Evolutionary Theory: Male and Female Nipples as a Test Case for the Assumption that Functional Features Vary Less than Nonfunctional Byproducts

Male and Female Nipples as a Test Case for the Assumption that Functional Features Vary Less than Nonfunctional Byproducts. Ashleigh J. Kelly et al. Adaptive Human Behavior and Physiology, https://link.springer.com/article/10.1007/s40750-018-0096-1

Abstract

Objectives: Evolutionary researchers have sometimes taken findings of low variation in the size or shape of a biological feature to indicate that it is functional and under strong evolutionary selection, and have assumed that high variation implies weak or absent selection and therefore lack of function.

Methods: To test this assumption we compared the size variation (using a mean-adjusted measure of absolute variability) of the functional human female nipple (defined as the nipple-areola complex) with that of the non-functional human male nipple.

Results: We found that female nipples were significantly more variable than male nipples, even after controlling for body mass index, testing-room temperature, bust size in women, and chest size in men.

Conclusions: Morphological variation in a feature should not be used by itself to infer whether or not the feature is functional or under selection.