In 1814, a few dozen British colonists founded a settlement on Tristan da Cunha, which is a group of small islands in the Atlantic Ocean. One of these individuals carried a recessive allele for retinitis pigmentosa, a retinal degeneration disorder. By the 1960s, the frequency of this allele was ten times higher on Tristan da Cunha than in the original population. This is an example of which genetic effect?