Genetic diversity is the way to go for resiliency. If a species lives under constant, high selective pressure, it becomes highly adapted for the current enviro

Genetic diversity is the way to go for resiliency. If a species lives under constant, high selective pressure, it becomes highly adapted for the current environment. If the current environment changes a little -- too hot, too cold, more rain, less rain, a new disease evolves -- then the entire species becomes unfit for its new environment, and it may not survive long enough for new mutations to arise that happen to be better suited to the new environment. If a species lives under weak selective pressure, it will only be weakly adapted to its current environment ... but it is much more likely that some part of it will be varied in a way that protects it or advantages it in the face of a changing environment. If selection pressure becomes harder -- if shit hits the fan -- it may be bad for a large chunk of the population, but it is more likely that at least some part of it will survive. Take potatoes -- you fill a field full of genetically identical potatoes which are perfectly suited to the climate and conditions you're growing in, and you can get just monstrous yields. But if the slightest thing goes wrong, if a new blight takes hold, the entire crop can be lost to the last. The population is strong, but fragile. And that is why eugenics is just so, so dumb, in addition to being, you know, ethically wrong.