Wednesday, October 16, 2013

Class notes on Darwin's theory of sexual selection

See here.  Some class notes on the theory of sexual selection:
Darwin suggested that the elaborate tails of male peacocks exist because females preferred to mate with males that had longer, brighter, or more beautiful tail If this is true, then the mating advantage of males with longer tails will compensate for the corresponding amount of reduced male survival.
This is, in my opinion, a very poor statement of what the theory must be.   How did females get to prefer to mate with a certain class of males?   That female preference is also subject to variation,  natural selection and evolution.

As a thought experiment, suppose there are genes that determine a male behavior of  tending to kill or not kill its own offspring.   Suppose some females somehow have a preference to mate with the males with the kill genes relative to the males without the kill genes.  It is plausible that this male-female sub-population would go quickly extinct.

The theory has to be asserting that the sub-population A of  {females that prefer to mate with males with longer, brighter tails, and males with such tails} outcompeted the sub-population B of {females that prefer to mate with males with small tails and males with such tails} or else that the two sub-populations diverged into separate species without out-competing each other.  The question would be why, in each case where sexual selection is supposed to apply, of the two species, the less flamboyant species has gone extinct.  

It is nowhere near obvious that sub-population A must outcompete sub-population B.  In fact one would think sub-population B always has the advantage, and only an accident could explain why sub-population A prevailed.  I think one would have to invoke some other mechanism if not happenstance.  For instance, suppose the expression of big tail genes is dominant, and small tail genes are recessive.  Then the supply of males with small tails would be reduced (because any male with mixed genes would display a larger tail) and even if females temporarily had a stronger mating preference for short-tailed males over long-tailed males, the smaller supply of short tailed males would make that a failing choice.  Sub-population A would tend to be naturally larger than sub-population B, and that is why it succeeds from the point of view of evolution.

As far as I have thought it through, female preference alone is never enough to make the theory of sexual selection to work.  But maybe further thought will make it clear.

PS: Darwin's theory as spelled out in Vol 2 of The Descent of Man is stated like the class notes. :(