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Genetic Drift Practical:Founder Effect vs. Bottleneck Effect

Background:Genetic drift is defined as the random change in allele frequencies that occurs in small populations. It is not due to natural selection. Genetic drift is strongly tied to population size and becomes important in situations where a small segment of an original population splits off or becomes isolated from the parent population. A change in allele frequencies as a result of this migration of a small subgroup of a population is known as ‘founder effect’. Other times this process occurs when the original population undergoes a devastating population reduction (i.e. by natural disaster) and the allele frequencies are changed as a result. This is referred to as the ‘bottleneck effect’.Evolution at the population level is defined as a change in allele frequency over time. This lab will demonstrate how genetic drift can be a strong evolutionary force in certain situations. We will see that a smaller population size can show a much more dramatic shift in allele frequencies over time due to these two events.

Initial Population:1. Pick up a cup of 24 Plain M&M’s and a paper towel; this represents the original population of a group of organisms known as Candus spectacularus. Each coloured M&M represents 1 allele found in the population.Note that there are 6 possible alleles represented in this population (multiple alleles).2. Count the alleles in your initial population and fill-in the data in Table#1.3. Use the following math formula to calculate Allele Frequency:

Founder Effect : 4. WITHOUT LOOKING remove 6 M&M’s from the cup and place them on the paper towel; this represents the founding group. The founding group has left the original population to start a new population. How did this emigration by the founding group affect the allele frequencies seen in this new population? (Discuss).Record all the data for Table #2.5. Now, put the 6 M&M's back into the cup. We will now study the bottleneck effect on this original population.

Bottleneck Effect:6. Without looking, remove 18 M&M’s. This removal represents a natural disaster that has hit the Candusspectacularus population. A hurricane has wiped out most of the members in this population. Only 3 individuals remain, each having 2 alleles (6 alleles). What do the allele frequencies look like after this disaster? (Discuss). Record all the data for Table #3.

Discussion Questions:

1) Is there more of a dramatic change in allele frequencies when alleles change in larger populations or when alleles change in smaller populations? Explain.

2) Let's assume that the M&M's are insects. A group of insects have moved to a new environment, which consists of lots of greenery with many bright red flowers. Birds love to eat insects.

a) Which colours in the founding population (blue, green, yellow, red) would have a higher fitness level in this new environment? Why?

b) What might happen to those alleles in the population (increase or decrease)?c) Which colours in the founding population (blue, green, yellow, red) would have a lower fitness level

in this new environment? Why?d) What might happen to those alleles in the population (increase or decrease)?e) Is natural selection acting on this founding population? Explain.

4) From the list below indicate whether it will lead to a founder effect or a bottleneck effect. Explain.a. Diseaseb. Immigration (organisms moving into an existing population)c. Stormd. Searching for food

5) Calculate percent change of allele frequencies after each event (founding effect and bottleneck) by using the following math formula: