Gene Distributions

The analysis of gene distributions throughout the world, called population genetics, is very important for estimating the date when human life originated and the timing and direction of human migrations throughout world history. Before DNA, the analysis of fossil remains was the major way of estimating human origins. With genetics, four major evolutionary forces are considered:mutations, genetic drift, gene flow, and natural selection.

Within anthropology, there has been an intense debate between the multiregionalists, who believe that humans evolved roughly simultaneously in multiple regions of the world, and the out-of-Africa proponents who believe that current human life originated in Africa. Based on their research on mutations, which shows much more genetic variation within Africa than within other regions of the world, geneticists support the African origins school. Because genetic mutations (and hence genetic diversity) happen randomly throughout history, by a process of extrapolation backward, the conclusion is, comparatively speaking, that a group with a larger number of mutations has had a longer time period in which to accumulate mutations and thus is older.

Mutations have also been very important in studying migrations and explaining gene distributions in different groups and regions of the world. Luigi Luca Cavalli-Sforza, a prominent population geneticist in the United States, has published much data showing the genetic distances between nationality groups and the genetic distance between ethnic groups throughout the world. The more genetic distance between groups, the more mutations have occurred to cause the distancing, and hence the further back one goes to find their common ancestors.

[Page 1033]Brian Sykes has classified nearly all women in the world into one of seven groups, each having genetic similarities from ancient history. He showed how these seven groups are distributed throughout the world and suggested the migration history of the groups based on differences and similarities between the genetic compositions of the groups. Genetic research has shown that males and females have very different migration patterns, with the female more likely to leave her parental family and follow the male's family pattern.

For males, a selected number of markers (specific positions on the Y chromosome that are unique and can be measured) can be grouped together into haplo-types, and a number of similar haplotypes can be grouped into haplogroups, to show genetic relationships between different groups or geographical areas of the world. There are strong correlations between haplogroups and regions of the world. Each marker is given a DYS#, which stands for DNA Y-chromosome segment number, numbers that are increasingly being standardized and used internationally so that comparisons between different studies can be made more easily. A world database using nine markers (http://www.yhrd.org),for example, has haplotypes for over 25,000 males, allowing any of these men to see where they have nine-marker genetic matches from over 200 sites throughout the world. The less two men have mutated, the closer they match, and the more recently their MRCA (most recent common ancestor) lived.

Genetic drift affects a small population that does not have sufficient population size to randomly reproduce all the genes of its parent population. A principle called the Hardy-Weinberg Law says that genetic frequencies will not change (that is, changes will balance themselves out) in an infinitely large population with random mating. But, a small group can become nonrepresentative of its parent generation because of inability to reproduce all the previous genes. Hence, two small groups that come from the same parent population will, over time, develop dissimilar gene distributions as each loses some genes and moves further from its original gene distribution. Different mutations can also add to the genetic distance between the small populations.

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