The problem is that every generation has new de novo mutations. Mostly they don't do anything, very rarely they're positive, but some are bad. It used to be t

The problem is that every generation has new de novo mutations. Mostly they don't do anything, very rarely they're positive, but some are bad. It used to be that only half of children survived to adulthood which produced enough selective pressure to remove new deleterious mutations, but that thankfully doesn't happen anymore. This isn't a problem in the short term but if we don't introduce something like pre-natal selection we'll eventually wind up back in a situation where half of children don't survive to adulthood despite modern medicine.