[b_sharp]

"Genetic drift is not an "effect", it's a result. Nor is it something that can be coherently described as happening "at the same time". It's what happens when differential reproductive sampling occurs across generations.

It can be reduced to simple mathematical models based on population size. If I remember correctly.

[mlc9852]

There are two simple equations for the frequency of two alleles of a given gene (see Hardy-Weinberg principle):

Equation 1:p2 + 2pq + q2 = 1

Equation 2: p + q = 1

Where p is the frequency of one allele and q is the frequency of the other allele. p2 is the population fraction that is homozygous for the p allele, pq is the frequency of heterozygotes and q2 is the population fraction that is homozygous for the q allele. Natural selection can act on p and q in Equation 1, and obviously affect the frequency of genes seen in Equation 2. It should be noted that the second equation can be derived from the first (or vice versa) since p2 + 2pq + q2 = 1 implies (p + q)2 = 1 and p and q are positive numbers.