The issue is that outside of modern society, you can't plan for future generations, as there is no stability. I mean even now, you can only help your offspring so much, and much of that comes in the form of guidance and teaching.
What if you built an underground bunker out in the woods. You got it all stocked up with a couple years worth of food, while you learn or re-learn nessecary survival skills. After three weeks out, your son gets mauled by a bear, and his leg has to be amputated. That supply of food is finite, and won't last long. Your son is no longer much good for hunting, and he probably wouldn't have a usefull trade that he can practice without a leg. Depending of the scale of the disaster we're talking about, your son probably won't get a mate (and thats what it WOULD be, a mate, not a wife, but a mate who would be with him because he could sire and provide for her offspring), as he won't be able to provide her or her offspring.
Point is that if you can't plan for future generations, then plan for the next 10 years. If you can't plan for the next 10 years, plan for the next 5. If you cant plan for the next 5, plan for just the upcoming year. If you can't plan for the next year, plan for tommarow. And if even your best laid plans don't survive the night, then focus on the next hour, the next minute, the next second. Focus on staying alive in the here and now. Becuase if you die right now, all of your plans, all of your preperation, were for naught.
