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
Department of Mathematics, Physics, and Engineering, Tarleton State University, Box T-0010, Stephenville, TX 76402-0010, USA.Human Biology (Impact Factor: 0.85). 07/2006; 78(3):295-305. DOI: 10.1353/hub.2006.0047
Our object in this paper is to analyze the opportunity for natural selection and gene flow in an isolated Zapotec-speaking community in the valley of Oaxaca, southern Mexico, that is undergoing a secular increase in body size. Surveys were conducted in the community in 1968, 1978, and 2000, including anthropometric and census data. No secular change was found in the growth status of schoolchildren and adult height between 1968 and 1978; subsequently, major secular gains in height occurred among children and adolescents between 1978 and 2000. The 1978 household data were used to compute gene flow (3.3%) and opportunity for selection intensity (I = 1.312). Migration and other demographic information was obtained from household census data for 1978 and 2000, and mortality information was extracted from community records and archives. These data were used to compute gene flow and opportunity for natural selection. Gene flow increased from 3.3% to 4.7% and intensity of natural selection decreased from 1.312 to 0.272 from 1978 to 2000. Variance in fertility increased slightly over time (12.25 to 13.69). Opportunity for selection was dominant during the prereproductive period in 1978, but approached 0 for the mortality component in 2000, resulting in a marked decrease in the mortality component (Im) of selection (0.626 and 0.019, respectively) and total opportunity for selection (I = 1.312 and 0.272, respectively). Secular increase in height and markedly decreased opportunity for natural selection (1) were associated with better health and nutritional conditions. Genotype-environment interaction and environmental influences are apparently the predominant causes of the secular trend. If natural selection plays a role in causing the secular trend, it is a small one.
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ABSTRACT: 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.
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ABSTRACT: Socio-economic transition with time offers us a way to understand the dynamics of evolution by studying the change in selection pressure. 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. 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. 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. 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.
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ABSTRACT: 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.
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