Not dieting, that’s something societal. I’m curious as to what the evolutionary benefit of developing a diet consisting of only one thing could possibly be. Example: Koala bears only eat Eucalyptus leaves, and pandas only eat bamboo. It seems that’s a real dumb thing for an animal to do from an evolutionary stand point. So, what’s the deal?
It’s not dumb until it goes wrong, a belief firmly held by civil engineers the world over.
Fact is, bamboo and eucalyptus are quite hardy, as plant life goes, and barring a huge environmental collapse, grow in large enough quantities to feed pandas and koalas comfortably. It may be unusual for a mammal to dine on a single food (notwithstanding some American teenagers who spend all their time at McDonald’s) but in the insect world, narrow diets are extremely common. Evolution allows this because there hasn’t been sufficient pressure to make them stop.
Now, if you want to talk evolutionary stupidity, ponder the appendix. Same deal: it persists because it hasn’t killed enough of the human population to be selected against.
The deal is that if you confine yourself to eating only one thing, and are terribly efficient at it, then you don’t have to worry as much about competition. It’s called “finding a niche”. The problem, however, is that ultra-specialization like that pretty much links your fate to your food source’s: if something happens to your sole food source, you’re a goner.
To be fair, although bamboo is their main nutrition source, pandas do eat other things:
http://www.animalinfo.org/species/carnivor/ailumela.htm
http://www.fonz.org/animals/af-pandas.htm
They even occasionally experience a time when their food supply is drastically reduced. Every 100 years or so, a species of bamboo will flower and subsequently die off. As long as there are other species of bamboo around the pandas can still survive.
Also, koalas do occasionally browse from non-eucalyptus trees. Eucalyptus are actually considered poisonous to some animals, so you might consider the adaptations of the koala to consume this poison would be a good thing since it has less competition for the food source.
I like that phrase. It captures the corporate environment in which I work. Not that I’m complaining about that, I fall back on that attitude myself sometimes. It’s the evolutionary verion of “Don’t fix it if it ain’t broke.”
I’m probably going to steal that quote from you. And, to be honest, I doubt I’ll credit you. Not that I don’t think you deserve credit, it’s just that it will lead to the question “Who the fuck is Bryan Ekers?” from my co-workers, and I see no reason to get into that.
But if I ever use the phrase on SDMB, I will cite you. 
Gosh, I’m famous.