Genetic Drift between two
generations in the Wright - Fisher
Model
Starting in generation t with 20
alleles for N =
20 haploid individuals, with f(A) = 8/20 =
0.40. Generation t+1
comprises the same number of alleles & individuals (20),
but a modified f(A)
= 5/20 = 0.25, due solely to random sampling
error, which Sewall Wright called genetic
drift.
The process of random sampling can be
visualized as a draw-&-replacement
exercise. Place 8 red and
12 blue marbles in a bag.
Draw one at random: note its color, and return the marble to
the bag. Repeat the process a total of 20 times. Create a new
bag representing the next generation, containing 20 marbles in
the proportions obtained by the random draw. Repeat.
The Wright - Fisher Model considers
haploid individuals, but can be adjusted for diploid
populations. Here, the 20 haploid individuals can
be treated as ten diploid individuals
with two alleles each. This is equivalent to the 'tide
pool ' model used to derive the Hardy-Weinberg
Theorem for random union of gametes. As a diploid model,
it is slightly unrealistic in that it ignores two-allele
mating combinations, and allows self-fertilization (if
the same marble were drawn twice in a row).