Quantitative genetics of modern human cranial variation

The anatomy and biology of the basicranial complex and its relationships to the other cranial complexes has long played a central role in functional and phylogenetic interpretations of the hominin fossil record. Cranial traits frequently populate character lists in phylogenetic analyses and are commonly described as comprising an anatomical or functional complex. Most often these traits are treated as individual independent characters of equal phyletic value. Implicit in such considerations, however, is the assumption that the genetic contribution to morphological variation is high and equivalent across traits, and that phenotypic and genetic correlations between traits, irrespective of their magnitude, introduce negligible confounding effects in phylogenetic reconstruction (Hlusko, 2004). While the ability to quantify and assess phenotypic variation in craniofacial variables has seen significant advances in recent years, the extent to which genetic and nongenetic factors influence this variation is typically not addressed. A key component to understanding the influences environmental and genetic factors have on the evolution of the craniofacial complex is a solid foundation of detailed knowledge of the genetic components underlying the development of normal craniofacial variation in modern populations. The aim of the present study is to elucidate fundamental aspects of the genetic architecture of normal variation in the human craniofacial complex. In the current context, genetic architecture refers to the characterization of: (1) the extent to which variation in a trait is under genetic control, and (2) the degree to which two traits are controlled by the same genes or set of genes. The first of these is the heritability (h2) of a trait, and the second is the genetic correlation (ρG) between traits. Heritability estimates provide important information regarding the potential evolutionary response of traits to selective forces—the rate of evolutionary change being determined by the product of the heritability and the selection coefficient (Lynch and Walsh, 1998). Genetic correlations provide a means for quantifying the shared effects of genes on two or more traits. Genetic correlations have become critical in understanding the phenotypic heterogeneity observed in many craniofacial syndromes (Cohen, 2002), and have significant implications for the evolution of the hominin craniofacial complex. Human cranial variation: evidence from family-based studies Craniofacial morphometrics of archaeological or museum osteological collections have provided the field of human evolution with a broad perspective of craniofacial variation. These studies, however, cannot address issues of the genetic influence on trait variation, or covariation between traits. In order to adequately approach such issues, a large number of individuals of known biological relatedness is necessary. One such population is the Fels Longitudinal Study, which includes over 3,000 individuals belonging to ~250 families. Participants of the Fels Longitudinal Study have been the focus of numerous research articles including many that have had significant anthropological and human evolutionary significance, such as several papers detailing the growth and development of cranial structures (Young, 1956, 1957; Garn et al., 1963; Lewis and Roche, 1972, 1974; Roche and Lewis, 1974, 1976; Roche et al., 1977; Lewis et al., 1982, 1985; Ohtsuki et al., 1982a,b) and a classic paper investigating ontogenetic changes in basicranial flexion (i.e., the saddle angle; Lewis and Roche, 1977). In this report we seek to continue and extend the analysis of the Fels Longitudinal Study cranial data by presenting the first quantitative genetic analysis of the cranial data available in the archives.