Starting in **generation ***t* with **20
alleles** for **N **=**
20 haploid individuals**, with **f(****A****) **=** 8/20 **=**
0.40**. **G****eneration ***t+1*
comprises the same number of alleles & individuals (**20**),
but a modified **f(****A****)**
= **5/20 **=** 0.25**, due solely to **random samp****ling
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).

The process of random sampling can be visualized as a

The

Figure after © 2013
by Sinauer; Text material © 2021 by Steven M. Carr