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.

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.

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.

Epidemiología y genética: alianza estratégica en el nuevo milenio? / Epidemiology and genetics: a strategic alliance in the new millennium?

Although the information derived from biological markers could conceivably be used to overcome some of the problems intrinsic to virtually all epidemiologic study designs­case definition, true exposure level, host susceptibility and resistance to factors of interest, the misclassification of study sub- 44 Rev Panam Salud Publica/Pan Am J Public Health 17(1), 2005 Opinión y análisis • Opinion and analysis jects (false positive and false negative test results), etc.­, we are still unable to resolve all such problems with the tools available at present. Biological markers seem more promising as potential indicators of the degree of susceptibility than as indicators of disease occurrence, an application requiring further technical refinement. Currently biological markers are employed in public health mainly to screen for particular diseases. Unfortunately, these markers have their limitations. For one thing, it is unlikely that they will completely eliminate the problem of false positive and false negative results, since DNA from solid tumors undergoes slight degradation due to necrosis and since genetic markers are susceptible to the effects of exposure to medication, diet, sex, ethnicity, and even the circadian cycle. And even if false positives and negatives were ultimately eliminated, it would be impossible to use many of the analytical tools based on two by two tables, such as the chi squared test, logistic regression, the Poisson regression, Cox' proportional hazards ratio, etc., since such tools rely on comparisons of the number of false positives and negatives in the exposed and non-exposed groups. Finally, albeit no less important, certain ethical issues must be carefully considered before allowing the massive use of human genetic markers, which could lead to violations of the rights of individuals, families, and communities if carried out in an indiscriminate, unregulated fashion. Epidemiology is rapidly broadening its scope, a trend that will continue into the future; new analytical tools will be developed, and the working hypotheses to which such tools will be applied will change. At present the scientific community is paying increased attention to this field of study, but more research and discussion are needed to respond to many of the questions for which we have no satisfactory answers yet