Peppered moth alternative theories
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Several alternative theories to explain industrial melanism, particularly noted in the peppered moth were proposed during the 1920s and 1930s. Some dissenters within the scientific community have criticised the peppered moth story, notably Sargent et al (1998), but the peppered moth researchers remain unconvinced.
Alternative selection mechanisms
Selection is differential
Several alternative selection mechansisms have been proposed. Note again that evolution can be defined as "a change in an allele frequency within a gene pool" (Dobzhansky, 1937). This change, be it caused by natural selection, mutation, migration or genetic drift by definition, is differential (see evolutionary differentiation). However, the magnitude of the changes observed can only be accounted for by natural selection. It can be seen from population genetics that a non-differential change will not cause evolution. If the allele frequencies are denoted by the algebraic terms p and q, and (say) p = 0.6 and q = 0.4, then a non-differential reduction in population size from say 2000 to 100 individuals, will still produce the same values of (approximately) p = 0.6 and q = 0.4.
The Phenotypic Induction Hypothesis
John William Heslop-Harrison (1920) rejected Tutt's differential bird predation hypothesis, on the basis that he did not believe that birds ate moths. Instead he advocated the idea that pollutants could cause changes to the soma and germ plasm of the organism. The origin of this hypothesis probably has its roots in the 1890s, when it was proposed as a form of Lamarckism. It is important to note its historical context.
The first to try to prove this hypothesis was Hasebroek (1925), who contended that air pollution altered lepidopteran physiology, thus producing an excess of black pigment. He exposed pupae of Lepidoptera to various doses of pollutant gases, namely hydrogen sulphide (H2S), ammonia (NH3) and "pyredin" (presumably his spelling of pyridine). He used eight species in his studies, four of which were species of butterfly that did not exhibit melanism. Ford (1964) contends that Hasebroek's illustrations showed that the abnormal forms that appeared were not melanics, and Hasebroek failed to study their genetics.
Heslop Harrison (Harrison and Garrett 1926; Harrison 1928) suggested that the increase of melanic moths in industrialised regions was due to "mutation pressure", not to selection by predators which he regarded as negligible. Salts of lead and manganese were present in the airborne pollutant particles, and he suggested that these caused the mutation of genes for melanin production but of no others. He used Selenia bilunaria and Tephrosia bistortata as material. The larvae were fed with leaves that had incorporated these salts and melanics subsequently appeared.
Similar experiments by Hughes McKenney (1932) and Thomasen and Lemche (1933) failed to replicate these results. However, the statistician and geneticist Ronald Fisher, showed that Heslop Harrison's controls were inadequate (Fisher, 1933). This theory however appeared to be falsified by breeding experiments. Further evidence, if it were needed, is likely to come from research into the biochemistry of melanism.
| Ecology | Genetics | Evolution | Taxonomy | Predation experiments |
| Researchers: Bernard Kettlewell (The Evolution of Melanism) | Mike Majerus (Melanism: Evolution in Action) | Laurence Cook | Cyril Clarke | Bruce Grant | E.B. Ford | Philip Sheppard J.W. Tutt |
| Alternative theories: Craig Millar | Ted Sargent |
| Creationism: Jonathan Wells (Icons of Evolution) | Judith Hooper (Of Moths and Men) |
| References |