According to the Hardy-Weinberg principle, a population in equilibrium will maintain the same allele frequency over many generations. For this to occur, however, several conditions must be met. One of these conditions is that no immigration or emigration can occur, or the allele frequency will change. Another condition is that the population must be large. A large population minimizes the effect of random fluctuation. If a small number of individuals in a population leaves and establishes a colony elsewhere, their combined alleles will establish the gene pool of the new colony. It is not likely, however, that allele frequencies in the new gene pool will be identical to those in the original gene pool. This is due to the fact that the new collection of alleles represents a random sample, which may not accurately represent the larger group from which is was taken. If a jar contains equal numbers of black and white marbles, and ten marbles are randomly selected from the jar and placed in another jar, it is likely that there will not be an equal number of black and white marbles in the new jar. This is the result of sampling error. In general, the smaller the sample, the less representative of the original ratio it will be.
In the most extreme case, one pregnant animal or one seed may establish a new colony. The gene pool that results will represent the allele frequencies of that single founder's progeny, not those of the founder's original gene pool. This phenomenon, called the founder effect, has two consequences. First, the new, or founder, population, has less diversity than the original population.
Second, the allele frequencies differ from those in the original population. The high frequencies of certain genetic disorders in founder populations illustrate this idea. In Holland, the frequency of the allele for Huntington Disease is one per million individuals. In about 1700, 100 Dutch people established a colony in South Africa. Since there has been almost no immigration to the Afrikaner population, they have bred among themselves. Apparently, one or more of the founders of the colony had the HD allele. In South Africa today, the frequency of the HD allele among the descendants of the Dutch settlers is one in twenty. Another example is found in the Amish community of Lancaster, Pennsylvania. This community was established by three German couples in 1770, and has remained a reproductively isolated population now numbering about eight thousand. Up to seven percent of the Amish population has Ellis van Creveld syndrome, a recessive form of dwarfism. This condition is extremely rare in other populations, including the population from which the original settlers came. These two examples show how the founder effect can dramatically increase the frequency of a rare allele.
A related concept is the bottleneck effect. In this situation, most members of a population are destroyed, perhaps by a natural disaster. Again, due to sampling error, the small percentage of the population that survives may have a gene frequency very different from that of the original population, and will almost certainly have a decrease in genetic diversity. This is the current explanation for the lack of genetic diversity observed in modern cheetahs.
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