Population Genetics – Genetic Drift & the Bottleneck Effect

INTRODUCTION

Genetic drift is a change in allele frequencies in small populations that appears to occur as a consequence of random chance. It’s like flipping a coin ten times and getting 8 heads and only 2 tails. You believe from your understanding of probability that if you flip the coin often enough, you will ultimately get heads and tails each with a frequency of 0.5 (or 50%) for all flips considered together. The problem with small populations is that they can only produce a limited number of offspring in each generation, and therefore may not produce the expected proportion of a particular genotype in a given generation. Unlike the coin flip example, the parent population can’t go back to the gene pool and have some more offspring until they get the expected proportions right. A consequence of this may be what is known as genetic fixation of an allele, wherein all forms of an allele but one are lost purely by chance within the population.

Genetic drift becomes a factor for evolution in populations that are formed from a small sample of a larger population by either the bottleneck effect or the founder effect. The bottleneck effect occurs when a population passes through a period in which most of the population is killed by natural disaster, disease, or excessive predator pressure. The limited genetic variability seen in the world’s cheetah population is attributed to the bottleneck effect of disease, habitat destruction, and overhunting by humans.

The founder effect occurs when a small portion of the population is transplanted to a new geographic locale where the fragment establishes its own niche in the new territory. A prime example of the founder effect occurs in the northeastern United States within the Old Order Amish Sect. This population was established by a few hundred migrants to this country and maintained as a closed genetic unit by their laws forbidding marriage outside of the sect. Let us now investigate what will be the effect on allele frequencies that results by subjecting a population either to the bottleneck or founder effect.

PROCEDURE

1. Using your beads and containers, establish a gene pool of 100 alleles in which the frequency of each allele will be 0.5. (In other words, add 50 red beads and 50 white beads to your cup.)

2. Remove 8 pairs of alleles to establish the allele frequencies of the bottleneck or founder population. Don’t do this by sampling with replacement! These are organisms in the existing population, not members of the next generation.

3. Use these eight individuals to establish the gene pool for the next generation. Prepare a container with 100 beads in the same frequency proportions. This is your gene pool for the next generation.

4. Mix the beads well and pick 8 pairs of alleles again as in step 2. The founder or bottleneck population is practicing replacement reproduction, i.e., having two offspring for each mating pair. Using these 8 pairs, establish a container of 100 beads with the allele frequencies that occurred in the 8 pairs to form the gene pool for the next generation of 8 offspring.

5. Continue this activity until one allele becomes fixed in the population (only one color is drawn) or you reach the 9^th generation. Record your results in the table on the next page.

DISCUSSION OF THE RESULTS

1. In what generation did one of the two alleles become fixed? _____________ If you did not achieve allele fixation in nine generations, what was the lowest frequency that one allele reached? ___________

2. Graph the change in frequencies of the two alleles over time on the graph paper provided.

a) Did the same allele go to fixation for every group in the class?

b) Based on the experimental design, why should either allele have an equal opportunity to go to fixation?

c) Did all the graphs in the class look the same? _____ If they did not, why didn’t they?