BB3140 B09
Genetic drift Chapter 10
These notes are based on a lecture given by Professor Lauren Mathews.
I. Random genetic drift
this is potentially an extremely important evolutionary
force.
Many evolutionary biologists feel that genetic drift plays a very large role
in generating biological diversity
Definition of Genetic Drift
: whenever there are two or more alleles for a gene
present in a finite population, the relative frequencies of the alleles change from
generation to generation in accordance with random sampling.
> HardyWeinberg (Chapter 9) suggests that there will be no change in
allele frequencies through the generations.
>BUT, even in the absence of selection, these alleles are subject to
random sampling error that can alter allele frequencies over time
II. How does genetic drift happen?
a. The importance of genetic drift in evolution is dependent on population size, or
the size of the gamete pool (the size of the sample that is drawn from the total
pool of alleles). Why is this?
(This section is covered in Chapter 10 pp. 256257)
Example:
finite population with 2 alleles, A1 and A2. p=0.5, q=0.5
Question:
What is the chance that a random sample of 4 gametes from a
pool that has p=0.5 are all A1? = (chance of each allele being the same) =
(½ x ½ x ½ x ½ ) x 2=
1/16
Question:
What is the chance that a random sample of 8 gametes from a
pool that has p=0.5 are all A1?
= (½ x ½ x ½ x ½ x ½ x ½ x ½ x ½) = 1/256
Another way to express this idea is to observe that in general, probability
theory says that
variance
in the sampling procedure is greater the
smaller the number of trials in the sample
 so the smaller the number
of genes sampled, the greater the chance that one allele will be
overrepresented by chance.
The textbook covers this idea in two different
ways:
coalescence and random walk.
See Figs. 10.1, 10.2, and 10.3 and
text, pp. 257260)
This means that changes in allele frequencies driven by genetic drift
are more likely to be an important (perhaps the most important)
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View Full Documentevolutionary force in small populations. Though genetic drift also
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 Spring '10
 politz
 Evolution, Population Genetics, Professor Lauren Mathews

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