Genetic Drift
Genetic drift is the gradual loss of genetic markers in small populations due to random events. A simple illustration is often used to teach this concept:
Fill a jar with 20 marbles—10 red, 10 blue. The jar represents a population, and the marbles represent people with different genetic profiles. Draw a marble at random from this population, record its color, and place it back in the jar. Each draw represents the birth of a child. Draw 20 times to simulate a new generation within the population. The second generation could have an equal number of each color, but more likely it will have an uneven number of the two colors.
Before you draw a third generation, adjust the proportion of each color in the jar to reflect the new mix of genetic profiles in the gene pool. As you continue drawing, the now-uneven mix will lead to ever more frequent draws of the dominant color. Over several generations, this “drift” toward one color will almost certainly result in the disappearance of the other color.
This exercise illustrates the inheritance pattern of genetic material over the course of several generations and shows how drift can result in the loss of genetic profiles. The effect of drift is especially pronounced in small, isolated populations or in cases where a small group carrying a distinct genetic profile intermingles with a much larger population of a different lineage.
A study in Iceland combining both genetic and genealogical data demonstrates that the majority of people living in that country today inherited mitochondrial DNA from just a small percentage of the people who lived there only 300 years ago. The mitochondrial DNA of the majority of Icelanders living at that time simply did not survive the random effects of drift. It is conceivable that much of the DNA of Book of Mormon peoples did not survive for the same reason.
Genetic drift particularly affects mitochondrial DNA and Y-chromosome DNA, but it also leads to the loss of variation in autosomal DNA. When a small population mixes with a large one, combinations of autosomal markers typical of the smaller group become rapidly overwhelmed or swamped by those of the larger. The smaller group’s markers soon become rare in the combined population and may go extinct due to the effects of genetic drift and bottlenecks as described above. Moreover, the shuffling and recombination of autosomal DNA from generation to generation produces new combinations of markers in which the predominant genetic signal comes from the larger original population. This can make the combinations of markers characteristic of the smaller group so diluted that they cannot be reliably identified.
The authors of a 2008 paper in the American Journal of Physical Anthropology summarized the impact of these forces succinctly: “Genetic drift has been a significant force [on Native American genetics], and together with a major population crash after European contact, has altered haplogroup frequencies and caused the loss of many haplotypes.” Genetic profiles may be entirely lost, and combinations that once existed may become so diluted that they are difficult to detect. Thus, portions of a population may in fact be related genealogically to an individual or group but not have DNA that can be identified as belonging to those ancestors. In other words, Native Americans whose ancestors include Book of Mormon peoples may not be able to confirm that relationship using their DNA.
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