speaking of does and grazing, imagine this scenario:

this bunch of deer has evolved for grazing. so they graze.

there is genetic variance within the population, so some deer are better at grazing than others. ie they manage to crop the grass down to a shorter height than other deer. (don't ask me how, maybe it's the shape of the mouth or length of the neck)

so they have a 'selective advantage', because they are able to consume more food and are less likely to starve, and have more energy to escape from predators etc. so they are more likely to pass those genes down. so after a few generations all the deer can crop grass down to a shorter height.

repeat this ad infinitum.

after many many generations all the deer would be able to pull out the grass by the roots. by which time the deer population would be able to eat up all the grass (which can't grow back because they're all pulled up by their roots) and their habitat would turn to desert. following which they all starve to death. (ceteris paribus)

but of course i'm discounting the possibility that the grass may have evolved some strange characteristics, like the ability of curling around a deer's neck and pulling it down the ground and choking it, in which case it would become the predator.

weird thought. though i can't take credit for originating it. ;)