Industrial melanism

A phenomenon in which dark mutated individuals of moths living in the vicinity of a city increased as the city became more industrialized. It is also called industrial melanism. It was discovered in Manchester, England in the late 19th century, where a moth called the peppered moth, which originally has light-colored wings, suddenly increased in number with dark mutated individuals. Similar phenomena were subsequently found in industrial cities in England, Europe, and North America, and the number of moth species believed to have undergone industrial melanism now numbers in the hundreds. It is not yet known in Japan.

All of these moths originally have light-colored wings, and they have the habit of perching on tree trunks with their wings open during the day to rest. In rural areas, where trunks have turned white due to the growth of lichen, the light-colored wings become camouflaged and are less likely to be preyed upon by small birds, but as industrialization progresses and the lichen dies due to the effects of soot, leaving the dark bark exposed, the light-colored wings become conspicuous. In such an environment, if a mutant individual with dark wings happens to appear, it will have a better camouflage effect than the light-colored type, giving it a survival advantage. This has been proven experimentally, and is thought to be the main reason for the increase in melanistic types in industrial areas. It has been found that the melanistic type in the peppered geometrie is caused by the presence or absence of a single dominant mutation gene, and industrial melanism is considered strong evidence for the modern evolutionary theory that evolution occurs through mutation and natural selection. However, there is still some debate as to whether the increase in the dark morphs is solely due to their camouflage effect.

[Tetsuyuki Ueda]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

都市の工業化が進むに伴い、その付近に生息していた一部のガに暗色の変異個体が増加した現象。工業黒化ともいう。19世紀後半イギリスのマンチェスターにおいて、本来は淡色のはねをもつオオシモフリエダシャクというガに暗色のはねをした変異個体が急速に増加したことから発見された。同様の現象は、その後イギリスをはじめヨーロッパや北アメリカの各工業都市でもみいだされ、工業暗化を生じたとされるガ類は数百種にも上っている。日本ではまだ知られていない。

　これらのガは、いずれも本来淡色のはねをしており、いずれも昼間は木の幹などにはねを開いて止まって休むという習性をもつ。田園地帯の地衣類が着生して白っぽくなった幹では、淡色のはねは隠蔽(いんぺい)色となり、小鳥などの捕食を受けにくいが、工業化が進み煤煙(ばいえん)の影響で地衣類が死滅し、黒ずんだ樹皮がむき出しになってしまうと、淡色のはねは目だつものになる。このような環境で、暗色のはねをもつ突然変異個体がたまたま生じると、淡色型より隠蔽効果に優れ、生存上有利となる。このことは実験的にも証明されており、工業地帯で暗化型が増加した主要な原因と考えられている。なお、オオシモフリエダシャクでは、暗化型は1個の優性突然変異遺伝子の有無により生じることがわかっており、工業暗化は、突然変異と自然選択により進化が生じるという現代の進化理論の有力な証拠とされる。ただし暗色型の増加が隠蔽色としての効果だけによるものかどうかについては、現在も議論の余地が残されている。

［上田哲行］

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