Notice how quickly and drastically the marble ratio changed: 1:1, 1:3, 0:1. Imagine that your bag is only big enough for 20 marbles (a tiny bag!) and that you can only draw four marbles to represent gene frequencies in the next generation. The marble-drawing scenario also illustrates why drift affects small populations more. Selection can only act on what variation is already in a population it cannot create variation. Selection cannot increase the frequency of the green gene, because it’s not there for selection to act on. If the green gene drifts out of the population, and the population ends up in a situation where it would be advantageous to be green, the population is out of luck. And with less genetic variation, there is less for natural selection to work with. The 10:0 situation illustrates one of the most important effects of genetic drift: it reduces the amount of genetic variation in a population. If the gene for green coloration drifts out of the population, the gene is gone for good - unless, of course, a mutation or gene flow reintroduces the green gene. The same thing can happen to populations. The cartoon below illustrates this process, beginning with the fourth draw. Imagine that our random draws from the marble bag produced the following pattern: 5:5, 6:4, 7:3, 4:6, 8:2, 10:0, 10:0, 10:0, 10:0, 10:0… Why did we keep drawing 10:0? Because if the green marbles fail to be represented in just one draw, we can’t get them back - we are “stuck” with only brown marbles. Through sampling error, genetic drift can cause populations to lose genetic variation. Information on controversies in the public arena relating to evolution. Alignment with the Next Generation Science Standards.The big issues – Pacing, diversity, complexity, and trends.Macroevolution – Evolution above the species level.
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