Nathaniel Pearson has an eminently readable post up on human effective population sizes. If you don’t know the importance of harmonic means in this domain, worth a read. He finishes though with an issue of practical importance, the proliferation of individually deleterious alleles at the large census sizes we see us today:
Along the way, our changing population size may shape public health in complex ways. In particular, a key question will be what happens to the likely sizable subset of newly arisen rare variants that pose health risks to people who carry them. As our population continues to skyrocket, more such variants will come into our midst.
At the same time, continued population growth should ultimately help natural selection purge such variants more efficiently than it can in a small population (where chance dominates the fate of variants, harmful or not).
But, to the extent that our future population growth itself depends on further advances in healthcare, we’ll also be altering the regime of such natural selection, ideally relaxing it in ways that help people live healthier lives no matter what variants they carry in their genomes.
As Mark Ridley observed in The Cooperative Gene natural operates in utero as well. Even assuming that natural selection is not purging deleterious alleles with great efficiency today, high human miscarriage rates are going to serve as a counterbalance to better healthcare for the genetically less fit.*
* On the order of ~50% of pregnancies miscarry, with the majority being cryptic, as women may not have known that fertilization occurred due to failure of implantation or problems in the first month).