title: Historical sampling reveals dramatic demographic changes in western gorilla populations.
abstract: BACKGROUND
  Today many large mammals live in small, fragmented populations, but it is often unclear whether this subdivision is the result of long-term or recent events. Demographic modeling using genetic data can estimate changes in long-term population sizes while temporal sampling provides a way to compare genetic variation present today with that sampled in the past. In order to better understand the dynamics associated with the divergences of great ape populations, these analytical approaches were applied to western gorillas (Gorilla gorilla) and in particular to the isolated and Critically Endangered Cross River gorilla subspecies (G. g. diehli).
  
  
  RESULTS
  We used microsatellite genotypes from museum specimens and contemporary samples of Cross River gorillas to infer both the long-term and recent population history. We find that Cross River gorillas diverged from the ancestral western gorilla population ~17,800 years ago (95% HDI: 760, 63,245 years). However, gene flow ceased only ~420 years ago (95% HDI: 200, 16,256 years), followed by a bottleneck beginning ~320 years ago (95% HDI: 200, 2,825 years) that caused a 60-fold decrease in the effective population size of Cross River gorillas. Direct comparison of heterozygosity estimates from museum and contemporary samples suggests a loss of genetic variation over the last 100 years.
  
  
  CONCLUSIONS
  The composite history of western gorillas could plausibly be explained by climatic oscillations inducing environmental changes in western equatorial Africa that would have allowed gorilla populations to expand over time but ultimately isolate the Cross River gorillas, which thereafter exhibited a dramatic population size reduction. The recent decrease in the Cross River population is accordingly most likely attributable to increasing anthropogenic pressure over the last several hundred years. Isolation of diverging populations with prolonged concomitant gene flow, but not secondary admixture, appears to be a typical characteristic of the population histories of African great apes, including gorillas, chimpanzees and bonobos.
authors: Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 Leipzig, Germany. olatha@utu.fi
authors: Olaf
authors: O
authors: Thalmann
authors: Daniel
authors: D
authors: Wegmann
authors: Marie
authors: M
authors: Spitzner
authors: Mimi
authors: M
authors: Arandjelovic
authors: Katerina
authors: K
authors: Guschanski
authors: Christoph
authors: C
authors: Leuenberger
authors: Richard A
authors: RA
authors: Bergl
authors: Linda
authors: L
authors: Vigilant
mesh_terms: D000354:Africa, Western; D000818:Animals; D017753:Ecosystem; D019143:Evolution, Molecular; D051456:Gene Flow; D014644:Genetic Variation; D006071:Gorilla gorilla / Q000145:classification / Q000235:genetics* / Q000254:growth & development; D018895:Microsatellite Repeats; D011156:Population Density; D011157:Population Dynamics