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A symbiotic relationship is an interaction between two or more species in which one species lives in or on another species.

  • It might help to understand it if you rephrase the sentence as "In a symbiotic relationship between two or more species, one species lives in or on another species." But if we want to make the sentence clearly about the definition of what a symbiotic relationship is, it's clearer if we can say "A symbiotic relationship is" which is why the author phrased the original sentence that way. – stangdon Apr 18 '16 at 12:42
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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...

A symbiotic relationship is an interaction between two or more species in which one species lives in or on another species.

...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:

A symbiotic relationship is an interaction between two or more species, in which one species lives in or on another species.

From here the two-concept sentence can be broken-down into...

1.) A symbiotic relationship is an interaction between two or more species.

2.) In a symbiotic relationship, one species lives in or on another species.

Or, more heavily...

2.) A symbiotic relationship is a relationship in which one species lives in or on another species.

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...

One species lives in or on another species.

...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...

A symbiotic relationship is an interaction between two or more species.

...leaves a very awkward construct:

A symbiotic relationship is an interaction between two or more species. One species lives in or on another species.

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:

A symbiotic relationship is an interaction between two or more species; [a relationship in which] one species lives in or on another species.

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:

A symbiotic relationship is an interaction between two or more species in which one species lives in or on another species.

(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.

  • Thank you for your answer, your answer makes the sentence thoroughly understandable. But could you tell me the difference between the point you have mentioned and the sentence below: "This is a pleasant city in which to live". I paraphrase it in this way: "This is a pleasant city to live in". Does my new example have any relation with the point presented by you in my previous example or not?Thank you again, sorry if my questions are too simple for you because I am an English learner. – Anfi Apr 18 '16 at 19:59
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    @Alireza : There's no language more difficult to learn than English, except maybe Icelandic. No apology needed. :) I do not see them as being quite the same application of "in which". However, your paraphrase is exactly right. Use of "in which", in both instances, is the more formal or archaic sounding way to go. But the way I would say the same idea is this: "I like this town." It implies what your example says, and is even used in New York City. I'm happy to help! – Alpinwolf Apr 19 '16 at 0:08
  • thank you again, for the very last question; To be sure, does "the host" at end of the sentence refer to "food": Parasitism is a kind of predator-prey relationship in which one organism, the parasite, derives its food at the expense of its symbiotic associate, the host. – Anfi Apr 19 '16 at 7:50
  • @Alireza : No, "the host" is another organism, but not the parasite's "food". Humans can have intestinal tape-worms as parasites. The person with the tape-worm is "the host". But the tape-worm does steal nutritional material from the host's digestive tract. The "Host" is not the food, but the host's own food becomes the parasite's food, and the host gets less. This is a bit more about biology than English, but I understand the confusion. – Alpinwolf Apr 20 '16 at 8:22
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    @Alireza , I'm not certain I understand your question. Perhaps it's the use of "host" here in general that's confusing? At a dinner party, there are guests, and there is a host (or hostess). With a parasite, the word is used to describe the organism that is (unwittingly) providing a safe and nourishing environment to the "guest" parasite. In a party scenario, "Host" is a flattering thing to be called. In science, it's a cold term. I apologize if I still do not understand the question. There are many grammar rules I use well, but do not strictly understand. – Alpinwolf Apr 20 '16 at 10:14
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Compare:

I saw a movie in which a company creates dinosaurs from DNA.

which there is a proxy nominal that refers back to a previous noun, movie, when connecting a new clause that pertains to or modifies the noun.

We use "in" because we want to say this:

In that movie, a company creates dinosaurs from DNA.

The action happens "in" the movie's plot.

Here is one without in:

I saw a movie which was called Jurassic Park.

We use in with nouns that express the idea of something ongoing or bounded. When the noun expresses something which is ongoing, then in is temporal, and means "during". When the noun expresses something which is bounded, then in is locative, and means "inside, within".

  • @TRomano to completely grasp what you said, would you present two distinct examples for both "ongoing" and "bounded" you mentioned? I assume that my sentence is "bounded" in your definition. – Anfi Apr 18 '16 at 19:19
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    ongoing and bounded are deliberately vague so as to encompass a wide range of things, including abstract notions like interaction, the word in your original question. Many of our abstractions have physical underpinnings which explain why we use particular prepositions with them. E.g. "In getting to know her, I discovered she held strong opinions on some matters." On a less abstract level: "We met in June" (temporal) and "We went for a walk in the park" (locative). – Tᴚoɯɐuo Apr 18 '16 at 20:51
  • "This is a pleasant city in which to live" Does This sentence differ from my previouse example regarding the point you presented? I paraphrase it in this way: "This is a pleasant city to live in".thank you. – Anfi Apr 18 '16 at 21:15
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    Your sentence is good; it uses the in which pattern correctly; and your paraphrase is accurate. – Tᴚoɯɐuo Apr 18 '16 at 21:20
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    the host is a noun phrase in apposition with the noun phrase symbiotic associate. The associate is the host. Compare: This is Mr Smith, the owner of the house. Mr Smith is the owner. – Tᴚoɯɐuo Apr 20 '16 at 10:26
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Here the preposition "in" doesn't refer to anything and it is used because it is required as the below two sentences show:

A symbiotic relationship is an interaction between two or more species.

One species lives in or on another species in an interaction (between two or more species).

If you omit the "in" in bold, the sentence will not be grammatical. The relative pronoun "which" replaced the second "interaction" as in:

A symbiotic relationship is an interaction between two or more species which one species lives in or on another species in.

It is personal style or preference where to place the preposition "in". You can place it either before "which" as in your example or at the end of the sentence. Some style guidelines and grammar books prefer placing it before "which" and I share this preference personally. Also, unless you place "in" before "which", it could confuse readers as there is another "in" in the sentence. "In which" could be replaced by "where" which is a relative adverb as in:

A symbiotic relationship is an interaction between two or more species where one species lives in or on another species.

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    No matter how many times I read it, the version with 'in' at the end sounds wrong and nonsensical. – DCShannon Apr 18 '16 at 12:46
  • @DCShannon I know what you mean, but out of curiosity, does "This is a house which we live in" sound non-sensical, too? Or do we have to use "This is a house in which (or where) we live"? – user24743 Apr 18 '16 at 13:16
  • That sounds pretty much fine, but it doesn't have a whole prepositional phrase in there making things crazy. – DCShannon Apr 18 '16 at 13:49
  • @DCShannon Yes, indeed. The previous prepositional phrase could cause confusion as I mentioned in my answer, but it is not that nonsensical. The two sentences have the same grammatical structure. The reason I showed the sentence with "in" at the end of the sentence is to show the in was there before it was moved before which. – user24743 Apr 18 '16 at 13:53
  • If you pause before "one species lives in or on another species", say it really fast, and then pause before overemphasizing the last "in", it's just barely readable. "A symbiotic relationship is an interaction between two or more species -- which, one species lives in or on another species, in." – DCShannon Apr 18 '16 at 14:37
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"in" here refers to the interaction: "It is an interaction in which..."

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    This answer would be a lot better and helpful if details would be added. – shin Apr 18 '16 at 9:40

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