When the sizes of finch beaks oscillates, it is because of an oscillating environment. The size changes within a species are large enough to explain the differences between the various species of Galápagos finches, species Charles Darwin initially thought belonged to several different families of bird. Not all of these changes oscillate, the evolution of Darwin's finches has been directional in some aspects.
The figure shown here illustrates that, as the climate in the Galápagos has changed due to the multiyear El Niño cycle, the finches have changed body size, beak size, and beak shape (a measure of width, length and depth). Some of those measurements have returned to levels that were seen historically (inside the faint horizontal lines in the figure), while other measurements continue to diverge.
As the Grants describe in "Unpredictable Evolution in a 30-Year Study of Darwin's Finches", "The temporal pattern of change shows that reversals in the direction of selection do not necessarily return a population to its earlier phenotypic state." This is the opposite of what Explore Evolution describes:
After the heavy rains of 1983, the depth of the average finch beak went back to its pre-drought size, and the so-called "evolutionary change" was reversed.Explore Evolution, p. 93
While one species did wind up at roughly the same beak size, the beaks at the end were sharper than they had been at the beginning, while the other species has a smaller and blunter beak than it had to begin with.
Does this demonstrate that evolution has limits? Not at all. We would not expect finches to evolve more rapidly over 30 typical years. Over the thirty years, the environment oscillated within limits, and the finches evolved and adapted to that changing climate. When the climate returned to the state it had been in at the beginning of the study, the finches became more similar, but not identical, to their initial state. Why students ought to assume that finches could not have changed more if the environment had changed more is not at all clear, especially since the size and shape of finches and their beaks continues to change and to cross historical limits.
Of course, the climate is not constant, either. Peter Grant explains:
The climate of the Galápagos has not remained stable over the last 50,000 years. This is known from an analysis of particles and plant products in cores taken from the sediment of El Junco lake on the summit of San Cristobal (Colinvaux 1972, 1984). Inferences can be made about changes in water level, cloud cover, and heat budget from the composition of the cores at different levels.
The present climate has persisted for the last 3,000 years, and it also prevailed about 6,200 and 8,000 years ago. In the intervening period of 3,200 years it was drier, and possibly hotter, than now. Going back further, it was drier before 8,000 years ago. The most different climate regime from the present one occurred from about 10,000 to 34,000 years ago; this was a time of little precipitation or evaporation.Grant, P. R. (1999) Ecology and Evolution of Darwin's Finches, Princeton University Press, Princeton, NJ, pp. 29-30
We know that finches can and do undergo significant morphological change when the climate changes. We know the climate changes. Explore Evolution simply insists that we should not follow the syllogism to its conclusion, and decide that the finches would have undergone large changes during periods of large environmental change. Explore Evolution invokes limits, but provides no actual evidence that inherent limits on evolution operate beyond those imposed by limited environmental variability. This is not, needless to say, how science proceeds, and it is not what we would expect from an inquiry-based approach to science. Unanswered questions are not places where scientists draw lines, they are opportunities to make new discoveries. An inquiry-based text should invite students to propose hypotheses about finch evolution, and provide teachers with a suite of data for students to test their hypotheses.