Global Geometric Morphometric Analyses of the Human Pelvis Reveal Substantial Neutral Population History Effects, Even across Sexes

Abstract

Recent applications of population genetic models to human craniodental traits have revealed a strong neutral component to patterns of global variation. However, little work has been undertaken to determine whether neutral processes might also be influencing the postcranium, perhaps due to substantial evidence for selection and plastic environmental responses in these regions. Recent work has provided evidence for neutral effects in the pelvis, but has been limited in regard to shape data (small numbers of linear measurements) and restricted only to males. Here, we use geometric morphometric methods to examine population variation in the human os coxae (pelvic bone) in both males and females. Neutrality is examined via apportionment of variance patterns and fit to an Out-of-Africa serial founder effect model, which is known to structure neutral genetic patterns. Moreover, we compare males and females directly, and the true versus false pelvis, in order to examine potential obstetrical effects. Our results indicate evidence for substantial neutral population history effects on pelvic shape variation. They also reveal evidence for the effect of obstetrical constraints, but these affect males and females to equivalent extents. Our results do not deny an important role for selection in regard to specific aspects of human pelvic variation, especially in terms of features associated with body size and proportions. However, our analyses demonstrate that at a global level, the shape of the os coxae reveals substantial evidence for neutral variation. Our analyses thus indicate that population variation in the human pelvis might be used to address important questions concerning population history, just as the human cranium has done.

The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation

“…Our earliest detailed knowledge of the hominin pelvis comes from fossils of Ardipithecus ramidus, from Ethiopia 4.4 million years ago (Ma) 3 [20,52]. Although the Ar.…”

“…The relationship between body size and shape, overall pelvic size and shape, and birth canal size and shape in modern humans is complex [3,15,17,18,79,[138][139][140][141]. With the evolution of the modern pelvis, the size of the birth canal was essentially decoupled from the size of the body: birth canal proportions in modern humans do not necessarily correlate with body proportions [138,141,142].…”

“…While it has been shown that thermoregulation is an important factor influencing variation in pelvic dimensions between geographically widespread human populations [2,3,12,13], the way in which obstetrics influences variation among populations, or even between males and females, is unclear. Evolutionary theory would suggest that because 'selection intensity and phenotypic variability are inversely related' ( [143], p. 59), there should be sexual dimorphism in the variability of pelvic morphology, with variation in the size and shape of the female birth canal limited by obstetric demands.…”

“…Evolutionary theory would suggest that because 'selection intensity and phenotypic variability are inversely related' ( [143], p. 59), there should be sexual dimorphism in the variability of pelvic morphology, with variation in the size and shape of the female birth canal limited by obstetric demands. However, the nature and extent of sexual dimorphism in the modern pelvis are particularly complex, and a large number of studies have produced no clear indication that obstetric constraints produced between-sex differences in variability [3,18,138,139,[141][142][143]. To further complicate matters, work by Betti and co-workers [3,18,141] has suggested that among modern humans, neutral evolutionary processes reflecting demographic history have also been important in determining variation in pelvic form (see also [17]).…”

“…Through the evolution of the uniquely shaped birth canal, modern humans have been able to birth larger-brained infants, while maintaining the flexibility to adapt body breadth to varying habitats and at the same time improving locomotor performance. Endnotes 1 Betti and co-workers [3,18] have shown that in addition to selection based on thermoregulatory and locomotor constraints, neutral evolutionary processes have also had a significant effect on pelvic shape variation among modern humans (see §3). However, the fossil record is insufficient to allow investigation of the extent of neutral evolution of the pelvis in extinct humans.…”