Two species of fruit flies occur on the volcanic, equatorial West African island of Sao Tomé that rose from the Atlantic Ocean 14 million years ago. Drosophila santomea occurs only on at higher elevations, while D. yakuba inhabits the lowlands. D. yakuba also occurs on the African mainland. On Sao Tomé a hybrid zone between the two species exists at middle elevations. Hybrid males, but not females, are sterile, and hybrids are greatly outnumbered by D. santomea. Studies of the two species' nuclear genomes reveal that D. yakuba on the island is more closely related to mainland D. yakuba than to D. santomea (2n = 4 in both species). Using this information, which of the following phenomena is the best initial hypothesis for how D. santomea descended from D. yakuba?

Question

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Given the information, habitat differentiation is the best initial hypothesis for how D. santomea descended from D. yakuba. This is because the two species occupy different elevations (habitats) on the island, with D. santomea at higher elevations and D. yakuba in the lowlands, suggesting that adaptation to different environments might have driven their divergence.

Two Species of Fruit Flies Occur on the Volcanic, Equatorial