The McDonald Kreitman tests of genes in the human and chimpanzee genomes showed many with an excess of nonsynonymous variation within species. Dr. Noor suggested this means that many mutations are sticking around but not getting fixed. In class, Dr. Noor suggested this was probably the result of the inability of our species to eliminate some detrimental mutations. Why is this elimination difficult

A mutation may be bad and good depending on the particular environmental conditions

Explanation:

For example, the falciform anemia is a hereditary disease caused by a mutation where the ability to transport oxygen of red blood cells is reduced; however, this genetic disorder is widespread in some African populations because it is a condition that confers resistance to falciparum malaria. In consequence, falciform anemia is a detrimental mutation that may also be beneficial in endemic malaria regions.

Moreover, it is important to note that the higher proportion of non-synonymous substitutions are not subject to selection (only a minor percentage are positively selected) because they are generally localized within non-coding regions, thereby having neutral status.