单选题 .  ①Recent studies of the Philippine fruit bat fauna have confirmed some previous hypotheses regarding bats on oceanic islands: for example, species richness [the number of different species in a particular area] and abundance are generally highest in the lowlands and decrease with increasing elevation. ②With few exceptions, each endemic species [species native to a particular place] is restricted to the modern islands that made up a single island during periods of low sea level, and genetic differentiation has been influenced by the ecology of the species and the current and past geographic and geological conditions. ③However, far more previous hypotheses have been overturned than supported. ④Some endemic Philippine species use disturbed habitat as extensively as nonendemic species that are widespread in Southeast Asia. ⑤Levels of genetic variation within all species are high, not low, and rather than showing evidence of an intrinsic vulnerability to extinction from natural causes, independent lineages of these bats have persisted in rather small areas for very long periods of time (often millions of years) in spite of frequent typhoons and volcanic eruptions. ⑥While colonization from outside areas has clearly contributed to the high species richness, speciation within the archipelago has contributed at least a quarter of the total species richness, including many of the most abundant species.13.  The primary purpose of the passage is to ______