I suspect I’m being unclear. I’m not discussing a state where we have good knowledge of the underlying mechanics, but one where we have some original frequency of a heritable condition, and then we make people with that condition / their relatives more likely to procreate than they were before. The equilibrium has shifted, and it has shifted upwards. We don’t need to know the strength of the selection pressures (positive and negative) or their mechanisms to make that prediction; we just know that the scales were probably balanced before, and we pulled some weight off of one side. The scales should tip away from the side we pulled weight off of.
Yes, you are being clear, and this doesn’t follow. It might help to reread my example. If we reduce a negative selection pressure it doesn’t mean that things will shift. In the example I gave there’s no real equilibrium, the allele just gets to stay under the radar of evolution because it is so rare evolution doesn’t get a chance to act on it. (This is by the way a well-known ev-bio issue, that bad recessive alleles can easily stay at low levels in a population.) Making the allele have a less negative selection pressure won’t necessarily change that state. If the pressure is moved to close to zero then one then expects neutral drift to occur as usual which can move things up or down, and if the pressure is still negative then it should stay about where it is unless neutral drift moves it a bit downwards.
I suspect I’m being unclear. I’m not discussing a state where we have good knowledge of the underlying mechanics, but one where we have some original frequency of a heritable condition, and then we make people with that condition / their relatives more likely to procreate than they were before. The equilibrium has shifted, and it has shifted upwards. We don’t need to know the strength of the selection pressures (positive and negative) or their mechanisms to make that prediction; we just know that the scales were probably balanced before, and we pulled some weight off of one side. The scales should tip away from the side we pulled weight off of.
Yes, you are being clear, and this doesn’t follow. It might help to reread my example. If we reduce a negative selection pressure it doesn’t mean that things will shift. In the example I gave there’s no real equilibrium, the allele just gets to stay under the radar of evolution because it is so rare evolution doesn’t get a chance to act on it. (This is by the way a well-known ev-bio issue, that bad recessive alleles can easily stay at low levels in a population.) Making the allele have a less negative selection pressure won’t necessarily change that state. If the pressure is moved to close to zero then one then expects neutral drift to occur as usual which can move things up or down, and if the pressure is still negative then it should stay about where it is unless neutral drift moves it a bit downwards.