Please see the image. Hypothesis: in Long Island Sound, mussels of various genotypes at various sites produce larvae which travel around the Sound. lap94-possesing larvae do better in higher salt conditions, in terms of survival, growth, and reproduction, leading to a higher frequency of that allele at higher numbered Sound sites as well as oceanic sites. The cost of expressing lap94 is a dsiadvantage at lower numbered/lower salt sites in the Sound, leading natural selection to reduce its frequency there.
Work Step by Step
Your estimate of the numerical allelic frequencies may vary a bit from those shown here-- not a big deal if you get the trend right. Use your imagination to match the saltiness of the water with the frequency of lap94, with its usefulness in tolerating salt but cost in lost-salt habitat.