Sorry, forgot to change the subject in my last post.

>You may be right for Budweiser, but one problem with
>this is that to create natural selection, a problem
>has to kill people before they reach childbearing age.

Not necessarily. You're scenario is selecting something
"out" of the population. However, it is possible to select
something "in" to the population. If, for instance, a
segment of the population found that they thrived on
Bud/McDonald's (or at least did not succumb to the McBud
Diet like their peers), they could concievably out-compete
the remainder simply by producing a greater number of
children. It happens with bacterial cultures.

Another possible scenario involves scarcity of resources.
It is concievable that in an area (like India or Africa),
where meat is scarce or in some cases taboo, that the
population could shift towards vegetarianism simply because
someone who requires less of the scarce food (in this case
meat) simply has more progeny than the meat reliant folks.
The food scarcity would force a decline in part of the
population, and the "others" would take over.