Haldane’s work was referenced by Kimura as support for the neutral theory of evolution, specifically the rather long time it took, about 300 generations, for a beneficial trait in a mammalian population (like cattle) to fix. The wiki entry is on Haldane’s dilemma:
Surely one generation can “fix” all sorts of traits, just kill off every cow except a founding couple, and repeat the killing process enough through several generations, and lots of traits will fix simply by drift rather than by selection in less than 300 generations. The idea being that traits get fixed by increasing the speed of drift by having small populations for a protracted time.
Alternatively, suppose there is a single trait that resides in one male and one female, and this trait is strongly selected for (like say resistance to an epidemic). It will have comparable effects to that described in the previous paragraph by making it easier to “fix” other traits that may not have had much if any selective favorable traits at all previously. That is to say, even though these other traits were previously deleterious, they rode the train of a strongly favorable trait that suddenly emerged. I call this a selection interference scenario. I don’t know if that’s the proper term.
Alternative, simple acts of God or natural disasters could effect similar outcomes in helping to fix traits.
My guess is that Haldane modeled the population with weakish selection so that one beneficial trait wouldn’t interfere with the growth of another beneficial trait – that is to say, to limit selection interference, or maybe to sustain the population size, or both. I’m trying to understand what the dilemma is.
Assuming substitution of genes to take place slowly, one gene at a time over n generations, the fitness of the species will fall below the optimum (achieved when the substitution is complete) by a factor of about 30/ n , so long as this is small – small enough to prevent extinction. Haldane doubts that high intensities – such as in the case of the peppered moth – have occurred frequently and estimates that a value of n = 300 is a probable number of generations. This gives a selection intensity of 0.1.
Is that selection intensity the same as the s-coefficient?
regarding seleciton intensity, I, From wiki:
I = ln (s0/S)