Fish with teeth specialized for scraping algae occur in both Flower Lake and Blue Lake. ████ ██████████ █████ ████ ███████ ████ ███████████ ███████████████ ███ █████ ████ ███████ ████ ████ ████ ██████ ██ ████████ ██ ██ ███████ ████████ ██ ████ ███ ███████ ████████ ████ ███ ██████████████ ██████████████ ███████ ████ █████ ███ ███████ █████ ████ ████ ███ ███ ██████████████ ████████ ████████ ████████ ████████ ███ ███ ███████ ████████ █████ ███ ██████████████ ██████████████ ███████ ████ ████ █████
The author concludes that the algae-scraping specialization of certain fish species in Flower Lake and Blue lake evolved more than once. This is based on the fact that if the fish in these lakes are closely related, then the algae-scraping specialization evolved only once. But the fish in these lakes are not closely related.
The author confuses a sufficient condition for a necessary condition. The fish being closely related is sufficient for the algae-scraping specialization to have evolved only once. But this doesn’t mean the fish being closely related is necessary. So even if the fish aren’t closely related, it’s still possible the specialization evolved only once.
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