Generations

Starting Frequency of allele A₁

Population Size

# Simulations

Fitness of A₁A₁ homozygotes

Fitness of A₁A₂ heterozygotes

Fitness of A₂A₂ homozygotes

In this simulation there is no selection. Any allele changes you see are only due to genetic drift.

Try making the simulation run for more generations. What eventually happens to each population?
Try changing the population size to investigate the relationship between population size and time to fixation.
Try running many simulations for 1000 generations, How many of the populations lose allele A₁? How many become 'fixed' for allele A₁?
How does this change if you change the starting frequency of allele A₁?

Default

In this simulation the new, beneficial allele (A₁) is recessive (the heterozygote has a fitness equal to the A₂A₂ homozygote)

What usually happens to a new rare allele via genetic drift, even if it is beneficial?
Why does the allele take so long to be fixed? Consider that the benefits of the beneficial allele is only felt by the homozygotes, and these are very rare when the frequency of the new allele is low.
Once the allele becomes common enough to be regularly seen in homozygotes, it quickly becomes fixed.

New allele is Recessive

In this simulation the new, beneficial allele (A₁) is dominant (the heterozygote's fitness is the same as the A₁A₁ homozygote).

The allele frequency becomes fixed for the beneficial allele very quickly.
The old ('bad') allele (A₂) stays in the population for a long time. The old allele is only selected against when it is in homozygotes, and these are very rare when the frequency of A₂ is low.
Alleles that cause disease can persist in the population even though selection is against them.

New allele is Dominant

In this simulation the heterozygote is the most fit. An example of this is sickle cell anemia:

A₁A₁ (often called 'AA') homozygotes are more susceptible to malaria than the heterozygotes. (A₁A₂ heterozygotes have some immunity to malaria)
A₂A₂ (often called 'SS') homozygotes usually die before they can reproduce.
Try changing the starting frequency of Allele A₁. What happens over many generations?
Imagine a population that moves to a region with no malaria (such as African populations moving to the United States). When there is no malaria, the A₁A₁ homozygotes no longer die of malaria and the heterozygotes no longer have an advantage. What will happen to the allele? (adjust the fitness of the A₁A₁ homozygote to '1').

Sickle cell (Heterozygote advantage)

Hints & Help

Hints:

The dark black line is the expected allele frequency when there is an infinitely large population (ie. with NO drift).
If you want to see the expected allele frequency with no Genetic Drift, enter '0' for the # of simulations.
Try the preset simulations to model the introduction of a new dominant allele, a new recessive allele, and heterozygote advantage in sickle-cell anemia.

For best results:

Select a combination of # Generations, Population Size, and # Simulations so that if multiplied together, they total less than 10,000,000. Longer, larger simulations will take more time.
If a simulation runs too long, you may need to close the browser tab and start again.

# Populations fixed for allele A₁:

# Populations fixed for allele A₂:

# Populations polymorphic:

# Populations fixed for either allele:

Average time to fixation:

Warning

Some populations remain polymorphic after the simulation. This will cause the average fixation time to be under-estimated.

To get a better estimate, run the simulation for more generations.

A good rule of thumb is to run the simulation for 10 times as many generations as the population size.

Genetic drift and Selection