FIGURE 4. BMP- and CaM-dependent signalling regulates growth along different axes, facilitating the evolution of distinct beak morphologies in Darwin's finches.
From the following article:
The calmodulin pathway and evolution of elongated beak morphology in Darwin's finches
Arhat Abzhanov, Winston P. Kuo, Christine Hartmann, B. Rosemary Grant, Peter R. Grant and Clifford J. Tabin
Nature 442, 563-567(3 August 2006)
doi:10.1038/nature04843
a, Developing avian beak is a three-dimensional structure that can change along any of the growth axes. b, A beak of the sharp-beaked finch reflects a basal morphology for Geozpiza. The model for BMP4 and CaM involvement explains development of both elongated and deep/wide beaks of the more derived species. Abbreviations: C, caudal; D, dorsal; R, rostral; V, ventral.
