Insights From a Quantitative Genetic Approach to Human Morphological Evolution I n recent years, a growing num- ber of studies have employed theory and methods drawn from quantitative and population genetics to advance our understand- ing of human morphological evolu- tion and diversification. 1–26 Quantita- tive genetics provides a framework for explicitly incorporating the dy- namics of the evolutionary process into analyses of phenotypic data. 27,28 This framework can be used to make quantitative predictions that can be compared with empirical observa- tions, such as distinguishing between the actions of neutral evolutionary forces (genetic drift, mutation, and gene flow) and adaptive forces such as diversifying natural selection. In addition, when neutrality can be assumed, empirical observations can be used to infer population histories and phylogenies or, in the case of natural selection, the strength of selective pressures. Recently, a sym- posium entitled ‘‘Quantitative genetic approaches to human phenotypic evo- lution,’’ held during the annual meet- ing of the American Association of Physical Anthropologists in Chi- cago 29 (1–4 April, 2009) brought to- gether an international group of researchers, showcasing current research in this area. Collectively, the talks highlighted the advances that have been gained through the application of quantitative genetic theory and methods to evolutionary questions in biological anthropology. WHAT HAVE WE LEARNED FROM A QUANTITATIVE GENETIC APPROACH TO HUMAN MORPHOLOGY? On the basis of numerous studies of modern human cranial variation, a consensus has emerged that neutral evolutionary forces have played an important role in shaping human cranial diversity. Evidence to support this view comes from four analytical approaches, which have been ap- plied to a range of craniometric data- sets. First, it has been repeatedly shown that cranial variation is apportioned similarly to presumably neutral genetic variation across the world. 4,7,9,11,13,24 Moreover, these apportionment statistics contrast with the very different apportionment of between-population to total-popula- tion variability for adaptive pheno- types such as skin color, which reflect the action of long-term diversifying selection. 7 Second, human population affinity or distance matrices based on craniometric data are congruent with analogous affinity or distance matri- ces based on neutral genetic loci. 12,14,15,23,25,26 It is worth noting that this congruence has been demon- strated in independent studies using at least four different global cranio- metric datasets and various neutral genetic systems (that is, autosomal microsatellites and classical genetic markers). Third, several studies have found that patterns of within-popula- tion craniometric variation fit a model of iterative founder effects with an African origin, which not only sup- ports an ‘‘out-of-Africa’’ model for modern human origins, but suggests that within-population patterns of human cranial variation reflect neutral evolutionary forces. 16,20,30,31 These studies have found a significant negative correlation between within- population craniodental variation and distance from sub-Saharan Africa, which is consistent with the pattern that has been shown for neutral genetic loci. 32–35 Fourth, cranial meas- urements show similar deviations from neutrality to microsatellites in the patterning of distances across dif- ferent traits and loci. 18 The emerging view that human cra- nial variation largely follows a model of neutral evolution leads to several important implications for future analyses, including the following. (1) Patterns of human cranial variation can be reliable indicators of past pop- ulation history and can, therefore, be used as a proxy for neutral genetic data in situations where genetic data are unavailable (for example, in osteoarcheological and paleontologi- cal contexts). One word of caution is that although patterns of variation at phenotypic traits or neutral genetic loci are expected to match population history or phylogeny, because neutral evolution is a stochastic process, this may not be true for individual traits or loci. Consequently, the ability to reconstruct population history accu- rately depends not only on whether the traits or loci are neutral, but also on how many independent traits or loci can be used to make inferences. (2) Cranial data, at least sometimes, are reliable indicators of divergence times between lineages. 19 AREAS OF ONGOING AND ACTIVE RESEARCH As ably highlighted by the presen- tations given at the Chicago sympo- sium, the application of quantitative genetic approaches within biological anthropology is an active, produc- tive, and growing field of research. Given his substantial and long-term theoretical, methodological, and em- pirical contributions to this research agenda, it was appropriate that John Relethford (SUNY, Oneonta) should set the scene with a paper on the NEWS Evolutionary Anthropology 18:237–240 (2009)