Developmental and Genetic Constraints on Neurocranial Globularity: Insights from Analyses of Deformed Skulls and Quantitative Genetics

Abstract

Neurocranial globularity is one of the few derived traits defining anatomically modern humans. Variations in this trait derive from multiple and complex interactions between portions of the brain and the size and shape of the cranial base, among other factors. Given their evolutionary and functional importance, neurocranial globularity is expected to present high genetic and developmental constraints on their phenotypic expression. Here we applied two independent approaches to investigate both types of constraints. First, we assessed if patterns of morphological integration are conserved or else disrupted on a series of artificially deformed skulls in comparison to nondeformed (ND) ones. Second, after the estimation of the genetic covariance matrix for human skull shape, we explored how neurocranial globularity would respond to putative selective events disrupting the normal morphological patterns. Simulations on these deviations were explicitly set to replicate the artificial deformation patterns in order to compare developmental and genetic constraints under the same biomechanical conditions. In general terms, our results indicate that putative developmental constraints help to preserve some aspects of normal morphological integration even in the deformed skulls. Moreover, we find that the response to selection in neurocranial globularity is pervasive. In other words, induced changes in the vault generate a global response, indicating that departures from normal patterns of neurocranial globularity are genetically constrained. In summary, our combined results suggest that neurocranial globularity behaves as a highly genetic and developmental constrained trait.

Developmental covariation of human vault and base throughout postnatal ontogeny

“…In agreement with our findings, many authors have supported that the skull is an integrated structure, where different regions interact dynamically (Lieberman et al, 2000b(Lieberman et al, , 2002Bookstein et al, 2003;Mitteroecker and Bookstein, 2008;Hallgrímsson et al, 2009;Martínez-Abadías et al, 2009Bastir et al, 2010). The processes responsible for this pattern may be multiple and may act at different spatiotemporal scales, overlapping their effects and producing a very complex structure of covariation (Hallgrímsson et al, 2009).…”

Studying morphological integration and modularity at multiple levels: concepts and analysis

“…Genetic integration between parts means that selection on one part will produce a correlated response in other parts, whereas genetic modularity can facilitate independent evolutionary changes in different parts [3,7,8]. Studies combining geometric morphometrics and quantitative genetics have investigated correlated responses by simulating selection for a highly localized shape change and observing whether the evolutionary response affects other parts as well [67,[71][72][73].…”

The Developmental Basis of Variational Modularity: Insights from Quantitative Genetics, Morphometrics, and Developmental Biology

“…Instead of identifying modules, several recent studies were aimed at modeling and visualizing the pattern of integration between a priori defined anatomical parts (e.g., Bookstein et al 2003;Rosas 2005, 2006;Gunz and Harvati 2007;Cobb and Baverstock 2009;Kulemeyer et al 2009;Martinez-Abadias et al 2009). Mitteroecker and Bookstein (2008) modeled factors of developmental integration in the primate cranium and expressed evolutionary differences between species in terms of these pleiotropic factors.…”