AbstractHybridization is common in nature, and widely used to create new breeds in agriculture. However, it can cause harmful consequences. Outbreeding depression can occur when adaptive gene complexes in one species (or population) are broken down by the immigration of genes that are adapted to some other environment. This causes mal-adaptation of progeny, lowering the future species (population) fitness. Genetic assimilation may dilute the genetic diversity of a small population if it hybridizes with a larger population of a different species. That is, no genetically “pure” progeny will be produced by small populations if their parents have a high probability of hybridizing. Hybrid offspring may be fitter than either parent due to heterosis, and may spread at the expense of the parent strain, and ultimately replace it. Through introgression, species or populations may also gain some adaptive traits such as resistance to disease, adversity, or pesticides, which they have never shown before, thus making them more difficult to control. Implications and suggestions concerning in situ and ex situ conservation of endangered species, ecological restoration and so on are proposed.