Patterns of variation and covariation in the shapes of mandibular bones of juvenile salmonids in the genus Oncorhynchus.

What is the nature of evolutionary divergence of the jaw skeleton within the genus Oncorhynchus? How can two associated bones evolve new shapes and still maintain functional integration? Here, we introduce and test a "concordance" hypothesis, in which an extraordinary matching of the evolutionary shape changes of the dentary and angular articular serves to preserve their fitting together. To test this hypothesis, we examined morphologies of the dentary and angular articular at parr (juvenile) stage, and at three levels of biological organization—between salmon and trout, between sister species within both salmon and trout, and among three types differing in life histories within one species, Oncorhynchus mykiss. The comparisons show bone shape divergences among the groups at each level; morphological divergence between salmon and trout is marked even at this relatively early life history stage.