of seeds are consistently available. This is natural selection in action, and it is not a matter of chance."
4. The allele (genetic state) responsible for sickle-cell anemia in African populations is also associated with a trait that confers resistance to malaria. Individuals who are totally free of the sickle-cell allele suffer high mortality from malaria, and individuals who inherit the sickle-cell allele from both parents tend to die early from anemia. Chances for survival are greatest when the individual inherits the sickle-cell allele from one parent but not the other, and so the trait is not bred out of the populaton. Futuyma comments that the example shows not only that natural selection is effective, but also that it is "an uncaring mechanical process."
5. Mice population have been observed to cease reproducing and become extinct when they are temporarily "flooded" by the spread of a gene which causes steriliy in the males.
6. Finally, Futuyma summarizes Kettlewell's famous observations of "industrial melanism" in the peppered moth. When trees were darkened by industrial smoke, dark-colored (melanic) moths became abundant because predators had difficulty seeing them against the trees. When the trees became lighter due to reduced air pollution, the lighter-colored moths had the advantage. Kettlewell's observations showed in detail how the prevailing color of moths changed along with the prevailing color of the trees. Subsequent commentators have observed that the example shows stability as well as cyclical change within a boundary, because the ability of the species to survive in a changing environment is enhanced if it maintains at all times a supply of both light and dark moths. If the light variety had disapeared altogether during the years of dark trees, the species would have been threatened with extinction when the trees lightned.
There are a few other examples in Futuyma's chapter, but I believe they are meant as illustrations to show how Darwinism accounts for certain anomalies like self-sacrificing behavior and the peacock's fan rather than as additional examples of observations confirming the effect of natural selection in producing change. If we take these six examples as the best available observational evidence of natural selection, we can draw two conclusions:
1. There is no reason to doubt that peculiar circumstancs can sometimes favor drug-resistant bacteria, or large birds as opposed to small ones, or dark-colored moths as opposed to light-colored ones. In such circumstances the populaton of drug-susceptible bacteria, small birds, and light-colored moths may become reduced for some period of time, or as long as the circumstancs prevail.
2. None of the "proofs" provides any persuasive reason for believing that natural selection can produce new species, new organs, or other major changes, or even minor changes that are permanent. The sickle-cell anemia case, for example, merely shows that in special circumstances an apparently disadvantageous trait may not be eliminated from the population. That larger birds have an advantage over smaller birds in high winds or droughts has no tendency whatver to prove that similar factors caused birds to come into existence in the first place. Very likely smaller birds have the advantage in other circumstances, which explains why birds are not continually becoming larger.