The evolution that took place among the finches was not as straightforward as these morphs make it seem. We have sped up the evolutionary process dramatically, by several million times, and left out intermediate forms that may have existed.
However, these morphs are not merely the stuff of fantasy. We based them on a molecular phylogeny (relationships determined from DNA) produced in the following research paper:
Petren, K, Grant, R B & Grant, P R 1999. A Phylogeny of Darwin's Finches Based on Microsatellite DNA Length Variation. Proceedings of the Royal Society of London, Series B: 266 (1417): 321-329.