Constraint, natural selection, and the evolution of human body form

Abstract

Variation in body form among human groups is structured by a blend of natural selection driven by local climatic conditions and random genetic drift. However, attempts to test ecogeographic hypotheses have not distinguished between adaptive traits (i.e., those that evolved as a result of selection) and those that evolved as a correlated response to selection on other traits (i.e., nonadaptive traits), complicating our understanding of the relationship between climate and morphological distinctions among populations. Here, we use evolutionary quantitative methods to test if traits previously identified as supporting ecogeographic hypotheses were actually adaptive by estimating the force of selection on individual traits needed to drive among-group differentiation. Our results show that not all associations between trait means and latitude were caused by selection acting directly on each individual trait. Although radial and tibial length and biiliac and femoral head breadth show signs of responses to directional selection matching ecogeographic hypotheses, the femur was subject to little or no directional selection despite having shorter values by latitude. Additionally, in contradiction to ecogeographic hypotheses, the humerus was under directional selection for longer values by latitude. Responses to directional selection in the tibia and radius induced a nonadaptive correlated response in the humerus that overwhelmed its own trait-specific response to selection. This result emphasizes that mean differences between groups are not good indicators of which traits are adaptations in the absence of information about covariation among characteristics.natural selection | ecogeographic variation | Bergmann's rule | Allen's rule | evolutionary constraints

The genetic variance but not the genetic covariance of life‐history traits changes towards the north in a time‐constrained insect

“…For example, although two leaf traits in Chamaecrista fasciculata showed substantial genetic variances, a strong negative genetic correlation between the traits explained considerably slower adaptive evolution of individual traits than would be expected if genetic covariance had been ignored (Etterson & Shaw, ). By contrast, femur length in humans was not a character under direct selection along a latitudinal cline but continued to show change in evolutionary time because of its correlation with other limb characters (Savell et al ., ).…”

Evolution of morphological integration in the skull of Carnivora (Mammalia): Changes in Canidae lead to increased evolutionary potential of facial traits

“…This suggests that the investigation of species divergence (i.e., ) alone can, at best, paint an incomplete picture of the evolutionary processes at work (see Grabowski ; Savell et al. for some empirical examples).…”

“…Its easy to see that, in the presence of nonzero genetic covariances (G 12 = G 21 = 0), the evolutionary response of a trait will be a function not only of genetic variance and direct selection, but also of indirect selection and genetic covariance (Lande and Arnold 1983;Agrawal and Stinchcombe 2009;Marroig et al 2011). This suggests that the investigation of species divergence (i.e., z) alone can, at best, paint an incomplete picture of the evolutionary processes at work (see Grabowski 2016;Savell et al 2016 for some empirical examples).…”

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

“…10 which an individual quantitative trait can respond to any given selective pressure. 7,[9][10][11]19,62,63 If traits are highly integrated, their ability to respond to a given selection gradient (β) is highly dependent on the nature and direction of that gradient (…”

“…In recent decades, evolutionary anthropologists increasingly assess among-group patterns of skeletal variation in human populations and other primate taxa within an explicit quantitative genetic evolutionary framework. [1][2][3][4] Utilization of this general framework has provided novel insights into the evolutionary history of the primate postcranium, [5][6][7][8][9][10][11] and into the evolution of cranial morphology in extinct and extant primate taxa. [12][13][14][15][16][17][18][19][20][21][22] Key to the success of this approach are three major intertwined conceptual and analytical frameworks: multivariate morphometrics, quantitative genetics, and neutral evolutionary theory (Figure 1).…”