The increase in phenotypic variance that occurs in some populations as
a result of bottlenecks and founder events can cause a dramatic incre
ase in the probability of a peak shift from one adaptive state to anot
her. Periods of small population size allow drift in the amount of phe
notypic variance. Increases in phenotypic variance, coupled with a con
stant individual fitness function with multiple peaks, can cause the m
ean fitness landscape to change from bimodal to unimodal, thereby allo
wing the population's mean phenotype to change deterministically by se
lection. As the amount of phenotypic variance is returned to an equili
brium state, the multiple peaks reemerge, but the population has moved
from one stable state to another. These variance-induced peak shifts
allow punctuational evolution from one peak to another at a rate that
can be much higher than that predicted by Wright's shifting-balance pr
ocess alone.