Studies that find small variations within a species over time, such as in the size of finch beaks or the coloration of moths, are sometimes used to try to prove Darwinian evolution. But such studies are sometimes flawed. And even if valid, they provide no such proof.
Adaptation within a species is called microevolution. It is the same phenomenon at work when the average height of men and women increased by several inches in the Western world over the course of the 1900s. Better health and nutrition played a large part in producing larger-sized people. In the same way, microevolution is at work when breeders produce varieties ranging from Chihuahuas to Great Danes within the one species Canis familiaris —the domestic dog.
These examples show, as in the rest of nature, that all species do have a margin of change available within their genetic pool to adapt to conditions. This trait is found in man, who can adapt to freezing weather, as the Eskimos do, or to the broiling sun in the desert, as bedouins have done. But bedouins and Eskimos are still human beings, and if they changed environments again, eventually their offspring would also go through minor changes to better adapt to their new environment.
What has never been scientifically demonstrated—in spite of many examples of wishful thinking—is macroevolution, or the change from one distinct species to another. Dogs have never evolved into birds or human beings.
Phillip Johnson goes to the heart of the matter: "Critics of evolutionary theory are well aware of the standard examples of microevolution, including dog breeding and the cyclical variations that have been seen in things like finch beaks and moth populations. The difference is that we interpret these observations as examples of the capacity of dogs and finches to vary within limits, not of a process capable of creating dogs and finches, much less the main groups of plants and animals, in the first place . . .
"As any creationist (and many evolutionists) would see the matter, making the case for 'evolution' as a general theory of life's history requires a lot more than merely citing examples of small-scale variation. It requires showing how extremely complex biological structures can be built up from simple beginnings by natural processes, without the need for input or guidance from a supernatural Creator" (Reason in the Balance, 1995, p. 74).
Thus some cited examples of evolution at work are really no proof at all of anything —much less how any of these creatures—moths, dogs, finches or human beings—came to exist.