This is pretty solid science, no warnings are necessary wrt the basic facts. As usual, I derive not just ‘should’ but ‘must’ from ‘scientfically is’, a philosophically suspect act at best.
Added later : I have just finished the section in Matt Ridley’s “Red Queen” on diseases driving sexual selection. It is very consistent with all of this, but my understanding was and still is primitive compared to what has been researched and is known. Basically, parasites and diseases evolve at 30x the rate of mammals and birds. That produces a Red Queen evolutionary arms race where being like your parents in biochemical vulnerabilities that allow diseases to grow on or with you is bad for your prospects of progeny. That need for genetic variety drives exogamy, which manifests in humans as girls marrying into other families and individuals preferring people who have different Major Histocompatibility Complex smells. And young, beautiful, athletic and intelligent are all markers of good genetics and good environment, which is why we are so delighted by them.
A small digress, on a topic which I repeat parts of often, because nobody gets the importance of epidemics in human history and why foreigners are good mates. Beyond being especially sexy compared to the people we grew up around, I mean.
The risk for humanity is, always and forever, disease, including parasites. Those can make the mega-deaths of mere human conflict seem trivial. The black death took away 30-50% of the population of Europe, whose population was relatively resistant to such things due to the many mixtures of ancient populations and the free movement of people through Asia, Africa and Europe. The Black Death changed absolutely everything.
To think that could not happen again, vaccines can handle it, is to adopt a losing strategy, centralized defense against many diseases simultaneously as they execute their distributed evolutionary arms race on the offense. There are always half a dozen diseases perking along in the background trying to become pandemic, and many more working on a transmission path from their current host specie into humans. We are a very populace target, subjected to very many many such attempts every year as people find new populations of animals and eat them, or breed viruses in the prolific duck-pig-people incubators.
Defense must lose, the only question is how often, how much, how fast defenses can be brought to bear from central supply or R&D.
You want to bet your life on such centralized human-managed processes? Don’t get me wrong, I think modern medicine and vaccines are very nice to have and vaccines may save a lot of pain and suffering (CDC safety data is too bad to know for sure), but risk is additive and protection is additive. So long as I don’t have to choose, I will do both and advocate both.
Biological defense in a an evolutionary arms race is to assume diseases will happen unexpectedly, and insulate your genetic self from the effects via as many as possible as wisely-composed geneotypes as possible. Your genes are going to be combined with other genes in your progeny. Your genes don’t much care about those genes, so long as the offspring have a shot at propagating their genes, in turn. That is a gene’s way of protecting itself, mix well for resistance from the next generation’s diseases.
As a carrier of those genes you think of as ‘yours’, but which, from their genes’-eye-view are merely using you as a test of ‘your’ particular combination, the old standards of genes in your troop plus a few your parents invented and passed along (most of those won’t do you any good), you are useful for your genes and they would much prefer that you as their carrier do well, but life is uncertain and they always choose to cover as many bets as possible, consistent with wise use of resources, of course.
Your genes can’t see the future, but they can like the look of the foreigner, the right mix of antibody genes in your offspring can have the great advantage of living through an epidemic. That is how humans came to be an inter-species hybrid race, like a mule, except not sterile like the mule. Inter-species hybrid vigor. Neanderthal r us. And there are at least 350x total Neanderthal genes in the world than ever before they went extinct, their gene’s big-time win. We and the Neanderthals extinguished each other by hybridizing, the hybrid out-competed both parent species via hybrid vigor, the human population subsequently became very large, both species have done well subsequent to our mutual extinction event.
Culture doesn’t leave so many detailed records, the way genes from old bones do, but our cultures continuously evolve and continuously combine. It isn’t possible to prevent that, humans apparently love novelty of all kinds, and invent much. We see us combining cultural elements into our lives, we all eat food that is very different than what we grew up with, if we can. We can’t avoid new music if we go out of our home. New things everywhere, our culture changes continuously.
New peoples assimilate, they always have, always will, we always did. Same reasoning, different cultures have different strengths and weaknesses, we are doing the selective breeding of our geneline and our culture. We win by producing hybrid vigor. Freedom has always been an important allele, don’t let it slip from the memome.
Added later : The ‘genetics as protection against disease‘ factor drives natural evolution, and of people also. Your best bet is spread the risk of susceptability to the next disease across as many sets of immune system variants as possible, as not all of them will catch any particular new disease. That is why outbreeding is such an advantage, and the reason none of us look like a ‘pure line’, no matter how perfect you think your people. Nature doesn’t make it easy or risk-free to be a pure line. Those aren’t natural.