Генетички дрифт

Генетички дрифт (allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene variant (allele) in a population due to random sampling of organisms. The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces. A population's allele frequency is the fraction of the copies of one gene that share a particular form.

Ако се од једне велике популације издвоји мања група јединки и оформи нову популацију, она не мора бити иста већ се чак може веома разликовати од матичне популације. У тако малобројној популацији су ефекти генетичког дрифта најизраженији. Илустративан пример за хумане популације је учесталост крвних група код Индијанаца у Северној Америци. Већина тих Индијанаца има крвну групу О, али једна изолована група услед генетичког дрифта има најучесталију крвну групу А. Ова учесталост је већа не само у поређењу са осталим северноамеричким Индијанцима него је већа и од учесталости у било којој хуманој популацији.

Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation. It can also cause initially rare alleles to become much more frequent and even fixed.

When few copies of an allele exist, the effect of genetic drift is larger, and when many copies exist, the effect is smaller. In the middle of the 20th century, vigorous debates occurred over the relative importance of natural selection versus neutral processes, including genetic drift. Ronald Fisher, who explained natural selection using Mendelian genetics, held the view that genetic drift plays at the most a minor role in evolution, and this remained the dominant view for several decades. In 1968, population geneticist Motoo Kimura rekindled the debate with his neutral theory of molecular evolution, which claims that most instances where a genetic change spreads across a population (although not necessarily changes in phenotypes) are caused by genetic drift acting on neutral mutations. In the 1990s, constructive neutral evolution was proposed which seeks to explain how complex systems emerge through neutral transitions.

Аналогија са кликерима у тегли

The process of genetic drift can be illustrated using 20 marbles in a jar to represent 20 organisms in a population. Consider this jar of marbles as the starting population. Half of the marbles in the jar are red and half are blue, with each colour corresponding to a different allele of one gene in the population. In each new generation, the organisms reproduce at random. To represent this reproduction, randomly select a marble from the original jar and deposit a new marble with the same colour into a new jar. This is the "offspring" of the original marble, meaning that the original marble remains in its jar. Repeat this process until 20 new marbles are in the second jar. The second jar will now contain 20 "offspring", or marbles of various colours. Unless the second jar contains exactly 10 red marbles and 10 blue marbles, a random shift has occurred in the allele frequencies.

If this process is repeated a number of times, the numbers of red and blue marbles picked each generation fluctuates. Sometimes, a jar has more red marbles than its "parent" jar and sometimes more blue. This fluctuation is analogous to genetic drift – a change in the population's allele frequency resulting from a random variation in the distribution of alleles from one generation to the next.

In any one generation, no marbles of a particular colour could be chosen, meaning they have no offspring. In this example, if no red marbles are selected, the jar representing the new generation contains only blue offspring. If this happens, the red allele has been lost permanently in the population, while the remaining blue allele has become fixed: ; all future generations are entirely blue. In small populations, fixation can occur in just a few generations.

In this simulation, each black dot on a marble signifies that it has been chosen for copying (reproduction) one time. fixation in the blue "allele" occurs within five generations.

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Спољашње везе

• Sheehy, Bob. „Population genetics simulation program”. Radford, VA: Radford University. Приступљено 2015-12-21. 
• Grimes, Bill. „Genetic Drift Simulation”. Tucson, Arizona: The University of Arizona. Приступљено 2016-08-25.