If an allele exists currently at frequency X, and the selection pressure on it changes upwards, what should we expect? The frequency to increase. Of course it is possible for the frequency to decrease, and I made no comments on the variance of that expectation.
No. This doesn’t follow. Consider for example an allele that is normally recessive and in the homozygous case is nearly lethal. Such an allele will generally be pushed to a very low frequency. The only way that such an allele stays at a substantial fraction of the population is if it is has a constant influx of new copies (For example Huntington’s disease is sort of this way. The allele is dominant and extremely negative in that form, and is homozygous lethal, but Huntingon precursor alleles are constantly mutating into new cases of Huntington’s and the specific biochem of the allele in question makes this much more likely). Now, if an allele has no impact in the heterozygous case. As the allele becomes extremely rare, the selection pressure will drop more and more to the point where it becomes negligible. Now, consider what happens if we discover a cure for this very rare disease that occurs in the homozygous case, or that we make it much easier to survive. What should we expect to happen to the frequency in the population? We should expect it to stay roughly constant, because there’s no positive selection pressure.
In general, decreasing negative selection effects does not increase the frequency of an allele.
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.
What should we expect to happen to the frequency in the population? We should expect it to stay roughly constant, because there’s no positive selection pressure.
If there is still an influx of new copies due to mutation, then the frequency will increase because there’s now less selection pressure driving the mutations out.
Influx of new copies for most alleles is generally negligible for any specific allele. Examples like Huntington’s are extremely rare. The probability that any mutation will arise more than once in the population is generally extremely small. Standard genetic models often don’t even bother taking into account the chance that a mutation will be matched because the chance is so small.
No. This doesn’t follow. Consider for example an allele that is normally recessive and in the homozygous case is nearly lethal. Such an allele will generally be pushed to a very low frequency. The only way that such an allele stays at a substantial fraction of the population is if it is has a constant influx of new copies (For example Huntington’s disease is sort of this way. The allele is dominant and extremely negative in that form, and is homozygous lethal, but Huntingon precursor alleles are constantly mutating into new cases of Huntington’s and the specific biochem of the allele in question makes this much more likely). Now, if an allele has no impact in the heterozygous case. As the allele becomes extremely rare, the selection pressure will drop more and more to the point where it becomes negligible. Now, consider what happens if we discover a cure for this very rare disease that occurs in the homozygous case, or that we make it much easier to survive. What should we expect to happen to the frequency in the population? We should expect it to stay roughly constant, because there’s no positive selection pressure.
In general, decreasing negative selection effects does not increase the frequency of an allele.
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.
If there is still an influx of new copies due to mutation, then the frequency will increase because there’s now less selection pressure driving the mutations out.
Influx of new copies for most alleles is generally negligible for any specific allele. Examples like Huntington’s are extremely rare. The probability that any mutation will arise more than once in the population is generally extremely small. Standard genetic models often don’t even bother taking into account the chance that a mutation will be matched because the chance is so small.