Conclusions

I'm not going to try to show you the formulas, but it shouldn't surprise you to learn that heterozygote advantage won't maintain a polymorphism indefinitely in a finite population. At best what it will do is to retard its loss.⁸ There are three properties of the interaction of drift and selection that I think you should take away from this brief discussion:

1. Most mutations, whether beneficial, deleterious, or neutral, are lost from the population in which they occurred.

2. If selection against a deleterious mutation is weak or $N_e$ is small,⁹ a deleterious mutation is almost as likely to be fixed as neutral mutants. They are ``effectively neutral.''

3. If $N_e$ is large, deleterious mutations are much less likely to be fixed than neutral mutations.