Sometimes it is disadvantageous for a population to eliminate its old genes completely. An old gene may possess the ability to meet an old crisis, if that crisis should return. Furthermore, the old gene and the new one with which it shares, as an alternate, its position on a chromosome may do things together that neither could do alone.
In genetic shorthand, AB may be better under some conditions than either AA or BB. The best-known example of this effect in man is probably the so-called sickling trait common among West African Negroes. This is expressed by the letters S and s. S means that you have the trait, s that you don't. The S gene curls the red corpuscles in the blood, impeding oxygen flow; the s gene has no known effect. The S gene alone resists malignant malaria, which kills many children. But an SS child may die of oxygen starvation, and an ss child of malaria, whereas an Ss child is likely to survive >diseases. The population profits by the retention of both genes, each of which has a disadvantage in that particular environment.
The example just cited may explain the presence of genetic variability in many populations even though we don't yet understand why it is there in each case. It may also in part explain the re-emergence of "types."