The brain, the braincase, and the morphospace

Abstract

Morphological integration deals with the functional and structural associations, at ontogenetic and evolutionary level, between anatomical traits. Current morphometric tools can be used to analyze anatomical systems in terms of the mutual relationships shared among their components. The brain has no fixed and rigid form, but rather it is largely shaped by a set of mechanical forces involving bones, connectives, and vessels. During morphogenesis, the brain and braincase exert reciprocal influences associated with size and shape changes of soft and hard tissues. The available evidence suggests that such influences are usually based on local interactions, more than on general schemes or long-range effects. The frontal, temporal, and cerebellar lobes have a direct spatial association with the facial block and with the endocranial base, sharing several morphogenetic factors and geometric constraints with these areas. The frontal, parietal, and occipital bones are more directly shaped by the cortical brain surface, but they have constraints associated with bone articulations and reciprocal spatial adjustments. The final phenotype, selected by evolutionary processes, is an admixture of adaptations, secondary consequences, and structural regulations. The set of rules that govern phenotypic variability can be revealed and quantified by using multivariate statistics. The occupation of multivariate morphological space (morphospace) depends on the underlying structural organization and on ecological and phylogenetic constraints. Therefore, the geometric study of morphospace occupation parameters can reveal the rules of variability behind the observed morphological diversity. These intrinsic properties of endocranial variation must be nonetheless interpreted taking into account information from brain, bones, connectives, and vessels, and the data resulting from these quantitative analyses should be used to plan specialized biological surveys.