Wandering genes

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High in the Himalayas, the people of Tibet have adapted to survive the challenges of living at altitude. In the rarefied atmosphere at the top of the world, Tibetans have a modified version of the EPAS1 gene that helps them to extract the limited oxygen available in the air much more efficiently than those of us who have spent our lives at sea level. On the face of it, this looks like a classic case of human evolution. A randomly generated mutation in EPAS1 gave an ancestor of Tibetans enough of an advantage that they were able to pass on more of their beneficial EPAS1 genetic variants to subsequent generations than those without the mutation. Over time the modified version of EPAS1 became more and more common across Tibet as those Tibetans with the mutation survived longer and procreated more than those without.

Evolution by natural selection needs genetic variation to act, and the prevailing view is that this variation either arises anew within an individual and spreads in a population under some sort of environmental pressure, or that neutral variation already present in a population becomes beneficial as individuals enter a different environment.

But this is not the case in Tibetans and high altitude. The modified EPAS1 in Tibetans is not the result of a genetic mutation appearing in their ancestors. Well, not in their human ancestors anyway. Comparisons between the DNA of Tibetans and the now extinct hominins Neanderthals and Denisovans, have shown that the modified EPAS1 gene in Tibetans was in fact inherited thanks to mating in Asia between the ancestors of Tibetans and Denisovans, a species closely related to but distinct from our own Homo sapiens lineage. So Tibetans are able to live at high altitude today thanks to a tryst between their ancestors and a closely related sister species, that brought the adaptation from another species into our own.

This process, known as adaptive introgression, has long been known to occur in plant species and is increasingly becoming viewed as an important evolutionary process that has occurred in the history of many species, including our own.

I find this concept fascinating. This project is all about documenting the evidence supporting the new and exciting view that evolution is shaped not just through the generation of mutations within a species’ own lineage, but through the introduction of new beneficial genetic variation through hybridisation and introgression events. Thanks to analyses of the enormous amounts of genetic data that are now being generated, adaptive introgression is being discovered across the tree of life, from malaria parasites, and their mosquito vectors, to butterflies, birds, people and maize, highlighting its key and previously underappreciated role in the evolution of life on earth. Genes may be selfish, but they also wander amongst the tree of life.