A symbiotic relationship is an interaction between two or more species
in which one species lives in or on another species.
Learn English – What does “in” refer to
prepositional-phrasesrelative-clausesrelative-pronouns
prepositional-phrasesrelative-clausesrelative-pronouns
A symbiotic relationship is an interaction between two or more species
in which one species lives in or on another species.
Best Answer
The highlighted use of "in" in your sentence is part of a two-word term "in which". That term is then referring to the "relationship" (or, arguably, the "interaction", but those words are interchangeable for this example).
This sentence...
...comprises two concepts, both of which regard "A symbiotic relationship", and joins them with the "in which" term. Personally, I find that the structure could benefit from the addition of a comma after "species", for pacing and clarity, so:
From here the two-concept sentence can be broken-down into...
Or, more heavily...
Of those last examples for the only second concept, the first example is cleaner and less redundantly worded, and an editor may suggest changing a sentence built like the second example to read like the first version instead. They mean exactly the same thing.
Joining them together, the use of "in which" is a sort of "call-back" to an earlier part of the sentence, so that you can further condense on the page the information you wish to convey, without losing detail. Without the first section describing the relationship, this break-away sentence...
...might be complete as far as it goes, but it leaves questions unanswered about what our over-all subject is. Merely including it as a second sentence after...
...leaves a very awkward construct:
While a reader can make sense of that, they have to pause to think about it, just a tad. That structure dissociates the concepts slightly, when the purpose is to make sure they go together. Another expanded way to build the sentence would be:
Note that the [bracketed] portion is what goes together, and would be said in one breath if it was being read aloud. There's a risk for a learner to see "which" and "one" next to each other, and take it as the question phrase "which one?", and that would certainly make a mess of things. Having the word "in" in that location makes a lot of difference, and is fused with "which" to make it all work.
Once more:
(And I still think that's missing the comma, but that wasn't the question.)
Reference: I'm a native speaker of American English, born and raised in California, with some college, and a parent who is a published author. But to be frank, the use of "in which" is a bit stuffy, and I associate such a construct with more-formal British English. It still works in America just fine, though.