EVOLUTION OF WARFARIN TOLERANCE IN RATS

    Warfarin is a poison that has been used control rat populations in Brittain since the early 1950's.  Soon after initial use, rat populations began to exhibit genotypes that were resistant to the adverse effects of warfarin.  Genetic studies have indicated that a single mutation can confer resistance.  The graph below illustrates the evolution of warfarin resistance in a population in Wales.  Note that soon after intensive warfarin poisoning in 1976 and 1977 (arrows), the frequency of resistance increased dramatically.  This change is consistent with the observation that in the presence of poison, genotypes carrying the resistant allele have higher fitness than genotypes not carrying the allele.  When warfarin poisoning was suspended, the number of resistant individuals declined precipitously.  This decline is expected because in the absence of warfarin, the resistant mutant is deleterious because it interferes with vitamin K metabolism.  Under these conditions, genotypes carrying the mutant allele have lower fitness than those not carrying it.
 

Figure from D. J. Futuyma.  1998.  Evolutionary Biology.  Sinauer, Sunderland, MA.

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