Laura Spinney in The Guardian:
Letting the virus that causes Covid-19 circulate more-or-less freely is dangerous not only because it risks overwhelming hospitals and so endangering lives unnecessarily, but also because it could delay the evolution of the virus to a more benign form and potentially even make it more lethal.
Though the data is still sketchy and the measures crude, this effect may already be influencing the difference in death rates between Sweden – which took a relaxed approach to containment until recently – and Norway, whose measures have been much stricter. Sweden has more than three times as many deaths per 100 cases as its neighbour.
The explanation for this startling gap may lie partly in natural selection, and the biological arms race between a pathogen and its host. Within any population, there is genetic variation. Viruses are no different. Some versions of the virus will be very slightly more dangerous to human health – more virulent – others less so. If the conditions are right, the slightly more virulent ones will begin to predominate and cause more damage.