Not a way for natural selection, just an idea ... do you know the Population Control Options agent from amaikokonut? You could use it to keep your norns invertile after a number of eggs or export the older norns or something.
I have that on, but even then, my population is so huge (hovering on 30-32, with 32 being max breeding limit) that if I don't turn on the Export if Old Age/Export after fathering/mothering 4 children options aren't on, I end up with more eggs than an easter egg hunt. With those options on though, the My Creatures folder is getting filled up quite quickly. lol