title: Cliff-edge model of obstetric selection in humans.
abstract: The strikingly high incidence of obstructed labor due to the disproportion of fetal size and the mother's pelvic dimensions has puzzled evolutionary scientists for decades. Here we propose that these high rates are a direct consequence of the distinct characteristics of human obstetric selection. Neonatal size relative to the birth-relevant maternal dimensions is highly variable and positively associated with reproductive success until it reaches a critical value, beyond which natural delivery becomes impossible. As a consequence, the symmetric phenotype distribution cannot match the highly asymmetric, cliff-edged fitness distribution well: The optimal phenotype distribution that maximizes population mean fitness entails a fraction of individuals falling beyond the "fitness edge" (i.e., those with fetopelvic disproportion). Using a simple mathematical model, we show that weak directional selection for a large neonate, a narrow pelvic canal, or both is sufficient to account for the considerable incidence of fetopelvic disproportion. Based on this model, we predict that the regular use of Caesarean sections throughout the last decades has led to an evolutionary increase of fetopelvic disproportion rates by 10 to 20%.
authors.affiliation: Department of Theoretical Biology, University of Vienna, A-1090 Vienna, Austria; philipp.mitteroecker@univie.ac.at.
authors.forename: Philipp
authors.initials: P
authors.lastname: Mitteroecker
authors.affiliation: Department of Logic and Philosophy of Science, University of California, Irvine, CA 92697.
authors.forename: Simon M
authors.initials: SM
authors.lastname: Huttegger
authors.affiliation: Konrad Lorenz Institute for Evolution and Cognition Research, A-3400 Klosterneuburg, Austria.
authors.forename: Barbara
authors.initials: B
authors.lastname: Fischer
authors.affiliation: Department of Pediatrics, Cincinnati Children's Hospital Medical Center, Cincinnati, OH 45229.
authors.forename: Mihaela
authors.initials: M
authors.lastname: Pavlicev
mesh_terms: D000465:Algorithms; D005075:Biological Evolution; D001696:Biomechanical Phenomena; D052178:Cephalopelvic Disproportion / Q000453:epidemiology*; D003198:Computer Simulation; D005260:Female; D006801:Humans; D015994:Incidence; D007231:Infant, Newborn; D007743:Labor, Obstetric; D008297:Male; D015233:Models, Statistical; D009774:Obstetrics / Q000379:methods*; D036801:Parturition; D010388:Pelvis / Q000033:anatomy & histology*; D010641:Phenotype; D011247:Pregnancy; D012306:Risk
keywords: cephalopelvic disproportion; human evolution; natural selection; obstetric dilemma; obstructed labor