the avoidance of breeding with close relatives in order to maintain genetic diversity. Several mechanisms that promote inbreeding avoidance have been identified. In some species, olfactory or vocal signals may provide information about genetic relatedness, but most often individuals simply avoid breeding with those who are most familiar. Some studies of mate selection provide evidence of optimal outbreeding: Neither closely related individuals nor those that differ greatly are chosen; those that are different enough to avoid inbreeding but not too different are regarded as optimal mates.