4. The same mutations keep popping up Once the virus enters a cell, it begins to replicate. The more copies it makes, the greater the likelihood that random errors, or mutations, will crop up. Most of these copying errors are inconsequential. A handful, however, might give the virus a leg up. For example, a spike-protein mutation known as D614G appears to help transmission of SARS-CoV-2. Another, E484K, might help the virus evade the body’s antibody response. If the viruses carrying these advantageous mutations get transmitted from one person to the next, they can start to outcompete the viruses that lack them, a process known as natural selection. That’s how the B.1.1.7 variant, which is more transmissible, became the predominant strain in the US. In the case of SARS-CoV-2, the mutations that improve the virus keep popping up in different parts of the globe, a phenomenon known as convergent evolution. “We are seeing the same combinations evolving over and over and over again,” says Vaughn Cooper, an evolutionary biologist at the University of Pittsburgh. Imagine a game of Tetris, Cooper writes in a recent story for Scientific American. “A limited number of building blocks can be assembled in different ways, in different combinations, to achieve the same winning structures.” Cooper and some other researchers see this evidence of convergent evolution as a hopeful sign: the virus may be running out of new ways to adapt to the current environment. “It’s actually a small deck of cards right now,” he says. “If we can control infections, that deck of cards is going to remain small.”