title: Natural selection and demographic transition in a Zapotec-speaking genetic isolate in the Valley of Oaxaca, southern Mexico.
abstract: 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.
  
  
  METHODS
  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.
authors.affiliation: Tarleton State University, Stephenville, Texas 76402-0010, USA. Little@tarleton.edu
authors.forename: Bertis B
authors.initials: BB
authors.lastname: Little
authors.forename: Robert M
authors.initials: RM
authors.lastname: Malina
authors.forename: Maria Eugenia Peña
authors.initials: ME
authors.lastname: Reyes
mesh_terms: D000293:Adolescent; D000328:Adult; D000383:Agriculture; D000886:Anthropometry; D049628:Body Size; D002648:Child; D003710:Demography; D005006:Ethnicity / Q000235:genetics*; D005260:Female; D005298:Fertility / Q000235:genetics; D051456:Gene Flow; D040961:Genetic Drift; D014644:Genetic Variation; D005828:Genetics, Population / Q000639:trends*; D006801:Humans; D008297:Male; D008800:Mexico / Q000208:ethnology; D009026:Mortality / Q000639:trends; D010865:Pilot Projects; D044382:Population Groups; D012641:Selection, Genetic