Bottleneck Effect
• The Bottleneck Effect occurs when there is a disaster of some sort that reduces a population to a small handful, which rarely represents the actual genetic makeup of the initial population. This leaves smaller variation among the surviving individuals.
Genetic bottlenecks are defined by the textbook as "events in which the number of individuals in a population is reduced drastically." Essentially, this is genetic drift at it's finest. It occurs when the true frequency of alleles of a certain population, is not accurately presented in the new population. There is no other way to avoid a bottleneck effect because having a certain phenotype or genotype does not matter. This can have an extreme effect on the genetic diversity of the population. To illustrate this point: if a large population, say of a bacteria, were to be growing on a surface and then one was to clean it with Lysol or the like, we would expect that not all the bacteria in the population would be killed and thus survive to pass on their genes. This dramatic population decrease would effectively cut out most of the genetic variation that existed within the population pre-Lysol attack. Now, on the same surface, we can expect that the remaining alleles frequencies have shifted dramatically, and all the forthcoming generations will be limited to this gene pool (barring the introduction of new genes to the population). This, effectively, can spread a trait like resistance through the entirety of a population in a few generations. And we know that bacteria run through generations quickly, leading to issues like MRSA.
An example of a bottleneck effect: Northern elephant seals have reduced genetic variation most likely due to being hunted. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Since then their population has rebounded to over 30,000 but the genes still carry the marks of their bottleneck. They have much less variation than a population of Southern Elephant seals that have not been hunted.
>Generation 2 an unusually small population creates a "bottleneck".
One of the most famous examples of bottleneck occurred in 1789, when the mutineers of a British ship set the captain and the crew adrift. Twenty seven adults and one baby arrived at island called Pitcairn and their descendants lived on that island for decades. This event caused a drastic reduction in the population size which decreased the genetic variation in the population (Evolution: Making Sense of Life).
• It is believed that there was a human bottleneck ~70,000 years ago, caused by the Toba catastrophe.
• The entire human population was likely reduced to 3,000-10,000 survivors in a small part of Africa.
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