I’ve got a better Idea. Instead of trying to second guess mother nature on what defines “Fitness” or “an acceptable population”, why don’t we just let it all sort out? If it’s really neccessary to reduce the population, It’ll happen naturally, and those who survive to reproduce will be the fittest. We call this process “natural selection” and it is the exact opposite of Eugenics.
You cannot predict with certainty what the future will hold, and what traits will perform well in it. Traits which seem maladaptive to us can, under the right circumstances, be survival traits. Sicle cell anemia is the perfecct example of this. People with two copies of the gine have a horrible genetic desease, but people with one copy are resistant to malaria. In areas where malaria is an issue, It’s adaptive despite the desease because a few people with sickle cell is better than everone with malaria.
What other traits might we eliminate that prove to be adaptive? Almost any trait can be adaptive in the right situation, even things like mental retardation. The strongest population is one that carries within it the greatest diversity of traits. In our hypothetical situation, the best long term selection method would be complete randomness, with as large a sample set as possible.
Come on. We’re rewilders. When we choose which animals survive based on traits we desire we call it domestication. We’re against that. We place the greatest value on biodiversity, which includes diversity within a species. We shouldn’t even have to have this conversation.