Simulating Random Events in Evolution: Genetic Drift
Integrated Science 4
Name
Per.
Introduction
The Vertebrate Evolution unit studied the results of evolution. In that context, we considered
changes in both traits and DNA for evidence of evolutionary change and evolutionary relationships.
The current unit on Islands focuses instead on the mechanisms producing observed evolutionary
change. We looked at evidence from finch beak sizes to evaluate natural selection as an evolutionary
mechanism. Natural selection is considered a deterministic process, one that yields predictable
results. In this activity, we use genetic evidence to model genetic drift as an alternative mechanism to
natural selection. Genetic drift is considered a stochastic process, producing results that are random.
In 1908 G.G. Hardy and W. Weinberg independently suggested a model that measured
evolution changes by the changes in the frequency of alleles in a population of organisms. (remember:
alleles are different copies of the same gene located on different chromosomes.) They reasoned that if
A and a are alleles for a specific gene and individuals in the population have two alleles, then the
allele frequency for both A and a can be measured in the population. If allele frequencies change from
generation to generation, then evolution has occurred.
Additional Background
Use the following resources to further develop your understanding of genetic drift and other related
phenomena as mechanisms for evolution.
Watch the following video introduction:
https://www.youtube.com/watch?v=mjQ_yN5znyk
View the following website:
https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-evolution-ofpopulations-19/population-genetics-131/genetic-drift-531-11736/
Summarize your understanding of genetic drift and other related phenomena in the space below.
Include specific examples or circumstances under which genetic drift and related phenomena are
likely to occur.
Simulation- Genetic Drift
For this simulation, you will evaluate the cause and effect relationship between population size and
gene frequencies over time. The simulation models changes in a population over 200 generations. The
allele frequencies in the starting population are p = .5 and q = .5. The genotype frequencies in the
starting population are p = .25, 2pq = .5, and q = .25. Each genotype has equal fitness, therefore
selection will not influence change over time.
2
2
1. Go to the website: http://www.radford.edu/~rsheehy/Gen_flash/popgen/
2. Before running the simulation, set the following:
• Population size: 100
• Number of Populations: 1
3. Once these parameters are set, click GO.
4. Record the allele and genotype frequencies in the table below. Take note of other relevant details
observed in the simulation – patterns in the data, whether or not an allele or genotype becomes
fixed or lost, etc.
Generation
Population
Size
p
q
p
2
2pq
q
2
Did evolution
occur? Y/N
1 (initial)
200 (final)
Observations:
5. Reset the simulation. Before running the simulation again, set the following:
• Population size: 10,000
• Number of Populations: 1
6. Once these parameters are set, click GO.
7. Record the allele and genotype frequencies in the table below. Take note of other relevant details
observed in the simulation – patterns in the data, whether or not an allele or genotype becomes
fixed or lost, etc.
Generation
Population
Size
p
q
p
2
2pq
q
2
Did evolution
occur? Y/N
1 (initial)
200 (final)
Observations:
8. Reset the simulation. Before running the simulation again, set the following:
• Population size: 100,000
• Number of Populations: 1
9. Once these parameters are set, click GO.
10. Record the allele and genotype frequencies in the table below. Take note of other relevant details
observed in the simulation – patterns in the data, whether or not an allele or genotype becomes
fixed or lost, etc.
Generation
1 (initial)
200 (final)
Observations:
Population
Size
p
q
p
2
2pq
q
2
Did evolution
occur? Y/N
Simulation – Migration/Founder Effect
For this simulation, you will evaluate the cause and effect relationship between founding populations
on islands and gene frequencies over time. The simulation models changes in a population over 200
generations. The allele frequencies in the starting population are p = .5 and q = .5. The genotype
frequencies in the starting population are p = .25, 2pq = .5, and q = .25. Each genotype has equal
fitness, therefore selection will not influence change over time.
2
2
11. Go to the website: http://www.radford.edu/~rsheehy/Gen_flash/popgen/
12. Before running the simulation, set the following:
• Population size: 100
• Number of Populations: 1
• Click the Migration box.
13. Once these parameters are set, click GO
14. Immediately after clicking GO, double click the Migration box to uncheck it.
15. Record the allele and genotype frequencies in the table below. Take note of other relevant details
observed in the simulation – patterns in the data, whether or not an allele or genotype becomes
fixed or lost, etc.
Generation
Population
Size
p
q
p
2
2pq
q
2
Did evolution
occur? Y/N
1 (initial)
200 (final)
Observations:
Simulation – Bottleneck
For this simulation, you will evaluate the cause and effect relationship between a large reduction in
population size due to a stochastic event and gene frequencies over time. The simulation models
changes in a population over 200 generations. The allele frequencies in the starting population are p =
.5 and q = .5. The genotype frequencies in the starting population are p = .25, 2pq = .5, and q = .25.
Each genotype has equal fitness, therefore selection will not influence change over time.
2
2
16. Go to the website: http://www.radford.edu/~rsheehy/Gen_flash/popgen/
17. Before running the simulation, set the following:
• Population size: 10,000
• Number of Populations: 1
• Click the Bottleneck box. Enter 50 for Start, 200 for End, and 100 for BN Pop.
18. Once these parameters are set, click GO
19. Record the allele and genotype frequencies in the table below. Take note of other relevant details
observed in the simulation – patterns in the data, whether or not an allele or genotype becomes
fixed or lost, etc.
Generation
1 (initial)
200 (final)
Observations:
Population
Size
p
q
p
2
2pq
q
2
Did evolution
occur? Y/N
Analysis
Use evidence from the simulations to summarize the cause and effect relationship between genetic
drift, the founder effect and the bottleneck effect and gene frequencies. Use your understanding of
these stochastic mechanisms for evolution to explain why the relationship exists. Present your
understanding in the following format:
•
•
•
What is your claim – is the hypothesis supported or refuted? Did evolution occur?
What is the evidence supporting your claim?
What is your reasoning for why these results occurred? How are the mechanisms of evolution
related to the experimental results?