What evidence must be considered when determining whether or not a trail is adaptation?

An adaptation is a trait that "evolved" for a particular condition that allows the animal to survive. Something is adaptive if it is a useful trait but not necessarily one that occurred in response to a particular selection pressure. Basically a trait is an adaptation if it is the product of selection, and adaptive if it is beneficial in the present time.

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The short-term effect of tar on the body is increased blood pressure.