Editing Genetic drift (section)

==Analogy with marbles in a jar==

The process of genetic drift can be illustrated using 20 marbles in a jar to represent 20 organisms in a population.<ref>{{cite web |url=http://evolution.berkeley.edu/evolibrary/article/samplingerror_01 |title=Sampling Error and Evolution |website=Understanding Evolution |publisher=[[University of California, Berkeley]] |access-date=2015-12-01 |url-status=live |archive-url=https://web.archive.org/web/20151208125724/http://evolution.berkeley.edu/evolibrary/article/samplingerror_01 |archive-date=8 December 2015}}</ref> 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&nbsp;– 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 (population genetics)|fixation]] can occur in just a few generations.

[[File:Random sampling genetic drift.svg|thumb|center|550px|In this simulation, each black dot on a marble signifies that it has been chosen for copying (reproduction) one time. [[Fixation (population genetics)|Fixation]] in the blue "allele" occurs within five generations.]]