See also: inbreeding

Outbreeding is breeding between organisms that are from different populations. Outbreeding depression occurs when a cross between two individuals from different populations, who themselves are phenotypically adapted for their environments in different ways, produces an offspring that is intermediate in phenotype and not suitably adapted to live in either environment. For instance, if an animal of large body size mates with an animal of small body size to produce a medium-sized animal, then it is not well adapted for either parent's environment and may be considered less 'fit' than its parents.

Sometimes the mixing of gene pools in this way is beneficial, producing a hybrid offspring that is genetically superior to either of its parents. This is called outbreeding enhancement.