It's a little more complicated. There's 2 kinds of mutations that happen for whatever reason - those that affect fitness and those that don't.Sensible Stephen wrote:I might be talking shit. Just what I have read. If Bindi disagrees then I have got it wrong.shanky wrote:OK
Even for very compact genomes like viruses have, it's easy enough to change a nucleotide here and there with no effect. This is due to how RNA is translated to protein. It takes 3 nucleotides (the RNA letters) to code for one amino acid. A chain of amino acids is a protein. There's a lot of redundancy in the 3rd RNA letter, so if it changes, you still likely get the same amino acid and therefore the same protein. When you read about how the origin of coronavirus isolates can be tracked, it's generally due to these kinds of mutations, which are happy to propagate as they aren't good or bad. This is how they could tell most of the isolates in the US had a European origin. It's probably how they'll work out where the new NZ outbreak originates from.
Mutations that affect fitness aren't evenly distributed across the genome. In coranaviruses generally (most don't infect humans), the biggest gene which helps it replicate barely changes - it already performs its job extremely well. The gene that changes the most is the spike protein. This is the one most vaccines are targeting and it's the one that allows it latch on to new cells and new organisms. Huge impact on fitness if the virus gains a new host.
As far as the coronavirus becoming less lethal, it's not necessarily the case. It spreads quite happily before symptoms show and it doesn't kill most people who get it, so being less lethal wouldn't have huge impact in a short time-frame (whereas the original SARS didn't have these advantages and died out on its own). There also has to be a functional way to get to whatever increases fitness. A mutation that makes it less lethal but less able to replicate is not helpful. Changes in the spike protein are a concern though - could stop vaccines working.
Anyway, all this stuff makes viruses interesting to study.