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A Population Bottleneck is a significant reduction in the size of a population that causes the extinction of many genetic lineages within that population, thus decreasing genetic diversity. Population bottlenecks have occurred in the evolutionary history of many species, including humans. Present-day bottlenecks are seen in endangered species such as the Yangtze River dolphin, whose numbers have dwindled to less than 100. Endangered species that do not become extinct may expand their numbers later on, but with a limited amount of genetic diversity with which to adapt to changing conditions. The genomes of future populations will reflect the narrowing of genetic possibility for thousands of years. [1]