The loss of q decreases the rarer q is in the population. When the recessive allele becomes rare, selection against it becomes very ineffective, making complete elimination of recessives very difficult. Modern medicine creates a moral dilemma by allowing individuals to live to reproductive age. This means there is no tendency to return to the equilibrium point once a disturbance has occurred. Instead, genetic processes lead to the fixation of one allele and elimination of the other.
Natural selection fixes one allele but drift determines which allele is fixed and which one is lost.
Once the allele frequency has been disturbed, the change in gene frequency is a function of allele frequency and selection coefficient. In the absence of other factors, most forms of selection eventually lead to genetic uniformity at one locus through fixation of one allele and elimination of the other allele. Natural selection tends to erode genetic variability. One important exception to this is the phenomenon of heterozygote advantage, which maintains variability. Return to Evolution Homepage.