Water snakes are common in the area around Lake Erie. They live on the mainland
ID: 19563 • Letter: W
Question
Water snakes are common in the area around Lake Erie. They live on the mainland as well as islands in the lake. The island and mainland populations are relatively isolated, but migration does occur. Interestingly, selection favors banded water snakes on the mainland and unbanded on the islands. Bandedness is controlled by one locus with two alleles: banded (B1) and unbanded (B2) with banded being dominant.
Each generation, some proportion of snakes migrates from the mainland to the island. On the island, the frequency of B1 is very low (0.2). On the mainland it is much higher (0.9). Only 12% of the mainland individuals migrate successfully to the island. After migration, random mating occurs (no sexual selection). Each population then returns to HW equilibrium in the next generation, and selection occurs before the next bought of migration. The selection coefficient against banded water snakes on the island is 0.15 and it is 0 for the unbanded water snakes. What is the frequency of B1 after migration and selection?
Explanation / Answer
FIRST, CALCULATE THE GENOTYPE FREQUENCIES ON THE MAINLAND AND ON THE ISLAND: MAINLAND: B1 = 0.9 B2 = 0.1 B1B1 = 0.81 B1B2 = 0.18 B2B2 = 0.01 ISLAND: B1 = 0.1 B2 = 0.9 B1B1 = 0.01 B1B2 = 0.18 B2B2 = 0.81 THEN, CALCULATE THE NEW FREQUENCY OF B1 ON THE ISLAND AFTER MIGRATION: B1 = p p’I = (1-m) (pI) +(m)(pM) p’I = (0.9) (0.1) + (0.1)(0.9) = 0.18 NEXT, CALCULATE THE STRENGTH OF SELECTION AGAINST THE WATER SNAKES AND THE ALLELE FREQUENCIES AFTER SELECTION p’ = p(1-s)/1-2sp+sp2 s = 0.2, p = 0.18 p’ = 0.15 HOW DO THEY COMPARE TO THE ORIGINAL FREQUENCIES? IS MIGRATION OR SELECTION STRONGER? Migration is stronger. The island frequencies became more like the mainland frequencies despite selection.
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