According to Darwinism there are two mechanisms through which a living thing changes:

  1. Natural selection and
  2. Sexual selection.

Turkey Cock's Plumage:

Darwin gave the example of entering a farmyard and being struck by the magnificence of a turkey cock's plumage. Might not, he asked, a turkey hen feel something of the same and choose to mate preferentially with this male bird? In this way fowls of striking feather leave more offspring and, by heredity, elaborate plumage evolves in.

Callipygous Women:

A second example given by Darwin was of an African tribe who line their women up in a rank. The man squints down the rank and the woman who sticks out the furthest behind (here Darwin lapsed into Latin, serving Victorian delicacy) is taken as the bride. In this way the interesting problem of the human female's callipygousness (Bottom, which see) was, he believed, resolved.

The form of the argument is:

  1. Men like callipygous women.
  2. Hence men mate preferentially with callipygous women.
  3. Hence callipygousness becomes more common among women.

The main problem is that it is not made clear - we are not given a "logic" - what it is that transforms statement 1 (Is it significant that i does not appear to be a proposition - that is it is unclear whether it is true or false?) into proposition 2. We are not, in other words, told why liking something implies mating with it.

It may be relevant that this may not be the same as saying the argument is untrue.

To get round this problem Run Away Sexual Selection (which see) was devised.

Peacock's Tail:

The Peacock's tail is perhaps the most famous example of alleged sexual selection; although Darwin himself felt he could not deal with it, saying he left the problem to future generations.