Humans are the only primate species in which mammary glands (breasts) remain large and prominent throughout adulthood. Other primates’ mammary glands swell only when they are nursing offspring; scientists are not sure why human females have noticeable breasts on a permanent basis. One hypothesis is that they increase a female’s ability to attract mates. If so, this would be an example of a trait favored by…

a. artificial selection

b. gene flow

c. sexual selection

d. natural selection

e. genetic drift