What is a population bottleneck and how does it affect genetic diversity and why do small populations lose genetic variability?
Dear Rk Naresh,
A bottleneck is a situation in which only a few individuals in a population survive a event or a part of the population is isolated. That's result in a shift in the population's allele frequencies. Small populations lose variability due to genetic drift (founder effect. The genetic variability of the new population is not representative of the original population.
Antonio
A population bottleneck arises when a significant number of individuals in a population die or are otherwise prevented from breeding, resulting in a drastic decrease in the size of the population. Genetic drift can result in the loss of rare alleles, and can decrease the size of the gene pool. Genetic bottlenecks are stochastic events that limit genetic variation in a population and result in founding populations that can lead to genetic drift. Following a population bottleneck, the remaining population faces a higher level of genetic drift, which describes random fluctuations in the presence of alleles in a population. In small populations, infrequently occurring alleles face a greater chance of being lost, which can further decrease the gene pool. A bottleneck implies a tightening of constraints on flow. In the case of the biodiversity bottleneck, flow refers to the survival of species through time. As the future unfolds and the techno sphere continues to grow, the possibilities for species to pass through the biodiversity bottleneck diminish. The bottleneck effect occurs when a population's size is reduced for at least one generation. Undergoing a bottleneck can greatly reduce the genetic variation in a population, leaving it more susceptible to extinction if it is unable to adapt to climactic changes or changes in resource availability.The bottleneck effect is an extreme example of genetic drift that happens when the size of a population is severely reduced. Events like natural disasters (earthquakes, floods, fires) can decimate a population, killing most individuals and leaving behind a small, random assortment of survivors. Even if endangered populations recover, severe bottlenecks may reduce genetic diversity and increase inbreeding as survivors are forced to mate with close relatives, resulting in lowered heterozygosity, increased genetic load, and increased expression of deleterious alleles. Inbreeding, genetic drift, restricted gene flow, and small population size all contribute to a reduction in genetic diversity. Fragmented and threatened populations are typically exposed to these conditions, which is likely to increase their risk of extinction. Drift is more pronounced in such populations, because smaller populations have less variation and, therefore, a lower ability to respond favorably that is, adapt to changing conditions. Small populations are at risk of losing genetic variation much faster than large populations. Species with high genetic diversity are generally more able to adapt to and reproduce under new conditions such as those brought by environmental changes. Simply adding new individuals to a very small population increases genetic diversity, and can immediately increase both the fitness of the population and it's capacity to adapt to changing conditions. Genetic drift occurs more frequently in smaller populations because there is less of a balance on allele frequency in smaller populations. When a population is smaller, the influx or efflux of individuals from that population has a larger affect on the allele frequency than in a larger population.
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