Peppered Moth Evolution - Genetics

Genetics

Evolution is defined as "a change in the frequency of an allele within a gene pool", an occurrence that causes a population's genetically inherited traits to change over successive generations. Evolution in the wild is chiefly caused by two mechanisms: natural selection, the process by which individual organisms with beneficial traits are more likely to survive and reproduce, and genetic drift, the statistical drift over time of allele frequencies in a population from random sampling effects in the formation of successive generations.

J.W. Tutt first proposed the "differential bird predation hypothesis" in 1896, as a mechanism of natural selection. The melanic morphs were better camouflaged against the bark of trees without foliose lichen, whereas the typica morphs were better camouflaged against trees with lichens. As a result, birds would find and eat those morphs that were not camouflaged with increased frequency.

In 1924, J.B.S. Haldane calculated, using a simple general selection model, the selective advantage necessary for the recorded natural evolution of peppered moths, based on the assumption that in 1848 the frequency of dark-coloured moths was 2%, and by 1895 it was 95%. The dark-coloured, or melanic, form would have had to be 50% more fit than the typical, light-coloured form. Even taking into consideration possible errors in the model, this reasonably excluded the stochastic process of genetic drift, because the changes were too fast. Haldane's statistical analysis of selection for the melanic variant in peppered moths became a well known part of his effort to demonstrate that mathematical models that combined natural selection with Mendelian genetics could explain evolution — an effort that played a key role in the foundation of the discipline of population genetics, and the beginnings of the modern evolutionary synthesis.

In peppered moths, the allele for dark-bodied moths is dominant, while the allele for light-bodied moths is recessive, meaning that the typica moths have a phenotype (visible or detectable characteristic) that is only seen in a homozygous genotype (an organism that has two copies of the same allele), and never in a heterozygous one. This helps explain how dramatically quickly the population changed when being selected for dark colouration.

The peppered moth Biston betularia is also a model of parallel evolution in the incidence of melanism in the British form (f. carbonaria) and the American form (f. swettaria) as they are indistinguishable in appearance. Genetic analysis indicates that both phenotypes are inherited as autosomal dominants. Cross hybridizations indicate the phenotypes are produced by isoalleles at a single locus.