Extreme variability in mammalian chemosensory receptor gene families. (a,b) The OR gene family (a) and V1R gene families (b) of 31 mammalian species differ vastly both in the number of genes and in the percentage of intact genes. Much of this variability is caused by neutral genomic drift. However, there are also signs of selective processes. Note, for example, the very small percentage of intact V1R genes in humans and other primates. The articles by Olender et al.  and Wynn et al.  suggest that the clear division into intact and pseudogenized genes is appropriate only for single individuals. If the entire species is considered, many of the genes are found in both an intact and a pseudogenised version. Data in (a) from , in (b) from .