Opportunity for Natural Selection and Gene Flow in an Isolated Zapotec-Speaking Community in Southern Mexico in the Throes of a Secular Increase in Size

Epidemiologic transition in an isolated indigenous community in the Valley of Oaxaca, Mexico

The objective of the present study is to analyze age-specific mortality in a rural indigenous community in the throes of a secular increase in size in the Valley of Oaxaca, southern Mexico, over 30 years, 1970-1999. Variation in mortality by age group was analyzed over time for evidence of an epidemiological transition. The seasonal rain pattern in the Valley of Oaxaca (83% from May through September) was evaluated for its relationship with mortality in wet and dry months. Mortality and causes of death changed markedly over the 30-year interval. Infant and preschool mortality, overall mortality, and causes of death changed from the 1970s through the 1990s. Prereproductive deaths (<15 years) predominated in the 1970s and were largely due to gastrointestinal and respiratory diseases, with periodic outbreaks of measles. Deaths of adults 65+ years predominated in the 1990s and were largely due to degenerative diseases usually associated with aging. The marked changes in age and causes of death over the three decades (epidemiologic transition from Stage I to Stage II) occurred concurrently with significant secular increases in body size in children, adolescents, and young adults, highlighting improved health and nutritional conditions in the community which is in early Stage II of the demographic transition. The demographic transition to Stage II is a leading indicator (15-25 years lag) for the onset of the secular trend, while the epidemiologic transition to Stage II is a predictor that the secular increase is in process in the study community.

Natural selection and demographic transition in a Zapotec-speaking genetic isolate in the Valley of Oaxaca, southern Mexico

OBJECTIVE

The study analyzed the potential for natural selection and the demographic transition in an isolated Amerindian population in the process of secular change in body size.

SETTING

A genetically isolated, Zapotec-speaking community located in the Valley of Oaxaca, southern Mexico, has been studied regularly from the mid-1960s to 2000. Children, adolescents and young adults have experienced a recent secular increase in body size since 1978 after a major period of no change.

METHODS

Potential for natural selection and the demographic transition were analyzed over a 100-year period, ca 1900-2000. National census data, results from anthropological surveys and community archives and reports were used.

RESULTS

Opportunity for natural selection changed markedly over the last century. Demographic transition to Stage II occurred ca 1955 and preceded a secular increase in body size. The crossover between curves for mortality (I(m)) and fertility (I(f)) occurred at approximately the time of onset of the secular trend among children, adolescents and young adults, i.e. those born since the early 1970s.

CONCLUSIONS

The 'classic' demographic transition occurred in the mid-1950s and preceded the secular increase in body size. A 'critical mass' of environmental improvement appears to be necessary to activate secular improvements in growth status, possibly turning on a gene complex that interacts with the improved environmental conditions. The lead time from the onset of demographic transition phase II to beginning of the secular trend is approximately 25 years (one generation) in this community.

Influence of socio-economic transition on genetic structure: a case study in Upper Assam, India

BACKGROUND

Socio-economic transition with time offers us a way to understand the dynamics of evolution by studying the change in selection pressure.

AIM

The present study aimed to explore the effect of temporal change in socio-economic conditions on the selection pattern of Sonowal Kachari, a major tribal population of Assam, north-east India, over a period of 31 years (from 1975 to 2006). It also attempts to locate the present position of the tribe in the demographic transition model on the basis of the prevalent selection pattern.

SUBJECTS AND METHODS

Data on differential fertility and differential pre-reproductive mortality were collected from 155 unrelated Sonowal Kachari post-menopausal women from Dibrugarh, Upper Assam. Opportunity for natural selection was calculated on the basis of the indices put forward by Crow (I(c)) and Johnston and Kensinger (I(j)). To compute the opportunity for natural selection, different parameters such as index of selection due to infant mortality (I(m)), index of selection due to fertility (I(f)) and index of selection due to embryonic mortality (I(me)) were taken into consideration. Standard statistical techniques were applied wherever relevant.

RESULTS

The different components of selection were found to be higher among the rural population than those in the urban one. Without considering the effect of regional variability, the Crow's index (I(c)) of opportunity for natural selection was found to decrease with time from 0.364 in 1975 to 0.244 in 2006. The Johnston and Kensinger index (I(j)) also decreased from 0.541 in 1975 to 0.284 in 2006. Irrespective of the methodology, the mortality component was found to decrease considerably with time without marked reduction in the fertility component.

CONCLUSION

The study, within its limitations, suggests that at present the Sonowal population is experiencing a selection relaxation due to the temporal improvement of socio-economic conditions. By comparing the fertility and pre-reproductive mortality components it can be argued that the population may have been passing through the initial period of post-transitional stage, which may increase the segregation load in the population.

Overweight and obesity in a rural Amerindian population in Oaxaca, Southern Mexico, 1968-2000

The purpose of this study was to evaluate secular change in the prevalence of overweight and obesity in a rural Zapotec Indian community in southern Mexico between 1968 and 2000. Cross-sectional surveys of children 6-13 years, adolescents 13-17 years, and adults 19 years of age and older resident in a rural community in Oaxaca were conducted in 1968/1971, 1978, and 2000. Individuals present in the 1968, 1978, and 2000 surveys provided a small longitudinal component. Height and weight were measured; the BMI was calculated. International criteria for overweight and obesity were used. Overweight and obesity were virtually absent in school children 6-13 years in 1968 and 1978 and in adolescents in 1978. Small proportions of children (boys, 5%; girls, 8%) and adolescents (boys, 3%; girls, 15%) were overweight in 2000; two children (1%) and no adolescents were obese. Among adults, 7% of males and 19% of females were overweight and <1% of males and 4% of females were obese in 1971/1978, but 46% of males and 47% of females were overweight; and 5% of males and 14% of females were obese in 2000. The trends for children, adolescents, and adults were confirmed in the longitudinal subsamples. In conclusion, overweight and obesity are not presently a major problem in children and adolescents in this rural Zapotec community. Overweight, in particular, and to a lesser extent obesity have increased in prevalence among adults since the late 1970s. The results suggest adulthood as a critical period for onset of overweight and obesity in this sample.