all living creatures are the products of evolution by natural selection. In this selection, new and randomly occurring variations, which first come about on account of a gene's mutation, are tested in living organisms in terms of their effects on living and raising children. This is measured as an individual's reproductive fitness. Variations that lead to improvements spread through populations, while those that cause no change simply hang around until, by chance, they either get eliminated or spread to each member of the population. Harmful variations get eliminated. But that takes time, and in between there can be a heavy price to pay.

At any given time, any population contains several individuals with harmful variations that are being weeded out. But the process is slow, and these individuals reproduce. Their children, too, carry the variant gene form,