Genetic abnormalities occur in all species, including insects. Each generation, the genes of those individuals that do not mate are eliminated from the gene pool. Over time, a phenotype best adapted to its environment in all respects is generated. Once a well adapted phenotype is established, mutations produce a phenotype that is less well adapted. These mutations are rarely inherited, must be generated anew and are present in low numbers.
Butterfly color patterns are consistent because they are essential to mate recognition, mating success and for defense in crypsis or aposematism. A radical change in coloration produces an individual with low probability of survival and reproduction. Collectors of insects prize mutants because they are so rare.
This weekend, I received the accompanying photo of a swallowtail butterfly with unusual markings that do not match the markings of any of our swallowtails in eastern North America. This is a mutant, possibly of the Giant Swallowtail or perhaps a polygenes/zelicaon mutant or hybrid. My first reaction was Papilio cresphontes, the Giant Swallowtail that is not expressing the black color in the middle of the wing. OTOH, it has some of the forewing markings of the Anise Swallowtail, Papilla zelicaon, although the hind wing markings don’t fit. The specimen was not collected and I have only this photo which makes investigation difficult. If anyone would like to weigh in on this specimen, comments are welcome.