Phenotypic evolution of human craniofacial morphology after admixture: A geometric morphometrics approach

Abstract

An evolutionary, diachronic approach to the phenotypic craniofacial pattern arisen in a human population after high levels of admixture and gene flow was achieved by means of geometric morphometrics. Admixture has long been studied after molecular data. Nevertheless, few efforts have been made to explain the morphological outcome in human craniofacial samples. The Spanish-Amerindian contact can be considered a good scenario for such an analysis. Here we present a comparative analysis of craniofacial shape changes observed between two putative ancestor groups, Spanish and precontact Aztecs, and two diachronic admixed groups, corresponding to early and late colonial periods from the Mexico's Central Valley. Quantitative shape comparisons of Amerindian, Spanish, and admixed groups were used to test the expectations of quantitative genetics for admixture events. In its simplest form, this prediction states that an admixed group will present phenotypic values falling between those of both parental groups. Results show that, in general terms, although the human skull is a complex, integrated structure, the craniofacial morphology observed fits the theoretical expectations of quantitative genetics. Thus, it is predictive of population structure and history. In fact, results obtained after the craniofacial analysis are in accordance with previous molecular and historical interpretations, providing evidence that admixture is a main microevolutionary agent influencing modern Mexican gene pool. However, expectations are not straightforward when moderate shape changes are considered. Deviations detected at localized structures, such as the upper and lower face, highlight the evolution of a craniofacial pattern exclusively inherent to the admixed groups, indicating that quantitative characters might respond to admixture in a complicated, nondirectional way.

Facial asymmetry and genetic ancestry in Latin American admixed populations

“…population) masks the effect of other allele in infl.uencing some trait (dominance), or when the action of one gene is modified by one or several other genes (epistasis). Variations produced in such ways often results in substantial variation (Falconer and Mackay, 1997;Ackermann, 2010), including hybrid morphology that is not intermediate (Martínez-Abadías et al, 2006) and/or asymmetries. In this context, the exploration of the phenotypic expression of asymmetries in an individual and the results of a population levei process like admixture would be useful to characterize individual facial asymmetries and population leveis of DA and FA.…”

Morphological integration in human skull and its possible role in etiogenesis of neuropsychiatric disorders: Deciphering the link between evolution, development and disease

“…The human skull shows tremendous inter-individual variations (25,26) in its anatomy and dimensionality of the landmarks, which may be pronounced in a population representing admixture of different ethnic groups (27), as for example, Indian (28-31) and American populations (32). Plausibly, these variations accumulated in the long course of evolution (33)(34)(35), through assimilation of co-variances (34,35) and epigenetic changes (36) in the gross and genetic structure of a population respectively, implied morphological integration (34,37).…”

A Geographic Cline of Skull and Brain Morphology among Individuals of European Ancestry

“…Human skull morphology varies greatly worldwide [8,9] largely due to the accumulation of genetic and environmental differences within populations but also between geographically separated populations [10,11] . Since the skull and brain are structurally and genetically linked during development [12,13] , it follows that the brain should also differ in size and shape among individuals.…”