‘Nothing makes sense in biology, except in the light of evolution,’ the splendidly named biologist Theodosius Dobzhansky wrote in 1973. It’s a good rule of thumb. Despite near-miraculous advances in medical science we remain biological beings, subject to biological laws. None is more central to our understanding of disease than evolution. Yet this theory remains poorly understood and poorly utilised in medicine. And an evolutionary perspective raises important questions about the drastic action we have been taking to confront Covid-19.
Most doctors are too busy dealing with the day-to-day deluge of cases to have much time for what they may consider abstruse academic ideas. I can see why: it’s hard enough to understand diseases, let alone think about how they might be changing as we watch. But we know a lot about viruses and their evolution, and this perspective seems relevant (if largely overlooked) now.
So far, in most commentary on this pandemic, it has been assumed that we are dealing with an unchanging threat. Sars-CoV-2 is a new type of virus that is attacking people, causing a disease called Covid-19. The extent of the threat is constant and can be assessed by examining what happened in places where the pandemic is ‘further ahead’. We know that a certain proportion of patients develop mild disease. Others suffer a much more severe infection. These figures are assumed to be a consistent feature of the new virus: it affects some worse than others.
But an evolutionary approach suggests a different view. What if there are different versions of this coronavirus — some less deadly than others? All viruses mutate. Typically, a rapidly spreading virus tends to become less harmful as it changes because this helps it survive. If the virus kills too many of its hosts, it is more likely to die out. If it becomes milder, it spreads further.