Homology
Paralogy: when a gene is duplicated to occupy two different places in the same genome, the two copies are paralogous. Paralogous genes often belong to the same species, but this is not necessary: for example, the haemoglobin gene of humans and the myoglobin gene of chimpanzees are paralogs. Paralogs typically have the same or similar function, but sometimes do not. At least one of the copies will be under less selection pressure, and may mutate and acquire a new function.