Introgression

Introgression is the transfer of genetic variants between populations through hybridization followed by repeated backcrossing. It can leave a population carrying DNA from another population despite retaining most of its ancestry.

Hybridization: Reproduction between genetically distinct populations or species produces offspring with ancestry from both. It creates the hybrids through which genetic variants can cross population boundaries.
Charles Darwin: An English naturalist whose theory of evolution by natural selection was published in 1859. Darwin discussed hybridization and variation before introgression became a formal genetic concept.
Adaptive introgression: The transfer of genetic variants between populations or species that increases recipient fitness. It is the case in which introgressed DNA spreads because it provides an adaptive advantage.
Hybrid speciation: The formation of a new species through hybridization between distinct parental lineages. Unlike introgression into an existing population, hybrid speciation makes hybrid ancestry foundational to a new species.
Backcrossing: Mating a hybrid or descendant to an individual from one of its parental populations. Repeated backcrosses return most of the genome to one population while retaining some donor variants.
Gregor Mendel: A nineteenth-century scientist whose pea-plant experiments established foundational rules of inheritance. Mendelian inheritance helped explain how particular traits could persist through repeated crossing.
Neanderthal introgression: The transfer of Neanderthal DNA into the ancestors of some present-day human populations through interbreeding. It is a well-studied example of archaic human variants persisting after repeated mating with modern humans.
Incomplete lineage sorting: The persistence of ancestral genetic variants across successive speciation events, producing gene trees that differ from species trees. It can mimic gene exchange in genetic data despite occurring without hybridization.
Recombination: The exchange or reshuffling of genetic material that creates new combinations of alleles. Recombination breaks donor DNA into segments as introgressed variants pass through generations.
Ronald Fisher: A statistician and geneticist who helped establish population genetics and the mathematical theory of natural selection. Population-genetic theory supplied tools for analyzing allele movement across generations.

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