1. LECTURE 7: Microevolution Part 5 Genetic Drift

2. What is Microevolution?
Evolution on a small scale
Change in allele frequencies from one generation to the next
A process that leads to a change in a species
What is a gene pool?
All the alleles that reside in a population
Microevolution is a change in the gene pool

3. What is Microevolution?
Natural Selection
- adaptation = promotes fitness
Genetic Drift
Change in allele frequencies in a small population due to random events
Gene Flow
migration
Mutation
Change in genetic structure

4. What is Genetic Drift?
Allele frequencies do not remain exactly the same from generation to generation, especially in very small populations.
A process called genetic drift - random changes in allele frequencies - occurs in all populations.

5. Figure 23.7 Genetic drift Genetic drift chance allele frequency change
CRCR
CRCW
CWCW
Only 5 of
10 plants
leave
offspring
Only 2 of
Generation 2
p = 0.5
q = 0.5
Generation 3
p = 1.0
q = 0.0
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3

6. Genetic drift
chance
allele frequency change
In any population, but especially small populations, allele frequencies change over time by chance.
An allele frequency graph

7. Genetic drift
chance
allele frequency change 1
0.8
If you follow allele frequency change in several small populations, you get a different result each time. One possible outcome is allele “fixation” - loss of one allele.
0.6
0.4
0.2 1 2 3 4 5 6 7 8 9
An allele frequency graph
generations

8. A real experiment with the b locus in flour beetles.
Genetic drift
chance
allele frequency change 1
0.9
0.8
N = 10 beetles in population
0.7
0.6
red
0.5
freqency b+ allele
0.4
b+ b+
0.3
0.2
0.1
A real experiment with the b locus in flour beetles. 2 4 6 8 10 12 14 16 18 20
generations (time)
brown 1
0.9
b+ b
0.8
0.7
0.6
0.5
frequency b+ allele
0.4
0.3
N = 50 beetles in population
black
0.2
0.1
b b 2 4 6 8 10 12 14 16 18 20
generations (time)
Note effect of population size

9. Genetic drift can be shown in the lab, using populations of different sizes and initial 50:50 frequencies of two “neutral” alleles.
1.0
0.5
allele A neither
lost nor fixed 1 5 10 15 20 25 30 35 40 45 50
Generation (500 flies at the start of each line or population)

10. In small populations one or other of the alleles is inevitably lost.
1.0
AA in five populations
0.5
allele A lost
from four
populations 1 5 10 15 20 25 30 35 40 45 50
Generation (25 flies at the start of each line or population)

11. How does Genetic Drift Happen?
2 examples:
Bottleneck Effect
Founders Effect

12. What is Bottleneck Effect?
Natural Disasters may cause death of a large amount of population
Therefore, only some alleles may be present, some may be wiped out
What do you think happens to population gene pool?

13. Figure 23.8 A special case of genetic drift: The bottleneck effect
chance
allele frequency change
Figure 23.8 A special case of genetic drift: The bottleneck effect
Original
population
Bottlenecking
event
Surviving
(b)
Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.
(a)
Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent.

14. Cheetahs are another example of a species that appears to have experienced a severe bottleneck in the relatively recent past, because their genetic variability is extremely low. For example, skin grafts from one cheetah to another are not rejected.

15. Polydactyly in Pennsylvania Old Order Amish (founding population approximately 200 people, almost always marry within the group)
Johns Hopkins University Press

16. What is Founder Effect? What do you think happens to each small group?
A small population moves to a new location
They do not represent all the alleles of the larger population from which they came
What do you think happens to each small group?

17. Some facts about genetic drift:
chance
allele frequency change
Some facts about genetic drift:
1. It occurs the same way if there are more than 2 alleles, it just produces a more complicated-looking graph.
2. It is most powerful in small populations, but is inevitable even in large populations.
3. If it goes on long enough, it is possible for one allele to become the only allele in the population. This allele is now said to be “fixed”.
The opposite of fixation is allele loss or allele extinction. New mutants run a huge risk of going extinct, but some do increase, and even become fixed, by chance.
Over truly long periods drift and mutation are the primary causes of changes in nucleotides sequence in non-coding parts of genomes.

18. What is Gene Flow?
Migration:
Movement of alleles between populations

19. What are Mutations? What do you think happens to the population?
Damages to chromosomes, causing changes in alleles
What do you think happens to the population?

20. What are the 3 types of Natural Selection?
Stabilizing Selection IQ
Directional Selection
Giraffe necks
Disruptive Selection
Finch beaks