> Problem is that reality may not be quite as black and white as the theory
> above.  What happens when those consumed toxic items are only seasonably
> available or available in small quantities?  Does a threshold amount, both
> volume-wise and periodic-wise, exist above which selection occurs?

Old toxicology principle: The dose determines the poison.

Mark points out one shining example: alcohol.  Beneficial in small amounts;
detrimental in large amounts (and, to a lesser degree, in no amount).

Virtually every substance has a threshold dose, above which ingestion is
detrimental.  With some "toxic" substances, the threshold dose is very low
(ex. benzene)

Rob