1. Evolution by Genetic Drift : Main Points (p. 231)
Allele frequencies fluctuate at random within a population;
eventually, one or another allele becomes fixed.
2. Genetic variation at a locus declines and is eventually lost;
the rate of decline in heterozygosity is used to estimate the
strength of drift: frequency of heterozygotes (H) = 2p(1-p).
3. At any time, the probability of allele fixation ~equals its
frequency at that time.
4. Evolution by genetic drift proceeds faster in smaller
populations; the average time to fixation is 4N.
5. Populations with the same initial allele frequency diverge; the
same or different allele maybe fixed but the average allele
frequency remains the same. The frequency of heterozygotes
declines.

2. In populations of finite size, sampling of gametes
Incorporating Genetic Drift
In populations of finite size, sampling of gametes
from the gene pool can cause evolution

4. Probability of Maintaining the
Same Initial Allele Frequency

5. The Ultimate Fate of Random Genetic Drift

6. The Effects of Drift are More Pronounced in Smaller Populations

7. 8 pops
The frequency
of heterozygotes
decreases under
drift.
Hg+1 = Hg[1-1/2N]

8. Effective population size
107 pops
N=9
N=16
Ne = 4NmNf / (Nm + Nf)
Effective population size

9. Rate of Evolution by Genetic Drift
equals rate that
an allele is fixed
at a locus.
Depends upon: (2Nu) number of mutations arising at locus
per generation, and initial frequency of new allele (1/2N)
K = 2Nm x 1/2N = m
rate of substitution = rate of mutation!

10. Neutralist view: allele substitution and polymorphism
are determined by the same evolutionary process.
Mutation provides a continual supply of new alleles.
Because many alleles are neutral
or effectively neutral, alleles
becomes fixed or lost from a
population as a result of
genetic drift.
Polymorphism is simply a snapshot of a continuous
process of mutational input and subsequent random
extinction or fixation of alleles.

11. Mootoo Kimura’s concept of neutralism is illustrated in the following diagram from his original paper.

12. Selectionist view: allele substitution and polymorphism
are determined by different, selective processes.
Mutation yields advantageous
alleles that are driven to fixation by
positive natural selection.
Two or more alleles are maintained
at a locus in a population by
over dominance.