Abstract
In a way, the DNA sequences contained within each of our cells can be thought of as the text in microscopic book. This text is amazingly long, approximately one million pages if 6,000 letters fit on both sides of a page. This DNA contains the codes used to build RNA and proteins, which in turn assemble all the tissues in our bodies and direct the chemical reactions of life. We inherit copies of this DNA genome book from our parents and pass on copies to our children. However, in each generation, small changes are made, approximately 175 letters out of the genome, which are also passed on to our descendants. We can infer how closely people are related by comparing these types of inherited changes, which are shared between individuals or groups of individuals. Most of these changes are of no functional consequence, such as using either American or British spelling for color/colour; the meaning is still the same. However, occasionally, there is a functional change that can result in a phenotypic difference among individuals.
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Notes
- 1.
Two lineages from this ancestor give a total of 40,000 generations between modern individuals, multiplying this by the per generation mutation rate, 2.5 × 10−8 (Nachman and Crowell 2000), gives 1 × 10−3 or one out of a thousand differences.
- 2.
This greater amount of variation implies that chimpanzees and other great apes have had a larger average population size over time than humans. Humans lost genetic diversity due to population bottlenecks and/or the extinctions of genetically distinctive human populations such as the Neanderthals.
- 3.
In populations that are separated to some degree, alleles may be at different frequencies. Say we have allele A and a at one gene and allele B and b at another gene. If A and B are at high frequency in one population and a and b are at a high frequency in another population, in a sample of individuals from both populations, A appears more often than expected with B and likewise for a and b. Few individuals contain A and b or a and B. This is a signal that the sampled individuals originate from more than one structured population.
- 4.
Principal component analysis is a statistical method used to summarize aspects of complex datasets. In an imperfect analogy, imagine viewing a city from directly above. The positions of the buildings can be seen but not the heights of the buildings. Now, imagine the skyline of a city when viewed from the east. The heights of many of the larger buildings can be measured but not their precise positions. These two views represent simplified aspects of a complex object. Combining the two views allows a more complete overall picture to emerge, but many details are still missing.
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Reed, F.A. (2012). Modern Human Migrations: The First 200,000 Years. In: Messer, M., Schroeder, R., Wodak, R. (eds) Migrations: Interdisciplinary Perspectives. Springer, Vienna. https://doi.org/10.1007/978-3-7091-0950-2_29
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