The probability of peak shifts in a founder population

We have analysed the role of sampling drift in inducing shifts between alternative adaptive peaks, in small and rapidly growing populations. Using a simple model of disruptive selection on a polygenic character, we calculate the net probabilityofapeakshift. If the growth rate is high, theprobabilityofashiftina growing population is insensitive to selection on the character. Assuming that the character is effectively neutral during the brief initial increase, we find that theprobabilityofapeakshift is given by theprobabilityof finding a standard normal variate greater than √2ΔV where ΔV is the reduction in additive genetic variance during the growth period. This result holds for arbitrary pattern of increase in size, provided that the rate of increase is high enough for selection to be negligible, and the character depends on a large number of loci. Comparing theprobabilityofpeakshiftsin founding populations with the rate ofshiftsin static and allopatric populations it appears that although strongly selected shifts are only likely to occur ina growing population, a static population is a more congenial setting for adaptive shifts.