SELECTION VERSUS RANDOM DRIFT - LONG-TERM POLYMORPHISM PERSISTENCE INSMALL POPULATIONS (EVIDENCE AND MODELING)

Our data on a subterranean mammal, Spalax ehrenbergi, and other eviden ce, indicate that appreciable polymorphism can be preserved in small i solated populations consisting of several dozens of, or a hundred, ind ividuals. Current theoretical models predict fast gene fixation in sma ll panmictic populations without selection, mutation, or gene inflow. Using simple multilocus models, we demonstrate here that moderate stab ilizing selection (with stable or fluctuating optimum) for traits cont rolled by additive genes could oppose random fixation in such isolates during thousands of generations. We also show that in selection-free models polymorphism persists only for a few hundred generations even u nder high mutation rates. Our multi-chromosome models challenge the hi tchhiking hypothesis of polymorphism maintenance for many neutral loci due to close linkage with few selected loci.