Climate change and evolution of Cross River gorillas
Max Planck Press Release
Two species of gorillas live in central equatorial Africa. Divergence between the Western gorillas (Gorilla gorilla) and Eastern Gorillas (Gorilla beringei) began between 0.9 and 1.6 million years ago and now the two species live several hundred kilometres apart. An international team of researchers including Olaf Thalmann of the University of Turku in Finland and Linda Vigilant of the Max Planck Institute for Evolutionary Anthropology in Germany found that the divergence of Western lowland gorillas and the Critically Endangered Cross River gorillas (Gorilla gorilla diehli) occurred more recently, about 17,800 years ago, during the Pleistocene era
An evolutionary model of the two subspecies of Western gorillas was generated using microsatellite genotyping of living gorillas and 100-year-old museum specimens. This data showed that, although Cross River gorillas diverged from Western lowland gorillas about 17,800 years ago, the two subspecies continued to intermittently interbreed. Olaf Thalmann and his co-authors suggest that climate change during the Pleistocene era caused the forests to expand, permitting the Western gorillas to expand their range. When the forest contracted again the gorillas were separated into two populations which began to diverge. Successive rounds of climate change resulted in periods when the two subspecies could interbreed followed by repeated episodes of isolation of the Cross River population.
The model indicates that gene flow finally stopped between the two subspecies approximately 420 years ago. Over the last 320 years there has been a 60% decrease in the numbers of Cross River gorillas causing a loss of genetic diversity within the population. Thalmann said, “The number of Cross River gorillas has continued to decrease, probably due to anthropogenic pressure, such as destruction of their habitat or hunting by humans. There are thought to be fewer than 300 individuals left.” He continued “It is unclear what effect this loss of genetic diversity will have on the long term viability of Cross River gorillas. But, given that this bottleneck occurred so recently, it is possible that if the population was allowed to expand the loss of diversity could be stopped.”
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Reference
Olaf Thalmann, Daniel Wegmann, Marie Spitzner, Mimi Arandjelovic, Katerina Guschanski, Christoph Leuenberger, Richard A Bergl and Linda Vigilant (2011) Historical sampling reveals dramatic demographic changes in western gorilla populations. BMC Evolutionary Biology. 11:85doi:10.1186/1471-2148-11-85
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.
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