Suchergebnisse

Populations residing for millennia on the high-altitude plateaus of the world started natural experiments that we can evaluate to address questions about the processes of evolution and adaptation. A 2001 assessment in this journal summarized abundant evidence that Tibetan and Andean high-altitude natives had different phenotypes, and the article made a case for the hypothesis that different genetic bases underlie traits in the two populations. Since then, knowledge of the prehistory of high-altitude populations has grown, information about East African highlanders has become available, genomic science has grown exponentially, and the genetic and molecular bases of oxygen homeostasis have been clarified. Those scientific advances have transformed the study of high-altitude populations. The present review aims to summarize recent advances in understanding with an emphasis on the genetic bases of adaptive phenotypes, particularly hemoglobin concentration among Tibetan highlanders. EGLN1 and EPAS1 encode two crucial proteins contributing to oxygen homeostasis, the oxygen sensor PHD2 and the transcription factor subunit HIF-2α, respectively; they show signals of natural selection such as marked allele frequency differentiation between Tibetans and lowland populations. EPAS1 genotypes associated in several studies with the dampened hemoglobin phenotype that is characteristic of Tibetans at high altitude but did not associate with the dampened response among Amhara from Ethiopia or the vigorous elevation of hemoglobin concentration among Andean highlanders. Future work will likely develop understanding of the integrative biology leading from genotype to phenotype to population in all highland areas.

▪ Abstract  The high-altitude Andean and Tibetan Plateaus offer natural experimental settings for investigating the outcome of the past action of evolution and adaptation as well as those ongoing processes. Both Andean and Tibetan high-altitude natives are descended from sea-level ancestors; thus both initially encountered chronic, lifelong high-altitude hypoxia with the same homeostatic "toolbox" that evolved at sea level for responding to brief and transient hypoxia. Yet now they differ phenotypically in many traits thought to be important for offsetting chronic high-altitude hypoxia. Compared on the basis of mean values of five traits, the characteristics of Tibetan high-altitude natives differ more than those of Andean high-altitude natives from the ancestral or unselected response to chronic hypoxia exhibited by acclimatized lowlanders. This suggests that different evolutionary processes have occurred in the two geographically separate areas, although it is not clear why or how those processes differed. Answers to those questions require better knowledge of the prehistory of human populations on the plateaus, as well as information on new phenotypes and the relationship between phenotype and genotype.

The authors examine the impact of China's post-1980 Tibet policy on Tibet's 500.000 nomadic pastoralists in Changtang (Northern Plateau), especially in Phala. They discuss the situation of the Phala nomads during the traditional era and the early period of direct Chinese rule 1959-80. The authors note that the new Chinese economic and cultural policies implemented in Tibet following Hu Yaobang's investigation tour in May 1980 have produced a major transformation in Phala. (DÜI-Sen)

This paper tests sociobiological hypotheses regarding the way practitioners of fraternal polyandry in a Tibetan population in Nepal may enhance their inclusive fitness. Demographic data demonstrate higher mortality and lower survivorship of offspring in fraternal polyandry than monogamy. Moreover, the probability of allele transmission for an ego is lower in fraternal polyandrous unions than in monogamous unions. On the basis of the measured parameters, Tibetan fraternal polyandry does not appear to enhance the fitness of individuals who practice it and, in fact, seems to entail substantial reproductive sacrifice. [sociobiology, fraternal polyandry, inclusive fitness, demography, Tibet]

This paper presents new demographic findings for a high altitude Himalayan population residing in Ladakh, India, and reviews problematic issues regarding the hypothesized relationship between fertility/fecundity and altitude in the Himalayas in light of these findings. It concludes that the low completed fertility ratio reported for the Sherpas of Khumbu, Nepal, is not caused by hypoxia‐induced low fecundity, but is the product of cultural factors affecting the exposure of females to the risk of intercourse, a critical confounding factor that has not received adequate consideration in previous studies. Contrary to earlier reports, the present study demonstrates that all high altitude Himalayan populations for which published data exist exhibit moderately high fertility and fecundity, and do not differ significantly in their fertility levels. Furthermore, it argues that the claims for a statistically significant difference in fertility between high, moderate, and low altitude Himalayan populations are groundless, and suggests that a parallel reevaluation of Andean findings is required. [fertility, fecundity, hypoxia, Himalayas, Andes]