Genetic Drift Notes
Genetic Drift: change in the occurrence of a ____________________ due to ________________ chance.
• Genetic drift _______________ the ______________________ in the population.
Remember: Natural selection is _______ random (it is mutations that are random). Natural selects for
_______________ phenotypes.
• So, the changes we see from genetic drift are ______ a result of natural selection, which means
they aren’t necessarily ________________ for the environment.
Types of Genetic Drift
1. Founder Effect
• A founder effect occurs when a _______ colony is started by a ______ members of the
original population. This _____________ population size means that the colony may have:
o ____________________ genetic variation from the original population.
 This can lead to a high occurrence of certain traits, even unfavorable ones.
• Example: A few birds are relocated to a new island and allowed to reproduce and populate the
island.
The top and dish and the bottom dish have no genetic
variation.
2. Bottlenecking
• Population bottlenecks occur when a population’s size is ______________. Undergoing a
bottleneck can ______________ a population’s genetic variation by a lot, even if the
bottleneck doesn’t last for very many generations.
Example – Northern Elephant Seals:
Northern elephant seals have reduced genetic variation because of a
population bottleneck humans inflicted on them in the 1890s. Hunting
reduced their population size to as few as 20 individuals at the end of
the 19th century. Their population has since rebounded to over
30,000—but their genes still carry the marks of this bottleneck: they
have much less genetic variation than a population of southern
elephant seals that was not so intensely hunted.
Example – Pingelap:
Pingelap is an island in the Pacific Ocean. In 1775, a typhoon swept through the island,
killing 90% of the inhabitants and leaving only approximately 20 people. One of the
survivors had the recessive genetic disorder achromatopsia, which results in complete
colorblindness. In normal human populations, the occurrence of achromatopsia is
0.003%. Six generations after the typhoon, the occurrence of achromatopsia was 4.92%.
The increase in the frequency of this trait is due to genetic bottlenecking.
Genetic Drift Activity
Each student will be given a bead. This bead represents your phenotype. Ms. Avery will use a random
number generator to randomly select several individuals to be the starting new populations. This will
mimic the randomness of genetic activity.
Starting Mainland Population:
Number of
Color
Individuals
Founding Island Population:
Number of
Color
Individuals
Blue
Blue
Red
Red
Green
Green
Yellow
Yellow
A disease is brought to the island. The disease affects the ____________ phenotype. Any individual
infected with this disease dies within a week.
1. What happens to the island population after infection with the disease?
2. What happens to the mainland population after the infection with the disease?
3. How does genetic variation differ between the two populations? How does this help with the survival
of the species after the infection?