In biology, outbreeding depression happens when crosses between two genetically distant groups or populations result in a reduction of fitness.
If the outcrossing is limited and populations are large enough, selective pressure acting on each generation can restore fitness.
[7] However, as the hybrid population will likely to go through a decline in fitness for a few generations, they will need to persist long enough to allow selection to act before they can rebound.
[1] Generally this mechanism will be more prevalent in the first generation (F1) after the initial outcrossing when most individuals are made up of the intermediate phenotype.
Examples of the second mechanism include stickleback fish, which developed benthic and limnetic forms when separated.