From the outset multiple causes have been suggested for changes in melanic gene frequency in the peppered moth
Biston betularia and other industrial melanic moths. These have included higher intrinsic fitness of melanic forms and selective
predation for camouflage. The possible existence and origin of heterozygote advantage has been debated. From the 1950s, as a
result of experimental evidence, selective predation became the favoured explanation and is undoubtedly the major factor
driving the frequency change. However, modelling and monitoring of declining melanic frequencies since the 1970s indicate
either that migration rates are much higher than existing direct estimates suggested or else, or in addition, non-visual selection
has a role. Recent molecular work on genetics has revealed that the melanic (carbonaria) allele had a single origin in Britain,
and that the locus is orthologous to a major wing patterning locus in Heliconius butterflies. New methods of analysis should
supply further information on the melanic system and on migration that will complete our understanding of this important
example of rapid evolution.