Adaptation, as defined by National Academy of Sciences is “a change in behavior, physiology, and structure of an organism to become more suited to an environment.” The critical takeaway here is even after the change, the organism remains the same kind as it originally was. Simply put, a housecat and tiger are both of the feline kind. Adaptation is reversible, so long as the gene pool remains diverse. The finch gene pool actually contains a large variety of beak styles.
In an area of limited food resources, a particular beak style, most suited to that food source becomes dominant, but the bird is still a finch. If other sources of food become available, other beak styles will arise, especially if access to a general gene pool of finches remains available.
This process can also be labeled as microevolution. Microevolution, as defined by NAS.edu is “a change in the traits of a group of organisms within a species that do not result in a new species.” Unlike evolution, we can witness adaptation and observe it, therefore, adaptation falls under the scientific method.
Evolution, however, claims that changes accumulate to the point that an organism is no longer the same species. A change in kinds. And that the changes become so great that breeding with organisms of the former type is no longer possible; that is, the change is not reversible.
There is lots of evidence of adaptation. There is no evidence of evolution.