the force of selection can be much stronger than was thought by the early population geneticists. the resistance to pesticides has grown quickly. resistance to "warfarin" in norway rats ("rattus norvegicus") grew rapidly because those that survived made up more and more of the population. research showed that, in the absence of "warfarin", the resistant homozygote was at a 54% disadvantage to the normal wild type homozygote.p182 this great disadvantage was quickly overcome by the selection for "warfarin" resistance.