Simulation · Biology · Class 12

Moths on a trunk: natural selection in action

From the lesson Natural selection seen in action in Evolution. Change the values and watch what happens.

The idea behind it

NCERT §6.3

In England, moths collected in the 1850s, before industrialisation, were mostly white-winged; dark-winged (melanised) moths were fewer.
A collection from the same area in 1920, after industrialisation, had more dark-winged moths: the proportion had reversed.
Explanation: predators spot a moth that contrasts with its background. Before industrialisation, trunks were covered by thick, almost white lichen, so white moths survived and dark ones were picked off.
After industrialisation, smoke and soot darkened the trunks and lichens could not grow; now the white moths were eaten and the melanised ones survived. Moths that could camouflage themselves survived.
Lichens do not grow where the air is polluted, so they can be used as indicators of industrial pollution.
Support: in rural areas without industry, melanic moths stayed few. In a mixed population, the better-adapted survive and increase, but no variant is completely wiped out.
Heavy use of herbicides and pesticides has selected resistant varieties within a short time. The same holds for microbes treated with antibiotics and for eukaryotic cells treated with drugs: resistance appears in months or years, not centuries.
These are examples of evolution by anthropogenic action. They also show evolution is not directed in a deterministic way: it is a stochastic process resting on chance events in nature and chance mutations in organisms.

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