Passage Summary: Some scientists thought a rare fish trait only happened once because the fish were family. Since DNA shows they aren't close family, the argument concludes the trait must have started twice.

Conclusion: The specialized algae-scraping trait must have evolved independently more than once.

Reasoning: Biologists argued that if the fish were closely related, the trait evolved only once; however, genetic testing proved the fish are not closely related.

Analysis: The argument fails because it incorrectly negates the sufficient condition to disprove the necessary one. The premise states that 'close relation' guarantees 'single evolution,' but it doesn't say that 'single evolution' *requires* a close relation. The trait could have evolved once in a very distant ancestor, or the biologists' initial assumption about the rarity of the trait could simply be wrong. Look for an answer that points out this logical gap in the 'if-then' relationship.