A model of developmental canalization, applied to human cranial form
Fig 8
Canalization of craniofacial shape from 2 to 3 years of age was explored by two-block partial least squares (PLS) analysis between the shape variables at age 2 (first block) and the shape changes from age 2 to 3 years (second block). (A) The first PLS dimension represents the shape pattern with the largest canalized variance. Individuals with a relatively large pharynx due to a retroflexed cranial base and a relatively short maxilla at age 2 (low scores along u1) tend to experience increased maxillary growth and basicranial flexion from age 2 to 3 years (low scores along v1). By contrast, individuals with a highly flexed cranial base and relatively long maxilla, both of which constrain the pharynx, tend to undergo a more than average increase in pharyngeal size (high scores along u1 and v1). Note that the left deformation grids represent deviations from the mean shape at age 2, whereas the right grids represent deviations from the mean shape change form 2 to 3 years. Hence, the maxilla does not shrink in individuals with a large maxilla, but it grows less than average. (B) The second PLS dimension represents canalization of facial height: individuals with a relatively high face (inferiorly placed maxilla) undergo reduced facial growth, whereas individuals with a relatively low face (including a small pharynx and nasal cavity) show a more than average increase in facial height. Together, this indicates strong canalization of the relative size of the upper airways in early postnatal development.