As long as enough generalist species are maintained, and studies of arctic and subarctic seas suggest that generalists are often dominant members of the marine communities there, it seems unlikely that the phylum is close to extinction.
21. In the passage, the author is primarily concerned with
rejecting an earlier explanation for the longevity of certain brachiopod species
reevaluating the implications of uniformity among existing brachiopod species
describing the varieties of environmental change to which brachiopods are vulnerable
reconciling opposing explanations for brachiopods lack of evolutionary success
elaborating the mechanisms responsible for the tendency among brachiopod species toward specialization
22. It can be inferred from the passage that many zoologists assume that a large diversity among species of a given class of organisms typically leads to which of the following?
Difficulty in classification
A discontinuous fossil record
A greater chance of survival over time
Numerical abundance
A longer life span
23. The second paragraph makes use of which of the following?
Specific examples
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