Does natural selection always result in an increase in genetic diversity?

Not always as organism tend to breed with there own kind.

Explanation:

In human anatomy, the thoracic duct or thoracic duct (also known as the left lymphatic duct) is an important part of the lymphatic system, in fact it is the largest lymphatic vessel. It is also known as the alimentary canal, chyliferous duct, Pecquet duct, left lymphatic duct or Van Hoorne's canal.

This large duct collects most of the body's lymph, except for the upper right quadrant. It drains into the blood circulation at the level of the left brachiocephalic trunk, at the junction of the left subclavian and left internal jugular veins.