Introgression describes the transfer of genetic material from one species into the gene pool of another. This process occurs through the repeated backcrossing of a hybrid organism with one of its parent species, a process that can take many generations to become significant. This movement of genes is distinct from other forms of gene flow because it happens between two different species rather than within the same species. The result is the incorporation of new genes and alleles into a population.
The Mechanism of Genetic Transfer
The process of introgression unfolds in a two-step sequence that begins with hybridization. The first step is the mating between two different species, which produces a hybrid offspring. For introgression to proceed, this hybrid must be fertile and capable of reproducing. This requirement distinguishes it from instances like the creation of a mule from a horse and a donkey, as mules are sterile and cannot pass on their mixed genetic makeup.
The second step of introgression is backcrossing. This occurs when the fertile hybrid offspring mates with an individual from one of the original parent species. This act is repeated over many generations, and with each generation, a portion of the “donor” species’ genetic material is integrated into the “recipient” species’ gene pool. The result is a population that is genetically very similar to the parent species but now contains specific segments of DNA from another.
Introgression as an Evolutionary Driver
Introgression is a source of genetic variation in natural populations that can contribute to adaptation and the development of new species. When the transferred genes provide a benefit to the recipient species, it is known as adaptive introgression. This process introduces genetic material that has already been tested by natural selection in another species, offering a shortcut to novel traits. This can be advantageous when a species faces new environmental challenges.
There are numerous examples of adaptive introgression in the natural world. In Heliconius butterflies, gene exchange between species has led to new wing patterns that aid in mimicry and predator avoidance. Some species acquired genes controlling red and orange color patterns from their relatives, allowing them to adopt warning coloration that matches other toxic species. In the plant kingdom, wild sunflowers have transferred genes for herbivore resistance to their crop relatives. Certain iris species have also exchanged genes that confer tolerance to different soil moisture levels, allowing them to expand into new environments.
A Look into Human Ancestry
Human evolution contains clear evidence of introgression from our closest extinct relatives, the Neanderthals and Denisovans. After modern humans migrated out of Africa, they encountered and interbred with these archaic hominin populations. These events, estimated to have occurred between 47,000 and 65,000 years ago for Neanderthals and 44,000 to 54,000 years ago for Denisovans, transferred archaic DNA into the Homo sapiens gene pool. Consequently, the genomes of most present-day non-African populations contain between 1% and 4% Neanderthal DNA.
This ancient genetic exchange left a lasting impact on human biology. Some archaic genes that remain in modern human populations are associated with the immune system, providing advantages in fighting off new pathogens encountered outside of Africa. For instance, specific Neanderthal-derived variants in genes related to the immune system are thought to have helped modern humans adapt to local diseases. A gene variant inherited from Denisovans, known as EPAS1, is common in modern Tibetan populations and is associated with adaptation to high-altitude, low-oxygen environments. Other introgressed genes have influenced skin and hair characteristics, which may have helped early modern humans adapt to different climates in Eurasia.
Not all transferred genetic material was beneficial, as some archaic DNA was likely detrimental to modern humans and has been gradually removed by natural selection. This is evident in the depletion of Neanderthal and Denisovan ancestry near genes that are highly expressed in the testes. This suggests that some of these archaic gene variants may have reduced male fertility in human-archaic hybrids.
Scientific Detection and Modern Uses
Scientists detect introgression by comparing the genomes of different species and looking for segments of DNA that stand out. These methods search for DNA in one species that is more similar to another species than would be expected from normal inheritance. Statistical techniques, such as the D-statistic, can identify an excess of shared genetic variants between populations, which is a strong indicator of past gene flow. The length and structure of these shared DNA segments can also help estimate when the introgression event occurred.
The understanding of introgression has practical applications in agriculture. Plant breeders can intentionally cross crop plants with their wild relatives to introduce desirable traits like disease or drought resistance. This process, often called pre-breeding, uses hybridization and backcrossing to move specific genes from a wild species into a crop line. For example, genes from wild tomato species have been introgressed into cultivated tomatoes to improve their flavor and nutritional content. This approach allows for the development of more resilient and productive crops.