Multivariate morphometrics, quantitative genetics, and neutral theory: Developing a “modern synthesis” for primate evolutionary morphology

Abstract

Anthropologists are increasingly turning to explicit model‐bound evolutionary approaches for understanding the morphological diversification of humans and other primate lineages. Such evolutionary morphological analyses rely on three interconnected conceptual frameworks; multivariate morphometrics for quantifying similarity and differences among taxa, quantitative genetics for modeling the inheritance and evolution of morphology, and neutral theory for assessing the likelihood that taxon diversification is due to stochastic processes such as genetic drift. Importantly, neutral theory provides a framework for testing more parsimonious explanations for observed morphological differences before considering more complex adaptive scenarios. However, the consistency with which these concepts are applied varies considerably, which mirrors some of the theoretical obstacles faced during the “modern synthesis” of classical population genetics in the early 20th century. Here, each framework is reviewed and some potential stumbling blocks to the full conceptual integration of multivariate morphometrics, quantitative genetics, and neutral theory are considered.

Selection for increased tibia length in mice alters skull shape through parallel changes in developmental mechanisms

“…As a result of integration, correlated responses to selection can result in phenotypic changes in some traits that are merely a consequence of covariation with other traits under selection (Gould and Lewontin, 1979;Wagner, 1984;Price and Langen, 1992;Parsons et al, 2015). Understanding how developmental processes lead to correlated responses to selection is pivotal to distinguishing adaptive changes from those that are non-adaptive, or potentially even maladaptive, in analyses of phylogeny, ancestral relationships and evolutionary change within lineages (Gould and Lewontin, 1979;Riska, 1986;von Cramon-Taubadel, 2019).…”

Morphological consequences of artificial cranial deformation: Modularity and integration

“…Although a recently proposed model-bound approach for understanding morphological evolution of the human skull considers not only statistical analysis of form, but also quantitative genetics and explicit evolutionary hypotheses, like neutral theory [10], morphological integration is typically studied by analyzing the covariation among morphological traits [11]. Nonetheless, this integration is not absolute but organized in units that are relatively independent while participating to generate a structure that act as functional whole.…”

“…These independent units are nevertheless integrated internally, and are operationally known as modules [7]. Even though the majority of the studies on modularity and integration have focused on variation among individuals within populations, there are more levels of variation that exhibit modularity and integration [9], deriving from distinct sources such as genetic variation, phenotypic plasticity, fluctuating asymmetry, evolutionary change, among others [10,17].…”

A paleogenetic perspective of the Sabana de Bogotá (Northern South America) population history over the Holocene (9000–550 cal BP)

“…The increase in morphological diversity during this period, coincident with the appearance of agriculture, was interpreted as evidence of gene flow (Delgado, 2012(Delgado, , 2015(Delgado, , 2017. The discrepancy between genetics and cranial morphology regarding the amount of diversity deserves attention since a certain degree of convergence would, in principle, be expected from both markers because they are neutral and are affected by similar evolutionary forces (Zichello et al, 2018;von Cramon-Taubadel, 2019). In this specific case, the discrepancy requires additional investigation but, given that some of the cranial phenotypes investigated are very plastic and could reflect a range of adaptations/plasticity rather than exclusively population history (Delgado, submitted), the evolutionary diversification viewed during the initial late Holocene would be likely an effect of natural selection (Delgado, 2017).…”