23.4 genetic drift
Ð Genetic drift is defined as a change in allele
frequencies in a population that is due to chance.
Paragraph 2
Ð Sampling error occurs when the allele frequencies
of a chosen subset of a population (the sample) are
different from those in the total population, by
chance (see BioSkills 3).
Simulation studies of genetic drift
Ð The following coin flips were done by a pair of
students in a recent biology class:
Computer simulation
Ð Figure 23.13 shows what happens when a
computer simulates the same process of random
combinations in gametes over time.
Ð The program that generated the graphs combines
the alleles in a gene pool at random to create an
off-spring generation, calculates the allele
frequencies in the off- spring, and uses those
frequencies to create a new gene pool.
Ð The graph in Figure 23.13a shows eight replicates
of this process with a population size of 4; the
graph in Figure 23.13b shows eight replicates with a
population of 400.
Ð Notice the striking differences between the effects
, of drift in the small versus large population and the
consequences for genetic variation when alleles
drift to fixation (frequency of 1.0) or loss (frequency
of 0.0).
Ð Although drift is more dramatic in small
populations, it still occurs in large populations and
can be an important factor in some cases.
Ð If you understand genetic drift, you should be able
to examine the MN blood group data in Table 23.1
on page 473 and describe how drift could explain
differences in genotype frequencies among
populations.
Key Points about Genetic Drift The data from the
simulations illustrate three important points about genetic
drift:
1. Genetic drift is random with respect to fitness. The
changes in allele frequency that it produces are not
adaptive.
2. Genetic drift is most pronounced in small populations.
In the computer simulation, allele frequencies changed
much less in the large population than in the small
population. And if the couple on the deserted island had
hypothetically produced 50 children instead of five, allele
frequencies in the next generation almost certainly would
have been much closer to 0.5.
3. Over time, genetic drift can lead to the random loss or
fixation of alleles. In the computer simulation with a
population of 4, it took at most 20 generations for one
allele to be fixed or lost. When random loss or fixation
occurs, genetic variation in the population declines.
Ð Genetic drift is defined as a change in allele
frequencies in a population that is due to chance.
Paragraph 2
Ð Sampling error occurs when the allele frequencies
of a chosen subset of a population (the sample) are
different from those in the total population, by
chance (see BioSkills 3).
Simulation studies of genetic drift
Ð The following coin flips were done by a pair of
students in a recent biology class:
Computer simulation
Ð Figure 23.13 shows what happens when a
computer simulates the same process of random
combinations in gametes over time.
Ð The program that generated the graphs combines
the alleles in a gene pool at random to create an
off-spring generation, calculates the allele
frequencies in the off- spring, and uses those
frequencies to create a new gene pool.
Ð The graph in Figure 23.13a shows eight replicates
of this process with a population size of 4; the
graph in Figure 23.13b shows eight replicates with a
population of 400.
Ð Notice the striking differences between the effects
, of drift in the small versus large population and the
consequences for genetic variation when alleles
drift to fixation (frequency of 1.0) or loss (frequency
of 0.0).
Ð Although drift is more dramatic in small
populations, it still occurs in large populations and
can be an important factor in some cases.
Ð If you understand genetic drift, you should be able
to examine the MN blood group data in Table 23.1
on page 473 and describe how drift could explain
differences in genotype frequencies among
populations.
Key Points about Genetic Drift The data from the
simulations illustrate three important points about genetic
drift:
1. Genetic drift is random with respect to fitness. The
changes in allele frequency that it produces are not
adaptive.
2. Genetic drift is most pronounced in small populations.
In the computer simulation, allele frequencies changed
much less in the large population than in the small
population. And if the couple on the deserted island had
hypothetically produced 50 children instead of five, allele
frequencies in the next generation almost certainly would
have been much closer to 0.5.
3. Over time, genetic drift can lead to the random loss or
fixation of alleles. In the computer simulation with a
population of 4, it took at most 20 generations for one
allele to be fixed or lost. When random loss or fixation
occurs, genetic variation in the population declines.
