On the Volcanic, Equatorial West African Island of Sao Tomé

question 2

Multiple Choice

On the volcanic, equatorial West African island of Sao Tomé, two species of fruit fly exist. Drosophila yakuba inhabits the island's lowlands and is also found on the African mainland, located about 320 kilometres away. At higher elevations, and only on Sao Tomé, is found the very closely-related Drosophila santomea. The two species can hybridise, though male hybrids are sterile. A hybrid zone exists at middle elevations, though hybrids there 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) . Sao Tomé rose from the Atlantic Ocean about 14 million years ago.
Using only the information provided in the paragraph, which of the following is the best initial hypothesis for how D. santomea descended from D. yakuba?

• A) allopolyploidy
• B) autopolyploidy
• C) habitat differentiation
• D) sexual selection

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Definitions:

Parietal Cells

Cells in the stomach lining that secrete hydrochloric acid and intrinsic factor, essential for digestion and B12 absorption respectively.

Active Transport

The movement of molecules across a cell membrane requiring energy input, typically against their concentration gradient.

Bicarbonate Ions

Chemical ions (HCO3-) that act as a buffer in blood and help maintain the body's pH balance.

Pepsinogen

Proenzyme formed and secreted by the chief cells of the gastric mucosa; the acidity of the gastric juice and pepsin itself converts pepsinogen into pepsin.