This is question number 34 from the 2014 USABO Open Exam:
- Black and white mice live on an island and have allele frequencies of B=0.20, b=0.80. On the continent, there is a much large population of mice with allele frequencies B=0.80,b=0.20. Ships begin moving back and forth between the island and the continent. Occasionally, a mouse finds its way onto the ship and leaves the ship and breeds with the mice of the other population. Equal numbers of mice ride in each direction. If the ships move continuously between the island and the continent, will the allele frequencies ultimately stabilize? (Assume that no other forces are affecting allele frequencies.)
A. Both populations will end up at approximately B=0.80, b=0.20.
B. The island and continent populations will stabilize with an allele frequency of B=0.50, b=0.50.
C. Black mice will move to fixation in both populations, because the B allele is dominant.
D. Both populations will end up at B=0.20, b=0.80.
E. The continent's allele frequencies will not change; the island will settle at an equilibrium that is somewhere between the continent's and the island's original states.
The correct answer is A. I guessed E, because I thought the difference in allele frequencies on the continent meant that the B allele was selected for there, and since the population was so big, I thought that the immigration and emigration wouldn't affect the allele frequency there so much, while the island's allele frequencies would change a little because of the smaller population.
Why is the answer A, why is E wrong (besides that A is right), and is there a math way or some sort of rule to solve this problem (or are you just supposed to reason it out)?