Essay On Population Bottleneck

Catastrophic natural disasters and epidemics of disease can lead to drastic reductions in population size. This phenomenon is called a population bottleneck (nature.com). The loss of reproducing individuals means a contraction in the number of alleles being contributed to the gene pool.

Species with low effective populations are subject to the influence of genetic drift – a stochastic evolutionary mechanism that moves an allele towards fixation, regardless of what fitness advantages or deleterious effects it may have (Wright, 1931). The bottle-necking of a population is expected to result in a decrease of genetic diversity, demonstrated by a decline in heterozygosity and the loss of alleles at a locus. These proxies for genetic diversity are influenced by different aspects of the bottleneck. While levels of heterozygosity are affected by how quickly the population grows following the release of the bottleneck, the number of alleles at a locus is affected by how narrow the bottleneck is (Nei et al., 1975). Although a bottleneck is expected to reduce genetic variation, there are several mechanisms that can skew the outcomes and limit this loss. Gene interactions (epistasis), dominance, and linkage disequilibrium can restrict the loss of, and, in some cases, inflate genetic diversity. Furthermore, inbreeding is common following a bottleneck, but many inbred families may be selected against if they are carriers of deleterious alleles (Cheverud et al., 1999).

The release of a bottleneck occurs when population size is allowed to increase again. This population growth is accompanied by increasing genetic diversity and movement towards the original levels of heterozygosity. The rate of return of genetic variation can depend on just how ...

... middle of paper ...

... several metrics did not provide the authors of this study with sufficient evidence to accept the idea of a population bottleneck roughly 70,000 years ago. However, they felt they had ample data supporting a bottleneck in human evolutionary history approximately 2 million years ago (Hawks et al., 2000).

Though it cannot be decided in absolute terms either way, there is a plethora of information to support the concept that a bottleneck occurred in the late Pleistocene era, following the eruption of Mount Toba. Knowledge of the effects of volcanic events allow for the recognition that harsh living conditions likely followed the eruption. From this it’s logical to infer even a small contraction in population size. Current genetic research indicating recent population expansions, as well as reduced genetic diversity throughout the human race further support the notion.