Question

Difference between genetic drift and natural selection

Answer 1

Explanation:

Genetic drift is the variation in population allele frequency due to random sampling errors over generations. This is more easily seen in small populations.

Natural selection changes allele frequency by favoring those alleles that give the organism a reproductive fitness advantage.

Let me give you an example. For simplicity, I'm ignoring diploidy and dominance-recessive behavior of genes. :

Consider a hypothetical insect with two alleles responsible for its color : Y(for yellow) and G(green).

We start the population with both alleles at 50%.

Case A:

Let's say they live on leaves that are green in color and these are preyed on by birds who operate by sight. Those insects that are yellow (Y) in color become more conspicuous as compared to the green colored ones. Hence, they get eaten and eventually we are only left with the G insects. This is natural selection, since the G allele provided a fitness advantage.

Case B:

Let's say the insects come out only at night and are preyed on by bats who operate by sound.

In this case, the allele does not offer any fitness advantage.

In this case, normally the allele frequency of G randomly fluctuates about 50%.

However, just like one can get 10 consecutive Heads in coin tosses with an unbiased coin (although the probability is low), just by chance, it is possible that all the G insects outperformed the Y insects in breeding, and the remaining few Y insects died out and finally we are left only with G insects. This is genetic drift.

Note that genetic drift is not possible for infinite populations.