Phenotypic integration and the organization of discrete facial variation

Facial morphology results from coordinated developmental and evolutionary processes that produce structured patterns of covariation among traits. Therefore, individual faces reflect integrated anatomical configurations rather than collections of isolated traits. However, discrete facial variation is often examined in a disconnected manner without explicitly accounting for these biological interdependencies. Given that such interdependencies shape which trait combinations are more or less likely, this study evaluates how modularity and integration structure the joint occurrence of facial features. Frontal facial photographs of adult males (n = 462) were analyzed. Twenty-nine anatomical landmarks were digitized and analyzed using geometric morphometrics. Procrustes coordinates were used to test three modularity hypotheses, to quantify integration between facial modules, and to explore patterns of continuous covariation through principal component analysis. Then, morphometric variables were categorized into discrete variables to evaluate. The results confirm that the face is organized into relatively distinct anatomical modules, with varying degrees of covariation. The highest integration observed was between the facial outline and the eyes modules. Observed frequencies of discrete trait combinations further support the structured co-occurrence of facial features by showing deviations from independence expectation. These findings highlight the importance of interpreting discrete facial phenotypes as integrated systems, providing a framework that complements trait-based approaches and insights for understanding facial discrete variation.