Learn English – What shoes are/is she wearing

As it is, and as others have pointed out, it's difficult to know the correct answer without knowing the rest of the sentence.

If you want to keep the same arrangement, mentally replace "which" with what's being described in the second part of the sentence. What's happening when using "which" is that you're replacing the subject in the second clause, so if you know what's being described, you'll know which tense to use:

Fruits are an evolutionary mechanism, [fruits] are...

Fruits are an evolutionary mechanism, [this evolutionary mechanism] is...

Alternatively, what might help is to simplify the sentence so that it's clearer to keep your subject more obvious.

The easiest way to do this is to remove "which"... and you have a couple of options:

If "fruits" is the subject of the sentence, the verb should agree with that. You have a couple of options

Rewrite it as an appositive (note, this will only work if "fruits" is the subject):

Fruits, an evolutionary mechanism, are the fleshy seed-associated structures of a plant that are sweet or sour and edible in the raw state.

[paraphrased from Wikipedia]

Part of the issue is that the two potential subjects are singular and plural... so if you rearrange the sentence, that will change:

Fruits are the fleshy seed-associated structures of a plant that are sweet or sour and edible in the raw state. These are considered an evolutionary mechanism.

Generally speaking, the word thanks is considered to be plural, so we would say "thanks are due," not "thanks is due".

That said, this Ngram provided shows a modest number of hits for thanks is due. So what's going on there?

Fact is, the word thanks is a little flexible, and can be used in various ways that would make it hard to provide a black-and-white answer to this question.

For example, if we put the indefinite article in front of thanks, we notice that "a thanks" is an informal way of saying "a thank-you", particularly when there is an adjective between them. So, neither of these would be considered incorrect:

A special thanks is due to FumbleFingers, who pointed out that Ngrams can be misleading.
A huge thanks is reserved for Linda and Penelope.

Moreover, the word thanks may be the object in a prepositional phrase, just before a verb. In those cases, the verb would agree with the subject of the sentence, not the object in the prepositional phrase. For example:

After all the work he has done for us, I think a token of thanks is due to Mr. Smith.
She worked her fingers to the bone; a word of thanks is due here.
A warm expression of thanks is due to those on both sides of the Atlantic.

^{(That last one is quoted from the book entitled Europe and America: The Prospects for Partnership by John Peterson, 1996)}

I managed to find a few instances where neither one of these were true, like this one:

Thanks is due to my parents, Charles and Susan, for their encouragement, help, and support.

Personally, I think "thanks are due" would have read better here, but this author may have deliberately opted to treat the word thanks as a mass noun – which is probably justifiable. After all, it's not like I can give you "one thank" (or, if I'm especially grateful, "two thanks").