Effects of genetic drift
Through sampling error, genetic drift can cause populations to lose genetic
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.
The same thing can happen to populations. 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 10:0 situation illustrates one of the most important effects of genetic drift: it reduces
the amount of genetic variation in a population. And with less genetic variation, there is less
for natural selection to work with. 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.
Selection cannot increase the frequency of the green gene, because it's not there for selection to
act on. Selection can only act on what variation is already in a population; it cannot create variation.