Polymorphisms for melanic form of insects may provide various selective advantages.However,melanic alleles may have significant/subtle pleiotrophic "costs." Several potential pleiotrophic effects of the W (=Y)-linked melanism gene in Papilio glaucus L.(Lepidoptera) showed no costs for melanic versus yellow in adult size,oviposition preferences,fecundity,egg viability,larval survival/growth rates,cold stress tolerance,or postdiapause emergence times.Sexual selection (males choosing yellow rather than mimetic dark females) had been suggested to provide a balanced polymorphism in P.glaucus,but spermatophore counts in wild females and direct field tethering studies of size-matched pairs of virgin females (dark and yellow),show that male preferences are random or frequency-dependent from Florida to Michigan,providing no yellow counteradvantages.Recent frequency declines of dark (melanic/mimetic) females in P.glaucus populations are shown in several major populations from Florida (27.3°N latitude) to Ohio (38.5° N).Summer temperatures have increased significantly at all these locations during this time (1999-2018),but whether dark morphs may be more vulnerable (in any stage) to such climate warming remains to be determined.Additional potential reasons for the frequency declines in mimetic females are discussed:(i) genetic introgression of Z-linked melanism suppressor genes from P canadensis (R & J) and the hybrid species,P appalachiensis (Pavulaan & Wright),(ii) differential developmental incompatibilities,or Haldane effects,known to occur in hybrids,(iii) selection against intermediately melanic ("dusty") females (with the W-linked melanic gene,b+) which higher temperatures can cause.