II. Origins of population diversity.

III. Origins of present-day populations.

Terms

A. Gene pools can be compared in terms of the frequencies of alleles at polymorphic loci, although for complex traits, such as skin color and body form, we cannot analyze specific loci and alleles readily as yet.

B. Large populations are specified in the definition of race as a matter of practicality. There are many very small populations that are quite deviant because of genetic drift and founder effect.

C. The number of races is arbitrary. The "splitters" can name several dozen. The "lumpers" may see only three (African, Caucasian, Asian). The urge to classify is a human trait; the results at best are an approximation to biological reality.

D. In any list of races, some races will be closer genetically to each other than they are to other races. E.g. Amerinds came from Asia and have many of the same alleles found in present-day Asians but rare or absent in Europeans and Africans.

E. Each of these races consists of local races or local populations.

F. Races are separated from each other more by our system of classification than by major genetic differences. A few, such as Polynesians and Australians, have been separated from other groups for tens of thousands of years. However, most have had some gene exchange, and often there are smaller populations that are intermediate genetically between major races.

A. A special form of genetic drift is founder effect, in which a population is founded by a small group that may be unrepresentative of the parent population.

B. During the tens of thousands of years that Homo sapiens has existed, the population lived in small tribes, where genetic drift and founder effect were strong forces.

C. As our ancestors spread over the continents, natural selection undoubtedly favored different gene pools in different environments.

1. Dark skin was favored in tropical areas because it protects against ultraviolet light. Light skin is favored elsewhere because some ultraviolet is necessary for synthesis of vitamin D.

2. A high ratio of body surface to volume is favored in hot areas so that body heat can be dispersed. A low ratio is favored in cold areas so that heat is conserved.

3. Most Africans and their descendants have an allele at the Duffy blood group locus that makes them resistant to vivax malaria.

D. Cultural practices also can alter fitness of different genotypes: e.g. lactase persistence.

A. Homo sapiens sapiens is thought to have evolved in Africa some 200,000 years ago. By 100,000 years ago, the populations were already diversified in Africa. A small group crossed into the Middle East at Suez and spread throughout Asia and Europe, reaching Australia some 40,000 years ago and America some 20,000 years ago (highly disputed date).

B. Mitochondrial DNA is particularly interesting, since it is maternally inherited and therefore does not recombine during meiosis. There are many versions of mtDNA today, all of which descended from a single female who lived some 200,000 years ago. There were many other female ancestors, but their mtDNA eventually was lost. Many think that female lived in Africa (though some argue for Asia).

C. The Y chromosome also does not recombine except for the pseudoautosomal region. Diversity in the Y chromosome is generated only by mutations, which accumulate over time. It is theoretically possible to identify a "founding" Y chromosome, just as in the case of mitochondria. The date given for such a founding father is of the same order of magnitude as mitochondria, some 200,000 years ago. Both dates are subject to large error because of the assumptions of rate of accumulation of mutations.

D. The virtual absence of polymorphic markers on the Y chromosomes of Finns indicates a very small population bottleneck in the past. Analysis of mitochondrial variation in this group suggests that the bottleneck occurred 4000 years ago, at about the time agriculture is known to have reached Finland.

E. Cultural events may also influence the gene pool of populations. For example, populations that are better able to exploit resources may have greater biological fitness and therefore contribute disproportionately to the gene pool. This is illustrated by the development of agriculture, which can support more people than the hunter-gatherer cultures. Present-day allele frequencies in Europe reflect the spread of agriculture from the Middle East.

F. Allele frequencies in present-day populations reflect their genetic origins and the relationships of the populations to each other.

1. The African-American population is an African-European hybrid, with about 70% of the genes having come from Africa and 30% from Europe. However, some populations (e.g. Charleston, SC, and the Sea Islands of Georgia) have over 90% African genes.

2. Polynesians came from mainland Asia in fairly recent times. This is in contrast to the theory that they came from mainland South America and are descendants of American Indians.

3. The American Indians came from Asia and appear to be most closely related to Mongolians rather than to the groups that currently occupy northeast Asia. The Hispanic populations of Texas have a high frequency of alleles that came from American Indian ancestry and hence Asia.

4. By analyzing simultaneously the variations at many loci, it is possible to construct phylogenetic trees that relate the many populations of the world.

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