Thema Nr. 4: Was unterscheidet Rassen voneinander (sehr ähnlich dem vorhergehenden Thema Nr. 3)? Fragen



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Thema Nr. 4: Was unterscheidet Rassen voneinander (sehr ähnlich dem vorhergehenden Thema Nr. 3)?

Fragen:


  • Was ist eine Rasse?

  • Was unterscheidet die Menschen-Rassen voneinander?



relevante Seiten (u.a.):
Buch-Kap. 6 und 7: bitte nach „Rasse“ und „Polymorphismus“ durchsuchen.

Erläuterungen:
Das Thema ist sehr ähnlich dem vorhergehenden Thema. Daher sollten sich die beiden Referenten zusammentun.
Ich habe das Thema nur deswegen aufgeteilt, weil das was wirklich zwei Rassen unterscheidet, sehr schwierig zu verstehen ist, Aber es ist für das Verständnis der Biodiversität sehr wichtig. Auch hat es ethische Konsequenzen für das menschliche Zusammenleben.

Literatur:
[was in den folgenden beiden Texten als „Populationen“ bezeichnet wird, ist im Wesentlichen dasselbe, was „Rassen“ sind]
Die beiden folgenden Texte genügen; die Originale brauchen nicht unbedingt eingesehen zu werden.

King,M.-C. and Motulsky,A.G. (2002). Mapping Human History. Science 298, 2342-2343.


Ref ID: 5341

The most recent contribution to this literature is by Rosenberg et al. in this issue (RM 5338). These investigators explored the genetic structure of human populations using highly variable markers on the human autosomes of individuals from different parts of the world. The genotyped markers were microsatellite short tandem repeat sequences that do not encode any expressed genes and are generally selectively neutral. The populations studied were defined by geography, language, and culture, and participating individuals were well rooted in their populations, with several generations of ancestors known to have lived in the same locale as the participant. Genotypes from more than a thousand individuals were evaluated by a statistical method that defines clusters of people on the basis of genetic similarity at multiple loci, without using prior information about ancestry. In this method, individuals are assigned to clusters probabilistically. Individuals may have significant probabilities of membership in more than one cluster due either to genetic similarities of groups or to ancestral intergroup matings. The world map (see the figure) illustrates variation at one microsatellite marker in 12 populations. This marker has four common alleles, each of which appears in all populations. Rare alleles are shared by fewer populations. Few alleles are unique to only one population. No allele is population specific.

Previous genetic analyses of human history have consistently suggested that most human genetic variation is due to differences among individuals within populations rather than to differences among populations. The Rosenberg et al. analysis of many more markers and many more people confirms this result: 93 to 95% of genetic variation is due to genetic differences among individuals who are members of the same population and only 3 to 5% of genetic variation is due to differences among the major population groups.

The power of the method lies in the construction of clusters on the basis of accumulated small differences in allele frequencies across many markers and many people. Statistical clustering of genotypes--composed of 4682 alleles from 377 markers in 1056 individuals from 52 populations--yields groups corresponding to major geographic regions of the world. Creation of two clusters reflects ancient human origins in Africa and rapid expansion throughout Eurasia, and migrations to the Americas from East Asia. Creation of five clusters yields groups corresponding to five major geographic regions of the world: Africa, Eurasia (Europe, the Middle East, Central and South Asia), East Asia, Oceania, and America. There is excellent agreement between membership of individuals in these clusters and their self-identified regions of origin. Similar results were obtained by the same statistical approach based on fewer populations and fewer markers.

Population substructure could be consistently identified within some geographic regions but not others. Within Africa, for example, analysis consistently yielded the same four subclusters: Mbuti Pygmies, Biaka Pygmies, San peoples, and speakers of Niger-Kordofanian languages (Bantu, Yoruba, and Mandenka populations). In contrast, within Europe, multiple analyses were not consistent. Many more individuals will need to be included to sort out European demographic history.



The identification of clusters corresponding to the major geographic regions may depend on the sampling of individuals from well-defined, relatively homogeneous populations. If individuals were sampled from a worldwide "grid" (or a worldwide grid weighted by population density), the clusters might be much less precisely defined. Does the correspondence of worldwide genetic clusters and major geographic regions suggest borders around genetic clusters analogous to the physical borders--oceans, mountain ranges, and deserts--separating geographic regions? No. Both the results of Rosenberg and colleagues and those of previous studies indicate that unlike separations between geographic regions, differences in allele frequencies are gradual, without discontinuities between clusters. After thousands of years--if enough markers and people are studied--allele frequency differences are collectively adequate to create clusters that correspond to the major migrations of human history.

_________________________________________________________________


Rosenberg,N.A., Pritchard,J.K., Weber,J.L., Cann,H.M., Kidd,K.K., Zhivotovsky,L.A., and Feldman,M.W. (2002). Genetic structure of human populations. Science 298, 2381-2385.
Ref ID: 5338

We studied human population structure using genotypes at 377 autosomal microsatellite loci in 1056 individuals from 52 populations. Within-population differences among individuals account for 93 to 95% of genetic variation; differences among major groups constitute only 3 to 5%. Nevertheless, without using prior information about the origins of individuals, we identified six main genetic clusters, five of which correspond to major geographic regions, and subclusters that often correspond to individual populations.


Most studies of human variation begin by sampling from predefined "populations." These populations are usually defined on the basis of culture or geography and might not reflect underlying genetic relationships. Because knowledge about genetic structure of modern human populations can aid in inference of human evolutionary history, we used the HGDP-CEPH Human Genome Diversity Cell Line Panel to test the correspondence of predefined groups with those inferred from individual multilocus genotypes (microsatellite markers).

The average proportion of genetic differences between individuals from different human populations only slightly exceeds that between unrelated individuals from a single population. That is, the within-population component of genetic variation, estimated here as 93to 95%, accounts for most of human genetic diversity. … This overall similarity of human populations is also evident in the geographically widespread nature of most alleles. Of 4199 alleles present more than once in the sample, 46.7% appeared in all major regions represented: Africa, Europe, the Middle East, Central/South Asia, East Asia, Oceania, and America. Only 7.4% of these 4199 alleles were exclusive to one region.

Despite small among-population variance components and the rarity of "private" alleles, analysis of multilocus genotypes allows inference of genetic ancestry without relying on information about sampling locations of individuals. We applied a model-based clustering algorithm that, loosely speaking, identifies subgroups that have distinctive allele frequencies.



Europe, with the smallest among-population variance component (0.7%), was the most difficult region in which to detect population structure. …Basque and Sardinian were identified as distinctive. Russians variously grouped with Adygei and Orcadians; Russian-Orcadian similarity might derive from shared Viking contributions. French, Italians, and Tuscans showed mixed membership in clusters that contained other populations.

The challenge of genetic studies of human history is to use the small amount of genetic differentiation among populations to infer the history of human migrations. Because most alleles are widespread, genetic differences among human populations derive mainly from gradations in allele frequencies rather than from distinctive "diagnostic" genotypes. Indeed, it was only in the accumulation of small allele-frequency differences across many loci that population structure was identified.



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