Fish with teeth specialized for scraping algae occur in both Flower Lake and Blue Lake  Some biologists argue that because such specialized characteristics are rare, fish species that have them should be expected to be closely related  If they are closely related, then the algae-scraping specialization evolved only once  But genetic tests show that the two algae-scraping species, although possibly related, are not closely related
Thus, the algae-scraping specialization evolved more than once
takes a sufficient condition as a necessary one