Modification of Dominance

Numerous experiments have shown that dominance relations can be changed experimentally by substitution of the genetic background or by selection. Therefore, certain genes have the ability to modify the character expression, and such genes seem to be abundant.

Most empirical evidence for evolutionary modification of dominance concerns wing patterns in lepidopteran species, mimicry in butterflies, and pesticide resistance. A classical example is provided by industrial melanism in the peppered moth Biston betularia, a lepidopteran species. During the second half of the nineteenth century, dark forms of this usually light-colored insect became more common in industrial areas and within about 50 years nearly replaced the pale form. One of the reasons why the melanic form gained a selective advantage was that the tree trunks on which these butterflies rested became blackened due to pollution which killed light lichen, and the pale form, originally cryptically colored, became better visible to their predators. Interestingly, the dark form became dominant during this process. Extensive experiments by H. B. D. Kettlewell in the 1950s and 1960s suggest that selection for an unlinked modifier occurred during the spread of the favorable mutant. Later, similar experiments were performed in related species with slightly different outcomes, which may be explainable by linked modifiers that, initially, had a different frequency as in B. betularia. Since the ecological setting is complex, other selective forces as well as migration-selection balance may also have played an important role. In all of these cases it seems that an originally recessive or intermediate allele became more or less dominant.

A quite different situation was observed in some cases of mimicry by P. M. Shepard, C. A. Clarke, and others in the 1950s and 1960s. There, frequency-dependent selection maintains overdominance, and one of the homozygous phenotypes is modified to resemble the heteroyzgotes.

Another well-studied class of examples for dominance evolution concerns insecticide-resistance genes. Often the genes causing resistance, and sometimes even those causing dominance, are known. Since pesticide applications are heterogeneous in space, and alleles conferring an advantage in treated areas tend to be disadvantageous in untreated areas, underdominance is often maintained by migration-selection balance.

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