The peppered moth remains the type example of rapid response to
human-induced environmental change, driven by selective predation
(Grant, 2012). Some of the issues with which it was once associated
now receive less attention. Kettlewell’s (1958) ‘ancient carbonaria’
remain to be properly examined at the genetic level, but there is
evidence that modification of expression at the carbonaria locus may
occur (see Grant (2004)). The question why melanic forms in so
many species are dominant still needs an agreed answer. Dominance
modification continues to be of interest in connection with the
evolution of genetic systems (Mayo and Bu¨rger, 1997; Bagheri, 2006).
HA is not necessary to explain the changing gene frequency in the
peppered moth, but there are other species where it could be
important.
Breeding results suggest that homozygous melanic peppered moths
may have a non-visual advantage, which, if true in the wild, would
rule out HA. Creed et al. (1980) noted that the data are uncontrolled,
and only part of the total data set is critical in establishing fitness
difference. Further work is certainly needed, but if there really is a
difference, melanics would have increased rapidly in polluted industrial
regions with even a quite small visual advantage once sufficient
melanic homozygotes had accumulated. At the end of the industrial
period, there must correspondingly be strong visual disadvantage to
overcome the non-visual fitness effect unless there is high migration.
The following possibilities then exist to explain why rural areas
continued to have low melanic frequencies: