Introgression, also known as introgressive hybridization, in genetics (particularly plant genetics), is the movement of a gene (gene flow) from one species into the gene pool of another by repeated backcrossing an interspecific hybrid with one of its parent species. Introgression is a long-term process; it may take many hybrid generations before the backcrossing occurs.
It is an important source of genetic variation in natural populations and major cause of speciation in the sympatric mode. It can have important effects on the dynamics of hybrid zones, speciation and adaptive radiation. The introgression differs of the simple hybridization. In the introgression the new species will have a complex mixture of parental genes, while in the simple hybridization this mixture will be of 50 to 50% of two parental species. The natural introgression does not have the human direct interference while the exotic introgression is induced intentionally (as for instance genetically modified organisms) or not (human activities affecting local races of crop, human disturbances like in introducing weeds).
An example of introgression is that of a transgene from a transgenic plant to a wild relative as the result of a successful hybridization leading to intentional or unintentional "genetic pollution". Another important example has been studied by Arnold & Bennett 1993: irises species from southern Louisiana.
An introgression line (abbreviation: IL) in plant molecular biology is a line of a crop species that contains genetic material derived from a similar species, for example a "wild" relative. An example of a collection of ILs (called IL-Library) is the use of chromosome fragments from Solanum pennellii (a wild variety of tomato) introgressed in Solanum lycopersicum (the cultivated tomato). The lines of an IL-Library covers usually the complete genome of the donor. Introgression lines allow the study of quantitative trait loci, but also the creation of new varieties by introducing exotic traits.