Genetic drift occurs when random chance alone causes changes in allele frequencies in a small population without selective pressures. A catastrophe reduced a beetle population to 25% green and 75% brown alleles randomly. If the last few green beetles fail to pass on the green allele, it may be lost forever through genetic drift. Genetic drift can reduce genetic diversity and occurs through genetic bottlenecks, where a population is drastically reduced, or the founder effect, where a small group from a population establishes a new population with low genetic variation.

1. Evolution Without Selection

2. Evolution Without Selection
Genetic Drift: Occurs when a population is so small
that allele frequencies may be altered by random
chance with no selective pressures.
Imagine a beetle population
where 40% are green and 60%
are brown

3. Evolution Without Selection
A random catastrophe has reduced the beetle
population's size.
Now 25% are green and 75% are brown.

4. Evolution Without Selection
If the last few green beetles fail to pass on their
green allele, the allele may be lost forever.

5. Genetic Drift
A certain allele may become extremely common or
disappear entirely.
(by chance – recall Punnett squares and
probabilities of passing down traits)

6. Genetic Drift
This results in reduced genetic diversity in the
population.
The low
worldwide
population of
white-collared
lemurs has
reduced their
genetic diversity.

7. Genetic Drift
There are 2 major reasons for population reduction
leading to genetic drift:
1) Genetic Bottleneck
2) Founder Effect

8. Think. Pair. Share.
Under what circumstances would a small group of
organisms become the foundation of a new
population?
Where did this small group of organisms come
from? Consider the terms below.
1) Genetic Bottleneck
2) Founder Effect

9. Genetic Bottleneck
1) A genetic bottleneck occurs when a species'
population is drastically reduced.

10. Genetic Bottleneck
The survivors must reestablish the population using
a very limited gene pool.

11. Genetic Bottleneck
The members of this new population exhibit little
genetic variation.
This is usually what happens in many endangered species such as the Iberian lynx.

12. The Founder Effect
2) The founder effect occurs when small group from
a population moves to a new location.

13. The Founder Effect
That small group will reproduce to populate the new
location. Genetic variation will be low.

14. The Founder Effect
Original
population 3 random animals with
this combination of alleles
wanders away
Alleles of original
population

15. The Founder Effect
Animals that
moved create a
new population in
a new location