Genetic drift refers to changes of gene frequency due solely to chance deviations from the expected ratios appearing when a small number of offspring and a small number of mating pairs in the population. The effect is not related to the advantage or disadvantages of the genes involved. It becomes much more pronounced in small isolated population than in larger groups, where the extent of the numbers would level off random fluctuations in the distribution of the gene pool.
As an example, if a new mutant, a appears in a single individual in the population it would be expected to be
A/a x A/A --> ½ A/A:½A/a.
However, if there are a limited number of offspring it might, by chance occur that they would all be A/A, rather than the anticipated ½:½ ratio. In this scenario the new gene would be lost.
A second example of genetic drift often termed the founder's principle, involves the chance of receiving a non representative distribution of genes among a handful of people, who may quit a large population to set up a new and isolated colony. If the original population was comprised of 50% B and 50% b, it's possible that 20 individuals who moved to an island may indeed be B/B or all b/b. In either situation, the new population would have a notably differing gene frequency distribution from the parent group. To a lesser extreme there would be the likelihood that the frequency of B among the members of the new colony could be 10 or 90 percent in place of the original 50%.
The Dunkers
Bently Glass demonstrated genetic drift among human populations among a group of people in Franklin County, Pennsylvania, known as the Dunkers. A German Baptist religious group they were descendants of a religious sect, the Baptist Brethern and were popularly known as Dunkards, Dunkers, or Tunkers, from the German for to dip, referring to their method of baptizing, by trine immersion, the candidate being immersed once for each member of the Trinity. Originating from the Rhineland region of Germany near Krefeld they had evolved from the Pietist movement. The first congregation was organized there in 1708 by Alexander Mack. Persecution drove them to America where,in 1719 under Peter Becker, they settled in Germantown, Pa. They have remained relatively isolated from others in the US, with their own manner of dress and custom distinct from those around them. They were essentially genetically isolated because they generally did not reproduce with people in nearby populations.
Amidst the characteristics of the Dunkers which were studied , the ABO blood groups illustrate the possible determinants of genetic drift. Comparisons were made with the people in Rhineland of West Germany today and with people in the eastern United States. A number of traits are either more or less frequent in the Dunkers than they are in Germany or other populations in Pennsylvania. Sixty percent of the Dunkers have a distinctly higher frequency of Type A blood, as compared with a 41% in Rhineland and 40% in the eastern US respectively. Along with the loss of type B among the Dunkers at a 3% population occurrence compared to 10 % in Rhineland and 11% in the eastern US. This appears to be the manifestation of genetic drift in an early generation after migration, when their numbers were small, the ancestors of today's Dunkers may have produced a generation of children with a larger abundance of the gene for the A antigen and a lower one for the B antigen. There is no reason to believe that selection or anything other than pure chance was involved. A gene pool with a high frequency of A and a low frequency of B has, as a result, come down through the generations.
Genetic Drift and Catastrophe
At times a huge catastrophe will wipe out the majority of a population, leaving only a few survivors. Such catastrophes could be a disease, plague or some terrific natural force such as a tidal wave or hurricane. The few survivors may not be representative of the originating population and would have descendants differing according to the chance distribution of genes in those who survived. At the University of Hawaii just such an example of this was revealed by N.E. Morton during his studies of the natives of the Micronesian Islands. On the island of Pingelap he found that 6% of the population had a serious inherited eye disorder, achromotophobia. The recessive gene for this condition, when homozygous, causes a degeneration of the retinal cones, and results in complete color blindness along with extreme sensitivity to bright light. The compared populations on surrounding islands showed a very low frequency of this abnormality, much less than one percent. An investigation into determining the cause of this notable deviation uncovered that a serious hurricane hit Pingelap in 1900 killing all but 20 inhabitants. By chance, it appears that this small group had a much greater chance than average occurrence of this gene. Or, it could be that the survivors with the gene had more children that those with the normal allele. Either factor, or more likely both, could have resulted in the high incidence of achromotophobia there today.
Summary
The relative importance of genetic drift and selection depends, in part, on estimated population sizes. Drift is much more important in small populations and is important to remember that most species consist of numerous smaller inbreeding populations called demes.
Studies of evolution at the molecular level have provided strong support for drift as a major mechanism of evolution. Observed mutations at the level of gene are mostly neutral and not subject to selection. One of the major controversies in evolutionary biology is the neutralist-selectionist debate over the importance of neutral mutations. Since the only way for neutral mutations to become fixed in a population is through genetic drift. This controversy is actually over the relative importance of drift and natural selection.
Sources
Columbia Encyclopedia:
http://www.bartleby.com/
Genetics: A Survey of the Principles of Heredity. Ed. H. Bently Glass, June Shepard. 4th ed. Boston: Houghton Mifflin Company, 1972.
The Talk Origin Archive:
http://www.talkorigins.org