The goal of the project is to demonstrate that intact koala retroviruses (KoRVs) are purged from the koala population and replaced by degraded versions potentially less harmful to the host species. What has been lacking is a natural system to directly and in a controlled fashion, examine birth, purging and fixation of KoRVs and degraded KoRVs and to test whether an empirical basis for the fixation of neutral KoRVs and purging of deleterious KoRVs exists. We are well placed to study such population genetic effects of endogenization using animals on the Island of St. Bees. Since this koala population is on an island and derived from few founders, we have the opportunity to directly observe purging effects due to deleterious KoRVs that exhibit effects as homozygotes or heterozygotes and to observe their birth and death.