Depending on other conditions (environmental and genetic), a particular gene may not be expressed in the same manner (or at all). You can find simple examples i

Depending on other conditions (environmental and genetic), a particular gene may not be expressed in the same manner (or at all). You can find simple examples in recessive alleles. The phenotype of such genes will only be expressed when both alleles are of a compatible type. Another striking example: Sickle cell disease will only be deadly if both alleles carry the unfortunate mutation. But the unhealthy allele is unlikely to be eliminated by evolutionary pressure because carrying one allele provides significant protection against malaria. The interactions between genes and their environment is usually more complicated that these simple examples, but I hope it illustrates that evolutionary pressure may not suffice to erase some apparently unsustainable alleles from the population.