By Bernie Douglas (January 7, 2008)
Jorde et al. in a study published in the American journal of Human Genetics (2000) compared worldwide genetic variation among 255 individuals using autosomal, mitochondrial, and Y-chromosome polymorphisms. Their finding was gene-diversity results showed higher levels of diversity in African populations than in non-African populations. Similar findings have been reached in numerous studies dating back more than a decade (Vigilant et al. 1991; Nei et al. 1993; Cavalli-Sforza et al. 1991; Deka et al. 1995b; Jorde et al. 1997), a long time in the field of population genetics. Antonio Salas et al. (2002) reported that Africa presents the most complex genetic picture of any continent with time and depth for mitochondrial DNA.
It has also been discovered that Africans have the largest total number of alleles, as well as the largest number of unique alleles – alleles are genes inherited from each parent that code for things such as eye color, body type (etc.) -- for most systems; a discovery that could possibly place Africans as being the most genetically fit people on Earth, as lower genetic variation is said to lower genetic fitness (Lacy 1987). Gst (Genetic diversity) values are 11%–18% for the autosomal systems and are two to three times higher for the mtDNA sequence and Y-chromosome RSPs. Africa has higher GST values than does either Europe or Asia for all systems except the Y-chromosome STRs (Short tandem repeats). Alu diversity (DNA sequences present in the human genome) is highest in Africans (0.349) and lowest in Europeans (0.297) (Watkins et al, 2003). All systems except the Y-chromosome STRs show less variation between populations within continents than between continents. These results are reassuring and offer broad support for an African origin of modern human populations.
A population with many different alleles at a locus may be said to have a lot of genetic variation at that locus. Genetic variation is essential for natural selection to operate since natural selection can only increase or decrease frequency of alleles already in the population. In a very real sense, genetic variation is the raw material for evolution (Reed amd Frankham, 2003). Without genetic variation, a population cannot evolve in response to changing environmental variables and, as a result, may face an increased risk of extinction.
The major proportion of the intercontinental FST (FST is the proportion of the total genetic variance contained in a subpopulation) value is contributed by the African populations. The between-population variance for all E. Asian, Indian, and European populations is relatively low and not significantly different from the between-population variance within Africa alone – that is, Africa alone has about as much genetic variation as the rest of the world combined. The observed patterns of between-population differentiation are consistent with a bottleneck or a successive series of bottleneck events that have reduced genetic diversity among non-African populations and may have coincided with emigration from Africa (Harpending and Rogers 2000; Jorde et al. 2000; Reich et al. 2001).
Europe may have less genetic diversity than other continents due in part to the wide spread war and disease that have ravaged the continent over the last a millennia. In 1347, for example, Black Death was rapidly carried throughout Europe from the Mediterranean Basin, and eventually, areas of European settlement as isolated as Viking settlements in Greenland were ravaged by plague. By the time these plagues had run their course in 1351, between 25 and 50% of the population of Europe was dead (Gottfried, 1983). Lower genetic variation depresses individual fitness, resistance to disease and parasites, and flexibility in coping with environmental challenges. Lower variation also decreases mean fitness of populations, resilience, and long-term adaptability (Lacy, 1987).
Antonio Salas, Martin Richards, Toma´s De la Fe, Marı´a-Victoria Lareu, Beatriz Sobrino, Paula Sa´nchez-Diz,1 Vincent Macaulay, and A´ ngel Carracedo (2002). The Making of the African mtDNA Landscape. American. Journal of. Human. Genetics. 71:1082–1111, 2002.
Cavalli-Sforza LL, Bowcock AM, Kidd JR, Mountain JL, Hebert JM, Carotenuto L, Kidd KK, (1991) Drift, admixture, and selection in human evolution: a study with DNA polymorphisms. Proc Natl Acad Sci USA 88:839–843.
Deka R, Jin L, Shriver MD, Yu LM, DeCroo S, Hundrieser J, Bunker CH, et al (1995a) Population genetics of dinucleotide (dC-dA)n.(dG-dT)n polymorphisms in world populations. Am J Hum Genet 56:461–474.
Harpending, H. and Rogers, A. 2000. Genetic perspectives on human origins and differentiation. Annu. Rev. Genomics Hum. Genet. 1: 361–385.
Jorde LB, Rogers AR, Bamshad M, Watkins WS, Krakowiak P, Sung S, Kere J, et al (1997) Microsatellite diversity and the demographic history of modern humans. Proc Natl Acad Sci USA 94:3100–3103.
Jorde, L.B., Watkins, W.S., Bamshad, M.J., Dixon, M.E., Ricker, C.E., Seielstad, M.T., and Batzer, M.A. (2000). The distribution of human genetic diversity: Acomparison of mitochondrial, autosomal, and Y-chromosome data. Am. J. Hum. Genet. 66: 979–988.
Lacy R.C. (1987). Loss of Genetic Diversity from Managed Populations: Interacting Effects of Drift, Mutation, Immigration, Selection, and Population Subdivision Conservation Biology 1 (2), 143–158.
Nei M, Livshits G, Ota T (1993) Genetic variation and evolution of human populations. In: Sing CF, Hanis CL (eds) Genetics of cellular, individual, family, and population variability. Oxford University Press, New York, pp 239–252.
Reed D.H.; Frankham R. (2003). Correlation between Fitness and Genetic Diversity. Conservation Biology, Volume 17, Number 1, February 2003, pp. 230-237(8).
Reich, D.E., Cargill, M., Bolk, S., Ireland, J., Sabeti, P.C., Richter, D.J., Lavery, T., Kouyoumjian, R., Farhadian, S.F., Ward, R., et al. (2001). Linkage disequilibrium in the human genome. Nature 411: 199–204.
Vigilant L, Stoneking M, Harpending H, Hawkes K, Wilson AC (1991) African populations and the evolution of human mitochondrial DNA. Science 253:1503–1507.
Watkins, W.S, Pradipta K. Das, Mark A. Batzer and Lynn B. JordeE. Brassington, Marion L. Carroll, Son V. Nguyen, Jerilyn A. Walker, B.V. Ravi Prasad, P. Govinda Reddy, Alan R. Rogers, Christopher T. Ostler, Steve Wooding, Michael J. Bamshad, Anna-Marie (2003). Genetic Variation Among World Populations: Inferences From 100 Alu Insertion Polymorphisms. Genome Res. 2003 13: 1607-1618.