Evolution by Genetic Drift : Main Points (p. 231)1. 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 itsfrequency 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.

In populations of finite size, sampling of gametesfrom the gene pool can cause evolution

Incorporating Genetic Drift

Probability of Maintaining the Same Initial Allele Frequency

The Ultimate Fate of Random Genetic Drift

The Effects of Drift are More Pronounced in Smaller Populations

8 pops

The frequencyof heterozygotesdecreases underdrift.

Hg+1 = Hg[1-1/2N]

Ne = 4NmNf / (Nm + Nf)

N=16

Effective population size

N=16N=9107 pops

Rate of Evolution by Genetic Drift

Rate of evolutionequals rate that an allele is fixedat a locus.

K = 2N x 1/2N =

Depends upon: (2Nu) number of mutations arising at locus per generation, and initial frequency of new allele (1/2N)

rate of substitution = rate of mutation!

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.

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

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.