Fig. 1

Hypothesis of developmental constraint in plumage pattern evolution on the basis of increasing complexity and the null (full) model of pattern evolution. 1) Model of developmental constraint: 1.1a) Scales - king eider (Somateria spectabilis), 1.1b) Bars - snow partridge (Lerwa lerwa), 1.1c) Spots - great argus (Argusianus argus). If there is developmental constraint in plumage pattern evolution on the basis of increasing stringency, then perhaps scales evolve from the ancestral state of uniform coloration, followed by bars, and finally spots. 2) The null model of plumage pattern evolution, also known as the full model, where there is no directionality and all evolutionary transitions occur. Images were taken at the University Museum of Zoology, the University of Cambridge, by T-L. Gluckman and are copyright of the University Museum of Zoology