In order to hand the allele a down to its offspring, the Aa individual has to pair with an AA individual. This means that in 90 percent of all possible cases, the allele is extinguished after 15 generations. A chance loss or gain of a non-adaptive allele by a population is called a genetic drift . Such a gene could, for example, enter a population via migration of individuals of foreign populations to a population. A genetic exchange between populations is also called gene flow .
Mutations are pretty common in natural populations.
On an average rate of mutation, the probability of exchange of one allele by another would be extraordinarily low. This example shows that the development of the abundance of forms as we know it in nature cannot be caused by the accumulation of neutral mutations. The situation changes drastically, when introducing selective advantages. An advantageous dominant allele spreads very fast, while recessive alleles have an only low ability to assert themselves even if they provide a selective advantage. The size of the population is decisive.