THE GENETIC mutations that cause evolution may arise at random. But natural selection often produces creatures that are similar, despite living in different environments. For example birds, bats, pterodactyls and insects all developed wings independently, because they were a nifty way to travel. This process is known as convergent evolution. Viruses evolve, too. SARS-COV-2, which causes covid-19, replicates while infecting its host. As it does, the virus's genetic information-a sequence of 30,000 RNA letters-is sometimes corrupted. These mutations can make SARS-COV-2 more dangerous in several ways. They can increase transmissibility, evade detection by tests, avoid immune responses (including from vaccines) and cause more severe illness. This makes tracking the evolution of SARS-COV-2 crucial. Of the 110m covid-19 cases found worldwide, scientists have sequenced and published the genomes of 600,000. By comparing these sequences and other viral characteristics, evolutionary biologists create phylogenetic trees-a set of hypothetical relationships between sequences which show how the virus has evolved over time.
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