In theory, quantitative characters are subject to three kinds of natural selection: directional, stabilizing and disruptive. In the first, fitness has a linear association with a character, and in the remaining two, a curvilinear association. In a moderately malnourished population from Andean Colombia, fertility—the number of surviving offspring—has a linear (positive) association with measurements of soft tissue. The wealthier are both fatter and have larger families in this population, suggesting that this trend is the result of socioeconomic factors. However, bony measurements tend to have a curvilinear relationship to fertility such that large and small boned individuals have fewer surviving offspring, the same pattern of stabilizing selection observed in samples from well-nourished industrial societies.
Mueller, William H.
"Fertility and Physique in a Malnourished Population,"
2, Article 6.
Available at: https://digitalcommons.wayne.edu/humbiol/vol51/iss2/6