Population Bottleneck

Knowledge of mutation rates also permits reconstruction of past population sizes. A small number of genetic differences between individuals in a population or species may indicate either a recent origin, or a population bottleneck. Which of these two possible causes is responsible can be determined by measuring the number of so-called pairwise differences (mismatch distributions) in the DNA sequences that occur between individuals. Population expansion times are earlier for populations with higher average pairwise differences. Irregular mismatch distributions indicate long-term populations that have been stable for long times.

As shown in Figure 3, humans have remarkably little genetic diversity, especially in comparison to our closest living relative, the chimpanzee. Figure 1. Pairwise sequence divergence distributions in humans and four geographic populations (subspecies) of chimpanzees. The broad and uneven mismatch distributions of central, Nigerian and Western chimpanzees indicate they have maintained relatively large effective population sizes for a very long time. The low genetic diversitites of eastern chimpanzees and humans reflect recent population expansions following the loss of many lineages during population bottlenecks. The small peak on the right side of the Eastern chimpanzee distribution corresponds to a population expansion 69 thousand years ago, and the larger peak reflects expansion around 20,000 years ago, near the end of the last ice age. Indeed, there is substantially more genetic difference among individuals within chimpanzee troops in West Africa than among all living humans on earth. As shown in Figure 1, this is due to a series of bottlenecks in human evolutionary history. Geneticists studying many different parts of the human genome have concluded that the past effective population size (that is, the number of reproducing females) averaged only 10,000 individuals over the last one million years, and was as low as 5,000 around 70,000 years ago. Compare this to the approximately one billion reproducing females alive today, and it becomes clear just how narrow these bottlenecks were.