Combine this with the dropping fertility of the potential parents, and this will be devastating. Even though you may have many potential parents among the survivors, after 25 years of a nuclear winter, worldwide, most of these parents will be past the point of fertility, or at least the mothers will be. Robust fertility, and infant viability, will come too late. This is the situation after every pole shift. With high tech food production and medical treatment, babies do survive.
Intermarriage is something that occurs naturally when young people live near each other.
In addition, if the parents of humans give The Call for their youngsters to be born with greater intelligence, genetic material will be available at the point of conception, just as it is today in some cases. As we have mentioned, there are humans being born today with Zetan genetic material, who are part Zetan, and they appear and act quite human. Just so, in the future, the genetic makeup of the human race will migrate in this direction. This kind of existence works when there is a perpetual source of goods to plunder, but as these types do not plan or put themselves out, they will find themselves suddenly without. Suddenly without clean water, any food at all, or the wherewithal to warm themselves until morning.