Question

13. Yeast are eukaryotic, single-celled fungus (this is not part of this homework question, but you could test yourself by describing the carbon cycle around yeast). Yeast reproduce quickly (under optimal conditions, yeast cells can double their population every 100 minutes). Yeast signal to potential partners by producing an attractive pheromone - a chemical smell, but the effect of the strong signal depends on the environment. In Environment A, yeast with the strong signaling allele are preferred as mates, but in Environment B the strong signal doesn't matter. Researchers grew 12 replicate populations of yeast in, 6 populations in Environment A and 6 populations in Environment B. Each replicate was a sealed flask. They measured the frequency of the strong signaling allele every day for 13 days. Initial populations began with 1% of the yeast having the strong signaling allele. Their results are shown below. Population 1 frequency of the strong signalling allele Population 2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Day Figure 1: Change in frequency of the allele that attracts mate in yeast. Each line represents a different replicate population (12 total populations). The solid lines are yeast populations in an environment that favors the strong signaling allele (Environment A). The dotted lines are yeast populations in an environment in which the trait does not affect survival or reproduction (Environment B). a. Is there evidence for evolution by natural selection in any of these 12 populations? Explain your answer in 1-2 sentences. 1 pt. b. What evolutionary processes could have caused allele frequency in Population 1 to decrease? Explain your answer in 1 - 2 sentences. 2 pts. c. What evolutionary processes could have caused allele frequency in Population 2 to increase? Explain your answer in 1 - 2 sentences. 2 pts.

Answer 1

Answer a)

Yes. One of the replicates (Population 1) in population A, out of the six, shows a rapid drop in the % frequency of the strong allele after Day 9. This means, it's trait for expressing the particular signal has slightly changed from the rest of the replicates under identical environmental conditions. This indicates there is a gradual evolution in that particular replicate.

Similarly, in population B, one of the replicates (Population 2) suddenly shows increase in % frequency of the strong allele after Day 10, compared to the other replicates under identical circumstances. This is also an example of evolution.

Answer b)

The reason for the decrease in % allele frequency in population 1 could be a 'loss of function' mutation in the particular gene.

Answer c)

The reason for the increase in % allele frequency in population 2 could be a 'gain of function' mutation in the particular gene.