Through sampling error, genetic drift can cause populations to lose genetic variation. 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. The cartoon below illustrates this process, beginning with the fourth draw.
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