How can we try to prevent extinction of particularly endangered species?

A  We can’t: Natural selection is already nature’s best mechanism for survival of a species.

B  By putting a handful of endangered animals in a Zoo, or freeze them to preserve some of their genetic material.

C  Among many measures, one consideration is to assure the population has a large gene-pool by enabling migrations and relocations to preventing isolation. That increases the standing genetic variation, and perhaps population size.