The mosquito in the Amber is an organism preserved over millions of years. This is perfect example of a

It is a perfect example of a natural selection.

(I'm not sure if this is the exact answer that you are looking for.)

Fossil.

Explanation:

Fossils may be defined as the impressions of dead living organism under the rocks. Amber is plant resin or dribbles of plant sap.

Plants and animals can trapped in the amber and can preserve for millions of years. The organisms froze in amber can be used as a fossil to study evolutionary history of an organism. Thus, mosquito preserve in amber is the perfect example of the fossil.