From further email from Wendy Orent:
Natural selection works on a genetic and individual level, not a population level. When you are talking about viruses, think of a swarm of strains, some of which are going to be more effective under the particular conditions they find themselves in (a host, or group of hosts, under particular ecological conditions.)
These influenza strains (say) are all madly jockeying, so to speak, to outreproduce each other (of course, this intentionality is strictly metaphorical.) Now, let’s say we are talking about a population of wild ducks who are infested with low-path H5N1. If there is a wide range of strains within duck A, those strains best at exploiting that duck’s body will reproduce better and faster and more effectively than milder strains. So, in the competition to use up the duck, so to speak, MORE virulent strains will win out.
Now, here’s the thing. That duck is dead – wiped out, gone. But duck B, which happened to get a smaller or a milder set of strains, doesn’t die; he lives to pass whatever virus he is dealing with to ducks C and D. So those milder strains are going to win out – and spread through the duck population. It has nothing to do with equilibrium – only with the balance between within host and outside-host competition. You sometimes do find dead ducks in the wild, because natural selection is blind as a cavefish and can’t see what’s going to happen a duck or so down the road. If you get a mutant that increases virulence, that will put virulent strains at a temporary advantage. But that virulent strain won’t spread – that’s why Ewald speaks of the “sieve of natural selection” when he talks about flu in wild migrating birds.
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Change the conditions, and you change the equation – that’s the point of “disease factory” conditions – you remove the penalties on viruses for being virulent.
Post edited by: martin, at: 2006/02/06 00:13