The interaction between genetics and climate on craniofacial variation : examining the causative forces of macromorphoscopic trait expression

Anthropologists have an extensive history using cranial form to measure group relatedness in past and present populations to answer a range of questions concerning population histories and cultural practices. However, most biological distance studies using skeletal remains do not consider extrinsic forces influencing modern human variation. Researchers have explored evolutionary and plastic responses in cranial form using measurements of the cranium and mandible, but these studies generally drew inferences through population comparisons or using inadequate statistical and biological models that so often lead to conflicting findings or confounding interpretations. To fill this gap in our current understanding of modern human variation, I have combined global craniofacial morphological, climatic, and genetic datasets to measure the magnitude and directionality of several climate variables on craniofacial form, while controlling for population structure (e.g., microevolutionary forces and population history). Craniofacial morphological data from the Macromorphoscopic Databank (MaMD) are used in conjunction with microsatellite data from Pemberton (2013), representing populations that overlap in geographic space with those in the MaMD. Finally, climate data from the National Oceanic and Atmospheric Association (NOAA) and Climate Research Unit (CRU) websites were obtained for weather stations in close proximity to populations under study. This combined dataset is used to explore the interaction between climate and genetics on craniofacial variation across 11 geographic regions using a mixed model approach known as Bayesian Sparse Factor Analysis of Genetic Covariance Matrices (BSFG). Data analysis follows the methods described by Katz and colleagues (2016) but expands their study through the exploration of selection processes using additional climate variables, including coldest month and driest month averages and annual ranges of temperature and absolute humidity. Overall, the study found significant correlation between genetic and phenotypic data indicating MMS traits can serve as genetic proxies in biodistance analyses. Several traits had higher heritability estimates (malar tubercle, zygomaticomaxillary suture course, postbregmatic depression and anterior nasal spine). Features associated with the nasal complex and facial breadth, particularly anterior nasal spine, nasal bone contour, and interorbital breadth, had strong associations to climate. These climate findings correspond to previous research on nasal form and environment where cold-dry environments select for high, narrow noses. Further evidence of selective forces in MMS traits are apparent with the reduction of these features in more variable climates where the respiratory system experiences less stress. The evolutionary mechanisms behind craniometric data have been explored extensively. Such studies use a full suite of traits that capture overall size and shape of the human cranium; however, MMS traits focus on macroscopic assessments primarily in the midfacial skeleton. MMS trait data are particularly important for expanding our understanding of natural selection whereby a large portion of cranial evolutionary research has centered around the neutral evolutionary processes. The wealth of research demonstrating the nasal complex is highly responsive to climate due to respiratory stress emphasizes the importance of exploring the proportion of genetics and environments on MMS trait manifestation. This project provides an evolutionary foundation of the neutral evolutionary and selection processes controlling systematic patterns of global craniofacial variation in the Macromorphoscopic Databank (MaMD).