Longevity: Selection

Longevity is difficult to select on because it is tied up with the action of natural selection. One way out of this difficulty is to turn it on its head. Since natural selection acts to mold survival automatically, then perhaps it can be used to do the work in selecting on longevity.

This basic strategy is the foundation of most successful schemes to select on longevity. It is implemented in the following way. Natural selection acts powerfully to screen out alleles that reduce survival before the onset of reproduction. The reason for this is simple. If organisms do not survive long enough to reproduce, then the genes that they bear will be eliminated from the population. In the case of organisms with lethal genetic disorders, when those disorders kill before the start of reproduction, the alleles that cause these disorders will be completely eliminated from the population in a single generation. An example of this is Hutchinson-Guilford progeria, a human genetic disease. This is one of the rarest of genetic diseases. Only a few dozen victims are alive at any one time. All of these progerics die before they can reproduce. It is thought that it is caused by a dominant allele, because the disorder does not increase in frequency with inbreeding, unlike disorders caused by recessive alleles. The alleles that causes progeria are eliminated every time they occur, so all cases represent new mutations. Natural selection's stringent elimination of all the victims of this disease reveals its power to maintain high survival before the onset of reproduction.

The key to this selective process is the point at which reproduction occurs. In laboratory populations, culture reproduction usually occurs at an arbitrary age that is convenient for the experimenter. The adults that have reproduced are then usually discarded. But the age at which cultures are reproduced can be changed by altering culture methods. When the age of reproduction is early, natural selection stops working to screen genes affecting survival that have effects at later ages. When the age of reproduction is later, natural selection will screen the genes affecting survival at all ages before the reproduction of the culture. This will take place automatically, without the experimenter having to measure the longevities of individual organisms. This allows selection for increased survival in very large laboratory populations, forestalling the problem of inbreeding.