Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Introgression is a long-term process, even when artificial; it may take many hybrid generations before significant backcrossing occurs. This process is distinct from most forms of gene flow in that it occurs between two populations of different species, rather than two populations of the same species. Introgression also differs from simple hybridization. Simple hybridization results in a relatively even mixture; gene and allele frequencies in the first generation will be a uniform mix of two parental species, such as that observed in mules. Introgression, on the other hand, results in a complex, highly variable mixture of genes, and may only involve a minimal percentage of the donor genome. Definition Introgression or introgressive hybridization is the incorporation (usually via hybridization and backcrossing) of novel genes and/or alleles from one taxon into the gene pool of a second, distinct taxon. This introgression is considered 'adaptive' if the genetic transfer results in an overall increase in the recipient taxon's fitness. Ancient introgression events can leave traces of extinct species in present-day genomes, a phenomenon known as ghost introgression. Source of variation Introgression is an important source of genetic variation in natural populations and may contribute to adaptation and even adaptive radiation. It can occur across hybrid zones due to chance, selection or hybrid zone movement. There is evidence that introgression is a ubiquitous phenomenon in plants and animals, including humans, in which it may have introduced the microcephalin D allele. It has been proposed that, historically, introgression with wild animals is a large contributor to the wide range of diversity found in domestic animals, rather than multiple independent domestication events. Introgressive hybridization has also been shown to be important in the evolution of domesticated crop species, possibly providing genes that help in their expansion into different environments. A genomic study from New York University Abu Dhabi Center for Genomics and Systems Biology showed that domesticated date palm varieties from North Africa show introgressive hybridization of between 5-18% of its genome from the wild Cretan palm Phoenix theophrasti into Middle East date palms P. dactylifera. This process is also similar to the evolution of apples by hybridization of Central Asian apples with the European crabapple. It has