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Note. Similar differences have been recorded in and Grant et al, 1995, 1996).

the United States (see text

Industrial Melanism in Reverse

In the 1950s, anti-pollution legislation was introduced in industrial countries on both sides of the Atlantic. This legislation led to declines in both sulfur dioxide and particulate soot emissions. Subsequently, the frequencies of the melanic forms have declined considerably in industrial regions in Britain (Table I). Current rates of decline are broadly in line with theoretical predictions using computer simulations. If the decline in carbonaria continues at its present rate, this form will be reduced in Britain to the status of a rare mutation by 2020. A similar decline in the frequency of the melanic form (f. swettaria) of the American subspecies of the peppered moth, B. betularia cognataria, has occurred in some parts of the United States.

Data on the declines of carbonaria in Britain and swettaria in America are important for three reasons. First, they show that evolution is not a one-way process. Evolutionary changes can be reversed if the selective factors that lead to them are reversed. Second, the data sets from different populations in Europe and America are, in effect, replicate natural experiments. The consistency in the patterns of increase and decrease in the frequencies of melanic forms correlated to pollution levels adds weight to the selective explanation of the evolutionary changes observed. Third, the accord between predicted decreases in melanic frequency and the observed frequency currently being obtained argues that the factors incorporated into the models are broadly correct.

Other Examples of Full Industrial Melanic Polymorphism

The case of the peppered moth is not unique. A small number of other examples of full industrial melanic polymorphism are known. The melanic forms in most of these cases are controlled by dominant alleles of single genes.

FIGURE 5 Nonmelanic and melanic forms of the brindled beauty, L. hirtaria.

An exception is that of the brindled beauty, Lycia hirtaria, in which the melanic form nigra (Fig. 5) is controlled by a recessive allele. That most recent melanic forms are genetically dominant is not surprising because a dominant mutation will be fully expressed as soon as it arises and will quickly be favored by selection if advantageous. Recessive mutations would not be exposed to selection until they occurred in homozygotes, in which their effects would be expressed phenotypically.

In some species showing full industrial melanic polymorphism, such as the lobster moth, Stauropus fagi (Fig. 6), melanism developed at roughly the same time as in the peppered moth. In others, industrial melanism has developed much more recently, as in the cases of the sprawler, Brachionycha sphinx, and the early grey, Xylocampa areola, in which industrial melanism developed only in the second half of the 20th century. The reason that industrial melanism did not evolve earlier in these species is probably serendipitous: a melanic mutation simply did not occur previously in an appropriate population.

The different timings of the initial occurrence of industrial melanics of different species help emphasize that natural selection cannot cause change unless phenotypic variation

1/2x

FIGURE 6 Nonmelanic and melanic forms of the lobster moth, S. fagi.

FIGURE 6 Nonmelanic and melanic forms of the lobster moth, S. fagi.

exists. This is manifest in the oak beauty moth, Biston strataria, the closest British relative of the peppered moth. The oak beauty has a melanic form, melanaria, which is a common industrial melanic in Holland, but has never been recorded, except as a rare mutation, in Britain. In terms of its ecology, behavior, and distribution, the oak beauty is similar to the peppered moth. However, the melanaria mutation seems never to have arisen in Britain in favorable circumstances nor has this form reached Britain from continental Europe as a migrant. Melanism in the oak beauty in Europe can be contrasted with that of another moth, the figure of eighty, Tethea ocularis. The melanic form, fusca, of this species was known in Belgium and Holland in the early part of the 20th century, but was absent from Britain. This form arrived in southern England, by migration, in the mid-1940s. Following its arrival, f. fusca spread to many industrial parts of Britain and increased in frequency rapidly, although its frequency is now declining again in response to reductions in pollution.

The current declines in melanism seen in the peppered moth, the figure of eighty, and several other species, following anti-pollution legislation suggest that future studies of industrial melanism may have to shift to countries in which industrialization is still increasing and anti-pollution measures are as yet limited.

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