Variation within a species with manmade selection pressures

It is, perhaps, not surprising that some of the most dramatic examples of local specialization within species (indeed of natural selection in action) have been driven by manmade ecological forces, especially those of environmental pollution. These can provide rapid change under the influence of powerful selection pressures. Industrial melanism, for example, is the phenomenon in which black or blackish forms of species have come to dominate populations in industrial areas. In the dark individuals, a dominant gene is typically responsible for producing an excess of the black pigment melanin. Industrial melanism is known in most industrialized countries and more than 100 species of moth have evolved forms of industrial melanism.

Variation Within Species

Figure 1.5 Sites in Britain where the frequencies of the pale (forma typica) and melanic forms of Biston betularia were recorded by Kettlewell and his colleagues. In all more than 20,000 specimens were examined. The principal melanic form (forma carbonaria) was abundant near industrial areas and where the prevailing westerly winds carry atmospheric pollution to the east. A further melanic form (forma insularia, which looks like an intermediate form but is due to several different genes controlling darkening) was also present but was hidden where the genes for forma carbonaria were present. (From Ford, 1975.)

Figure 1.5 Sites in Britain where the frequencies of the pale (forma typica) and melanic forms of Biston betularia were recorded by Kettlewell and his colleagues. In all more than 20,000 specimens were examined. The principal melanic form (forma carbonaria) was abundant near industrial areas and where the prevailing westerly winds carry atmospheric pollution to the east. A further melanic form (forma insularia, which looks like an intermediate form but is due to several different genes controlling darkening) was also present but was hidden where the genes for forma carbonaria were present. (From Ford, 1975.)

The earliest recorded species to evolve in this way was the peppered moth (Biston betularia); the first black specimen in an otherwise pale population was caught in Manchester (UK) in 1848. By 1895, about 98% of the Manchester peppered moth population was melanic. Following many more years of pollution, a large-scale survey of pale and melanic forms of the peppered moth in Britain recorded more than 20,000 specimens between 1952 and 1970 (Figure 1.5). The winds in Britain are predominantly westerlies, spreading industrial pollutants (especially smoke and sulfur dioxide) toward the east. Melanic forms were concentrated toward the east and were completely absent from the unpolluted western parts of England and Wales, northern Scotland and Ireland. Notice from the figure, though, that many populations were polymorphic: melanic and nonmelanic forms coexisted. Thus, the polymorphism seems to be a result both of environments changing (becoming more polluted) - to this extent the polymorphism is transient - and of there being a gradient of selective pressures from the less polluted west to the more polluted east.

The main selective pressure appears to be applied by birds that prey on the moths. In field experiments, large numbers of melanic and pale ('typical') moths were reared and released in equal numbers. In a rural and largely unpolluted area of southern England, most of those captured by birds were melanic. In an industrial area near the city of Birmingham, most were typicals (Kettlewell, 1955). Any idea, however, that melanic forms were favored simply because they were camouflaged against smoke-stained backgrounds in the polluted areas (and typicals were favored in unpolluted areas because they were camouflaged against pale backgrounds) may be only part of the story. The moths rest on tree trunks during the day, and nonmelanic moths are well hidden against a background of mosses and lichens. Industrial pollution has not just blackened the moths' background; sulfur dioxide, especially, has also destroyed most of the moss and lichen on the tree trunks. Thus, sulfur dioxide pollution may have been as important as smoke in selecting melanic moths.

In the 1960s, industrialized environments in Western Europe and the United States started to change again, as oil and electricity began to replace coal, and legislation was passed to impose smoke-free zones and to reduce industrial emissions of sulfur dioxide. The frequency of melanic forms then fell back to near pre-Industrial levels with remarkable speed (Figure 1.6). Again, there was transient polymorphism - but this time while populations were en route in the other direction.

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