In co-evolution, one organism evolves to its own benefit in response to the other organism. As one changes, the other changes. So what's the endpoint for all this change?
In antagonistic interactions, the end point could be that one organism eliminates the other. In mutualisms, the end point would be when there's no further selection for a tighter association. In the example of the long-flowered orchid and the long-tongued moth, unless something changes to make the association less or more beneficial for either organism, the flower's length and the moth's tongue are plenty long enough. Other times, the end point is just that an organism has reached the limits of how tall or fast or whatever it can be.
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