Figure 1

Conceptual model of ERV evolution. Circles represent individual ERV loci and the tree their genealogical relationships. Newly integrated, active loci will be present initially only in single host individuals and must eventually be inactivated (and rendered neutral if originally harmful) by mutations acquired during host replication. A small proportion of these loci (= 1/2N_e where N_e is the long-term effective population size) will eventually drift to fixation in the host population and thus appear in any published genome sequence. If active ERV loci are harmful, they will be found at even lower frequencies (until inactivated or lost due to negative selection).