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 ...
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... 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.
Rantala, M. J., and Roff, D. A. 2006. Analysis of the importance of genotypic variation,
The literature does provide evidence for my hypothesis and also provides a clearer picture as to how frequent and to what extent the interbreeding is believed to occur. Examining these articles will introduce the new findin...
“ignored population growth,” since writing their book in 1968, is one reason “collapse now seem[s] ever more likely” (63). They suggest we can avoid collapse if we reduce our population to an “optimal number” and they reinforce the message of their book, earth has a finite capacity and a growing ...
The main purpose of this lab was simply to study two factors of Natural Selection: Genetic Drift and Fertility. The first of these factors in the study was Genetic Drift. In this section of the lab, there was one main question to answer: What effect does population size has on random mechanisms? For this question, I hypothesized that if the population is smaller, then the random mechanisms will have a greater effect on the populations. I believed that this was because basic math principles would allude to the idea that any bad random mechanism to a small population would harm a greater percent than a larger population. With a greater portion of their population gone, then the amount of turtles reproducing would lessen significantly.
[ 8.1 ] Will the allele frequencies in the pens tend to move toward the frequency in the larger field population?
There are few factors that can alter the proportion of homozygotes and heterozygotes in a population: genetic drift, gene flow, mutation, and natural selection. Genetic drift is the change in the gene pool that occurs in a small population due to chance. For example, let's say we are talking about a species of fish called Drew fish, well the Drew fish population was hit by an oil spill and many of them died. The remaining survivors form new colonies, and this results in genetic drift, the founder effect. Gene flow is about the movement of alleles. Well let's say a Prow Fish began to mate with the Drew fish this will bring about some gene flow because these Prow Fish are just the same species as Drew fish but a different population with different alleles, so they have yellow fins as oppose to blue.
“No time has been found at which every genetic locus resides in a single African population.” (Wolpoff et al. 2000)
Population bottleneck is an evolutionary event in which a large part of a population or species is died or otherwise cannot reproduce, which increases genetic drift that can cause big losses of genetic variation for small populations. However, the amount of drift is inversely proportional to the population size, population bottleneck result a smaller population with reduced genetic diversity. Dropped genetic variation implies that the population will most likely be unable to adjust to new selection pressures, such as a shift in available resources (food, shelter, etc.) or climatic change, because the genetic variation that selection would act on may have already drifted out of the population. This genetic drift can affect the proportionate distribution of an allele and even cause to fixation or damage of alleles. Also, population bottleneck increases inbreeding and genetic homogeneity as there are some animals left in the gene pool which lead to unfavorable alleles can accumulate. Due to the smaller population size after a bottleneck event. population bottlenecks inform us that intraspecies diversity is similarly very vital
Lewis, Ricki, (2014), Human Genetics, 11th Edition, Chapter 15 Changing Allele Frequencies, pp 293. [VitalSource Bookshelf Online]. Retrieved from
There are three main factors that fuel rapid population growth in the world. Firstly, the increased production and distribution of food has now enabled more people to live longer lives. Initially, starvation caused high death rates, therefore inhibiting population growth. In our modern society, there is an abundance of food to feed the population. Improved distribution channels have enabled food to reach into dry places, thereby eliminating the risk of starvation. Secondly, there is an overall improvement of public health care. Poor sanitation and lack of clean water were the main causes of deadly diseases like malaria. These diseases often wiped
The data obtained confirms that there is a significant difference in expected and obtained allele frequencies, which results in the conclusion that the populations of fruit flies disrespect the Hardy-Weinberg equilibrium because of the sexual selection and/or genetic drift genetic factors. The system is closed; meaning that there is no migration, nor gene flow and natural selection, due to how no offspring die off from a lack of reproductive fitness. Moreover, mutation is not a disruptive force here, since it seems that there is no harsh environmental conditions that would bring about such an effect. Under small populations, the visible impacts of genetic drift can be observed. The small population size of the fruit flies reflects how random chance of genetic drift caused the significant difference conclusion of our
Raven, Peter H., George B. Johnson, Kenneth A. Mason, Jonathan B. Losos, and Susan R. Singer. "Quantifying Natural Selection." Biology. Tenth Ed. New York: McGraw-Hill Companies, 2013. Print. 8 Apr. 2014.
...roach by combing all three of these mechanisms might be required to fully balance the two-fold cost of sex. (West, Lively, Read) The mutation accumulation theory requires mutation rates to be high, (Kondrashov, 1993 Deleterious mutations and the evolution of sexual reproduction), each deleterious mutation will lead to a decrease in log fitness then the previous one, and population sizes have to be large for it to work properly. Even though some models may not be able to fully explain the two-fold cost of sex, it just might play an important role. A pluralistic approach helps “shift the emphasis of empirical work away from the search for discriminating prediction to parameter estimation”. This approach also “emphasizes environmental and mutational mechanisms interact synergistically in a number of ways and outweighs each other’s weaknesses”. (West, Livley, Read)
... as the tropics, and there is evidence that higher species diversity reduces the risk of infectious diseases in humans. For these reasons, it is critical that genetic diversity is preserved.
AGenetic Drift is the variation in a population’s allele frequencies from one generation to the next as a result of chance events. Genetic Drift may cause some genes to disappear, and overall reducing the genetic variation in a certain population. There are two types of Genetic Drift: Bottleneck Effect and Founder Effect . An example of Genetic Drift would be the American Bison, which suffered a huge reduction in population numbers, after succumbing to the bottleneck effect . Due to the quick killings of the Bison, many alleles died with their carriers, and genetic variation decreased exponentially. The American Bison has been gaining numbers in the past couple decades but the genetic variation amongst the different animals is very small. Another example of Genetic Drift would be that of the Northern elephant seals. Also being the target of hunters, the N.Elephant seals population reduced to a shocking 20 individuals at the end of the 19th century . Even though their population is steadily increasing, their genes still carry the effects of the bottleneck. They N. Elephant