Fish with teeth specialized for scraping algae occur in both Flower Lake and Blue Lake. Some biologists argue that be...

shunhe on May 13, 2020

Hi @isorom19,

Thanks for the question! Let’s take a look at the stimulus first. We’re told that there are fish with special teeth in two different lakes. We’re then told that if the fish are closely related, then the trait should have only evolved once. But we know that the fish aren’t closely related, and so we conclude that the special teeth evolved more than once.

To diagram it out, we are given the following argument

Closely related —> Evolved once
~Closely related
Conclusion: ~Evolved once

But this is clearly a case of mistaken negation, which is a classical logical mistake. Note that the following would make the argument valid:

Evolved once —> Closely related
~Closely related
Conclusion: ~Evolved once

And so we can see here that what happened is that the argument mixes up the sufficient and necessary conditions, or which side of the arrow the two words are. This is what (C) tells us, and so (C) is the correct answer.

Hope this helps! Feel free to ask any other questions that you might have.