Fig. 1.

Contribution of the different skeletal structures to beak variation in Darwin's finches. (A) The large, medium, and small ground finches have deep beaks used to crack seeds. In contrast, the large cactus and cactus finches use their elongated beaks to feed on pollen and nectar from flowers. (B) At stage (st.) 27 the prenasal cartilage (pnc) condensation, labeled with Col2a1, occupies a significant portion of the beak primordia in the large and medium ground finches. By st. 30, the pnc, labeled with Col2a1, occupies a smaller proportion of the beak relative to the developing premaxillary bone (pmx). (C) Interspecific variation in Darwin's finches is caused mainly by differences in the amount of the pmx. The large and medium ground finches have larger pmx volumes than their size-matched large cactus and cactus finches, respectively. (D) These differences in adult pmx volume correlate with the time and place of expression of the osteogenic marker alkaline phosphatase (Alk. Phosp.) during embryonic development. Arrow colors in B–D indicate species that have comparable body sizes but differ in beak morphology. [Scale bars: 0.15 mm (stage 27) and 0.2 mm (stage 30) in B; 0.2 mm in D.] Molecular tree is from ref. 26.