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 the repeated backcrossing of an interspecific hybrid with one of its parent species. Purposeful introgression is a long-term process; it may take many hybrid generations before the backcrossing occurs.
Introgression is an important source of genetic variation in natural populations and a major cause of speciation in the sympatric mode. It can have important effects on the dynamics of hybrid zones, speciation and adaptive radiation. There is evidence that introgression is a ubiquitous phenomenon in plants, animals, and even humans, in which it may have introduced the microcephalin D allele.
Introgression differs from simple hybridization. Introgression results in a complex mixture of parental genes, while simple hybridization results in a more uniform mixture, which in the first generation will be an even mix of two parental species. Natural introgression does not have human direct interference while the exotic introgression is induced intentionally (as for instance genetically modified organisms) or not (human activities affecting local races of crops or human disturbances such as by 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.