- 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.
is a good article explaining how the founder effects affect genetic drift.
An example of a bottleneck effect
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 if 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.
The Founder Effect is defined as a type of bottleneck, "a type of genetic drift describing the loss of the allelic variation that accompanies founding of a new population from a very small number of individuals (a small sample of a much larger source population). This effect can cause the new population to differ considerably from the source population." Examples of this showing up in human populations is polydactyly among Amish communities or the Blue people of Kentucky. Unlike bottlenecks, which persist as long as the effective population size remains low, a founder effect is an instant event. Here is a link to a website that delves into the genomic heritage of the people of Quebec. Quebec is a community whose genomic heritage is a result of founder effects.
- The founder effect occurs when there is a lack of genetic variation due to a small mating population. The Founder Effect happens when there is a dramatic decrease in genetic diversity caused by the development of small colonies of individuals, from the original population, that remain isolated to other colonies.
- Only a small subset of the genetic diversity of the source population is likely to be included in the new population, and the relative frequencies of these alleles may be very different from what they had been before (book, 165).
- An example of the founder effect comes from the Afrikaner population of Dutch settlers in South Africa. This population has an abnormally high count of Huntington's Disease (nerve cells in brain degenerate) because the original Dutch settlers carried the gene with a higher frequency that the rest of the Dutch population.
- Huntington's Disease also occurs in unusually high frequency near Lake Maraciabo, Venezuela. After much research, scientists were not only able to discover the mutation that causes the devastating disease, but also trace it back to the founder herself. 200 years ago, a women had 10 children all of whom stayed in the area to raise their own children. Unfortunately, Huntington's is a dominant allele meaning that if one parent carries it, there is a 50:50 chance it will be passed to the offspring. The disease has been able to persist for so long because natural selection no longer acts after reproduction, which is when Huntington's begins to take its effects.
- Ichthyosis in golden retrievers affects 50% of the population in Europe and 30% in America. It's an autosomal recessive disorder that causes the skin to look scalely, hence the name. Geneticists, who are searching for answers to human forms of ichthyosis, found that the disorder can be traced back to one dog. By the late 1990s/early 2000s, ichthyosis was recognized as a separate disorder from regular flaking skin. While the effects, are usually mild, it is still a major concern for breeders who are now testing their dams and sires for the ichthyosis mutation prior to breeding. Caution is being exercised, however, to not eliminate mates from the gene pool too soon because of one disorder. Doing so would result in a severe bottleneck of the golden retriever population, particularly in Europe.
Both bottlenecks and founder effects provide perfect opportunities for genetic drift to take over, as they both involve small populations. After a few generations, the population may look very different from the original population. In rare cases, a bottleneck may even lead to speciation.
has a great comparison between the Founder's effect and Genetic Drift.
is an article relating Genetic Drift and Effective population size. This article explores a new formula that might estimate the effective population size. The accuracy of the equation depends on the number of loci or alleles, number of generations involves, and ratio of sample size to effective size.
There are essentially three main theories for the african origin of human beings and their subsequent dispersal. 1. Replacement 2. Weak Garden of Eden & 3. Multiple Dispersals. Throughout our history as a species we have been subject to numerous bottle necks.
- Bottleneck effects isolates genes
- Specifies the founder effect when people left Africa
there's this really cool video on bottlenecks and founder effect here
gives another outlook on how bottlenecks effect the drift.