A very simple description of natural selection:
In spite of its revolutionary philosophic impact, Darwin’s concept of natural selection is quite easy to understand (and to misunderstand). It was based on observations of artificial selection, the results of selective breeding by farmers and animal fanciers. Darwin, for example, referred to all the different breeds of pigeons that had been produced by artificial selection.
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Presumably, that’s the way birds saw it, too, back in the 1850s. The darker moth stood out, but the lighter one was camouflaged against the mottled gray lichen that encrusted the trees back then. As a result, birds ate mostly dark moths, and light moths made up over 98 percent of the population.
But then pollution killed the lichen on the trees, revealing the dark color of the bark. As a result, the dark moths were more camouflaged than the light ones. Thus, the dark ones had a better chance of surviving and leaving more offspring to grow into dark moths in succeeding generations. Sure enough, just as Darwin would have predicted, the population shifted. The “dark environment” just naturally selected the dark moths as more likely to survive and reproduce.