Daren periodically undergoes dilation of his esophagus for a stricture suffered from swallowing a poisonous substance. the esophagus is located in which body cavity?

Thoracic  body cavity

The esophagus is located in the thoracic body cavity.  

The thoracic cavity is the big chamber of the body that is enclosed by the thoracic wall. The thoracic cavity is covered by the ribs, the spinal column, and the sternum. Several structures are present within the thoracic cavity. They include; structures of the digestive system, including the esophagus; which is the passage through which food is moved from the throat to the stomach, structures of the respiratory system,  such as lungs and bronchi, endocrine glands etc.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".