Might help if you look at fixation as a property of population size. (Different groups that don’t have contact with each other, can vary in size and thus fixation difficulty/speed. Smaller populations have stuff reach fixation faster (positive and negative*).)
*I haven’t heard a lot about interactions between genes, and how ‘positive/negative’ a gene is is a function of what other genes you have (and environment). (For some, what others have is also relevant, and I have heard about one of those.)
Why shouldn’t the effects then cancel out?
This is about a baseline. When you consider that the baseline takes for granted a certain level of performance, then, maybe there are beneficial mutations which are taken for granted now, but took time to develop. (i.e., what a species looks like now, can be very different from what came before, if you go back far. Change can be slow...though, so ‘a long time ago’ might not be as long for, say, flies.)
Might help if you look at fixation as a property of population size. (Different groups that don’t have contact with each other, can vary in size and thus fixation difficulty/speed. Smaller populations have stuff reach fixation faster (positive and negative*).)
*I haven’t heard a lot about interactions between genes, and how ‘positive/negative’ a gene is is a function of what other genes you have (and environment). (For some, what others have is also relevant, and I have heard about one of those.)
This is about a baseline. When you consider that the baseline takes for granted a certain level of performance, then, maybe there are beneficial mutations which are taken for granted now, but took time to develop. (i.e., what a species looks like now, can be very different from what came before, if you go back far. Change can be slow...though, so ‘a long time ago’ might not be as long for, say, flies.)