A symbiotic relationship is a close interaction between species,
where one of the species lives on or in the other.

Parasitism is a relationship in which one organism (the parasite) obtains its food at the expense of another organism (the host).
The parasite is usually smaller than the host. Natural selection works on both the parasite and the host. The host has also produced deffensive adaptations that the host resist parasites.

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Mutalism is a relationship where both the host and the parasite benefit from the symbiotic relationship.
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Commensalism is a relationship where one organism benefits while the other organism is neither harmed or helped. True commensalism in nature is very rare.
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