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 two hundred miles away. At higher elevations, and only on Sao Tomé, is found the very closely related Drosophila santomea. The two species can hybridize, 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 fourteen million years ago.
The observation that island D. yakuba are more closely related to mainland D. yakuba than island D. yakuba are to D. santomea is best explained by proposing that D. santomea _____.
A. arrived on the island before D. yakuza
B. descended from a now-extinct, non-African fruit fly
C. descended from a single colony of D. yakuba, which had been introduced from elsewhere, with no subsequent colonization events
D. descended from an original colony of D. yakuba, of which there are no surviving members. The current island D. yakuba represent a second colonization event from elsewhere.
Which of the following describes the most likely order of events in allopatric speciation?
A. divergence, genetic drift, genetic isolation
B. genetic isolation, divergence, genetic drift
C. genetic isolation, genetic drift, divergence
D. divergence, genetic isolation, genetic drift
E. genetic drift, genetic isolation, divergence
Fossils of Lystrosaurus, a dicynodont therapsid, are most common in parts of modern-day South America, South Africa, Madagascar, India, South Australia, and Antarctica. The animal apparently lived in arid regions, and was mostly herbivorous. It originated during the mid-Permian period, survived the Permian extinction, and dwindled by the late Triassic, though there is evidence of a relict population in Australia during the Cretaceous period. Some dicynodonts had two large tusks, extending down from their upper jaws. The tusks were not used for food gathering, and in some species were limited to males. Food was gathered using an otherwise toothless beak. Judging from the fossil record in sedimentary rocks, these pig-sized organisms were the most common mammal-like reptiles of the Permian.
Which of the following is the most likely explanation for the modern-day distribution of dicynodont fossils? The dicynodonts were_____.
A. most abundantly distributed throughout Gondwanaland
B. carnivores that traveled widely to find prey
C. amphibious and able to swim long distances
D. evenly distributed throughout all of Pangaea