When Veron hypothesised reticulate evolution in corals, he had advantages that Haeckel and Darwin did not. He knew about what is called the Modern Synthesis of Evolution, which holds that, along with natural selection, factors such as genetic mutation and genetic drift play major roles in evolution. But Veron’s hypothesis came from observations and mind experiments, not genetic data.
Since Veron’s eureka moment, genetic tools have become more widespread and sophisticated. Does data support the hypothesis that species don’t just separate, they also merge? The answer is a resounding yes.
‘It’s not just rare freaks or accidents, it’s happening all the time. And in quite divergent species too,’ said Nielsen. Roving genes have been found in every branch of the tree of life where geneticists have looked. Today, the technical terms for the process of genes moving between populations are introgression or admixture.
Introgression occurs in plants such as maize and tomatoes. In mosquitoes, the entire genome except for the X chromosome can be swapped with other species. In a tropical genus of butterfly called Heliconius, gene jumping has been found to cause critical changes in the patterns of their colourful wings. Introgression has been documented in finches, in frogs, in rabbits, in wolves and coyotes, in swine, in yaks and cows, in brown bears and polar bears. And in us.
Nielsen and his colleagues found that Tibetans (and a few Han Chinese) carry a very beneficial gene called EPAS1. The protein EPAS1 gives a boost to haemoglobin, the molecule that ferries oxygen in our blood. EPAS1 makes high-altitude living easier. In 2014, the researchers discovered that the EPAS1 gene was also in the DNA of an extinct group of humans called Denisovans, known from bone fragments in Siberia and Tibet.
The prevailing hypothesis is this: ancient humans left Africa moving northward along temperate plains. When they encountered the Himalayas and their cold, high altitudes, it literally took their breath away. Those oxygen-poor conditions should have kept humans near the base of the mountains. But the ancient humans also encountered Denisovans and interbred with them, receiving the EPAS1 gene. Only those humans with the EPAS1 gene moved up the mountains, and their offspring also carried the EPAS1 gene, giving the ancestors of today’s Tibetans a critical advantage at higher altitudes.
‘I think that process of splitting up and merging back together again, and getting a bit of DNA from here to there, that’s happening all the time, in all of the tree of life,’ Nielsen said. ‘And it’s really changing how we’re thinking about it, that it really is a network of life, not a tree of life.’
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