Partial protection from fluctuating selection leads to evolution toward wider population size fluctuation and a novel mechanism of balancing selection

Classical theory predicted that natural selection favors a variant causing smaller fluctuation of population density. It is also known that if population is partially protected from fluctuating selection, as in the case of seed bank, the variance of fitness is further reduced and therefore the reproductive success of population is ensured. This study, exploring a mathematical model for coupled demographic and evolutionary dynamics, finds that such a ‘refuge’ from fluctuating selection even causes positive selection for a variant increasing the amplitude of population size fluctuation under weak or moderate regulation of population density. Under strong density regulation and constant carrying capacity, long-term maintenance of polymorphism known as the storage effect emerges. However, if the carrying capacity is changing either cyclically or randomly and the density regulation is strong, variants whose fitness fluctuates in phase with population size are positively selected to either fixation or oscillation at intermediate frequencies. The latter dynamics, arising when fitness fluctuates as expected under a simple life-history trade-off, is a novel form of balancing selection. These results highlight the importance of allowing in models the joint demographic and population genetic changes, the failure of which prevents the discovery of important and novel eco-evolutionary dynamics.