Comparing whole genomes:
It is somewhat unfortunate that the classical way of measuring effective population size does not do us much good. As I pointed out, in the time it takes a population to die out from excessive size, there might be about one new mutation introduced into the population in a region of the size generally measured by the classical Wright-Fisher approach to estimating population size.
It is also somewhat unfortunate that just about all the animals for which population size has been estimated, humans included, seem to come up with about the same size. With little or no variation in size, there is little or nothing to explain by measuring population sizes. The great mystery has of course been why no variation. The answer I think is clearly that a population that gets too large has a population crash or goes extinct, be it human or other animal. Since the basic rules are the same across the board, the size estimates are about the same. Since to my knowledge nobody has done the math to see what happens to estimates of population size under circumstances of outbreeding depression, by which I mean reduced fertility because the population is too large or too genetically diverse, I suspect there will have to be some revisions to the population sizes already calculated by classical means. My suspicion is that the revised calculations will imply much smaller effective population sizes.
There is another page that I think could be effectively borrowed from the field of population genetics. By now we have analyzed the DNA of specific, human beings down to the last base pair, which means down to the last atom. In fact, there is a rather expensive cottage industry in existence for analyzing genomes, and we frequently hear of some new species of interest that has had its DNA read out. The cost, I think, was about a million dollars for a genome read out, and dropping fast. I have even heard it proposed that costs might drop far enough so that one could get his or her own genome read out for something like a thousand dollars.
That is a lot of money, but considering what gets spent on courtship and marriage, it is not inconceivable. Most people in developed parts of the world, where infertility is worst, could afford it once in a lifetime.
Given the data base of genomes for everybody, there are things that could be done. The most obvious is this: The autosomes, all of the chromosomes except the sex chromosomes, come in pairs. Two almost identical ones in each cell. It would be a simple enough matter to compare them all, base pair for base pair. The computer I am sitting at has a processor speed that can handle 3 billion calculations a second and, fortunately in light of the programs I have run at full speed, can carry on for hours. I think it would be possible to run through all the autosomes in a few minutes and find out what the total heterozygosity of a person was. (At how many locations one chromosome differed from the other.) There would be no need to do some sort of calculation of gene pool size. That was just a surrogate for saying how chewed up a person’s own genome was from ancestral mating choices. At a thousand dollars a person, you can just measure it. Then with maybe a hundred couples over fifty years old, and presumably not likely to have more children, it would be possible to establish the relationship between heterozygosity and fertility. Just ask them how many children they had had and graph it against their heterozygosity. I should be most surprised if there was not a rather clean curve of the shape we have looked at so often.
That would, I think, be another very strong proof of the principle that too big and diverse a population destroys fertility.
But you could do more. You compare the genomes of husband and wife and come up with a genetic distance. Comparing that with reproductive success would let you know what kinds of distances were compatible with having children and what were not.
That analysis would not be completely straightforward. Just as the Iceland study shows that marrying second cousins produces the maximum number of offspring but fewer grandchildren, so the Denmark study seems to suggest that marriages of those less related to each other produce more children but fewer grandchildren. There seems to be a first generation hybrid vigor as well as a first generation inbreeding vigor that needs to be taken into account.
With sufficient data and analysis on hand, it would be possible to offer some rational genetic advice for couples contemplating marriage.
It could even be a sort of a massively expensive dating service. As far as how well it would be received and how effective it would be, I have no idea. But people will do a lot in order to survive, and do a lot to protect their children. It might even work.
At least it might inspire some more dystopic science fiction, in case there is ever a shortage.
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