Usually, we only see the outcome of natural selection after hundreds or thousands of years. In a new @NatureEcoEvo study, researchers were able to watch it almost in real time. They even know the date of the event that set it in motion: March 11, 2011, the day of the devastating Tohoku tsunami in Japan. 🌊🦈 🧬 🇯🇵 The tsunami created new freshwater ponds and brought together two stickleback species, one adapted to freshwater and the other to marine life. Just one year later, 38% of sampled fish were hybrids. But within about ten generations, almost all marine ancestry had disappeared from the population. The process happened in layers. Marine alleles disappeared especially fast around loci involved in hybrid male sterility, freshwater adaptation, migration and reproductive isolation. Selection first removed some of the strongest barriers and then continued across much of the genome. And some marine alleles may have literally swum out of the population. Fish carrying marine ancestry were more likely to migrate toward the sea and leave the freshwater habitat. So ancestry could disappear not only because its carriers survived less well, but also because they simply left. Even after the strongest loci were removed, marine ancestry continued to decline. Simulations showed that genetic drift alone was not enough to explain this. One possible explanation is hundreds or thousands of weak Dobzhansky–Muller incompatibilities spread across the genome. But not every marine region disappeared. A few survived and some may even have been beneficial, although the evidence for adaptive introgression is still limited. Similar patterns were also found in other populations created by the tsunami. So tsunami first broke the barrier between two species and triggered massive hybridization. The genome removed this admixture layer by layer. It is a remarkable example of how even after strong hybridization, natural selection can push genomes back toward established species boundaries in just a few generations! nature.com/articles/s41559-0… #selection #hybridization #tsunami #adaptation