If fertility is below replacement for each individual, then the increase in the number of copies of a favorable gene becomes unlikely. Say that a mutation occurs in an individual. If from that individual onwards the number of descendants at each generation in the entire tree of descendants is always 2, and we consider that replacement, then its unlikely for that mutation to spread out and take over.
In this scenario, there is almost no selection going on anyhow as we have set it up, at least explicitly. But aside from that, the number of gene copies will tend to be the same in each generation. So a favorable mutation, e.g. higher intelligence, doesn’t blow up and take over.
We need a frequency distribution of offspring that includes more than 2. We also need those with favorable mutations and accumulated branches of them to have fertility greater than 2.
Modernity is working against that. Modernity doesn’t have much selection and it doesn’t have long chains of above 3 fertility by those with better genes in the sense of intelligence, social skills, etc. So we have constructed a Liquidity Trap. Good genes don’t have the liquidity they need in the sense of above 2 fertility to spread through the population.
We need to construct that by societal selection. We could impose penalties for having children by those with lower genetic potential. This would mean charging the parents for school instead of providing it free. It would mean no welfare. We might also give benefits to high IQ children or other favorable traits. Children who pass tests would be free in school. Scholarships to college are already a form of that.
We need to have parents with high genetic value to have more than 2 children and those with low to have less than 2. We have moved in the opposite direction during the last 50 years under the forces of higher labor force participation rates by women and immigration.
Steve Sailer has written on whether evolution has been faster during the increase in human population over the last X-thousand years, where X can vary.
John Hawks, Eric T. Wang, Gregory Cochran, Henry C. Harpending, and Robert K. Moyzis
Genomic surveys in humans identify a large amount of recent positive selection. Using the 3.9-million HapMap SNP dataset, we found that selection has accelerated greatly during the last 40,000 years
Here’s the link for “Recent acceleration of adaptive human evolution” by Cochran, Harpending, Hawks, Moyzis, and Wang:
For evolution to happen, we need chains of above 2 fertility for a gene to spread out, unless we had a lot of luck.
These chains might be in stages. So we might have a contraction or funnel stage where the good genes survive in the funnel, even though fertility is below replacement, and the bad genes don’t. So the population falls but the percentage of a gene rises.
But then the population increases again from that point with above replacement fertility spreading the gene. Before the funnel, there had to be enough above replacement fertility to spread the gene to the group that survives the funnel.
But to spread a gene out, requires liquidity. Evolutionary liquidity is in the form of 3 or more offspring surviving to the point of having offspring. To spread a favorable gene, there need to be long chains with above replacement fertility for the good gene.
These are not happening anymore in the modern world among the white population or much of the East Asian population. We are instead getting this only in the lower tiers, in the third world population. Thus we are losing the genetic inheritance of mankind and stopping evolution from the high level it has reached in the white and Asian populations.
So the white and East Asian worlds have arrived into a genetic Liquidity Trap. They lack the fertility liquidity needed to spread good genes. The answer is to make having 3 or more children the tendency in those having good genes and 1 or 0 the tendency in those having the worst.
Society needs to structure itself to spread good genes not bad. Right now its spreading bad with welfare, affirmative action, third world immigration into the developed world, etc. We should have immigration into the 3rd world to spread the better genes in the developed world and below replacement fertility in the 3rd world. The Genetic Liquidity Trap is a bigger danger to humans then global warming.