Natural selection is the process by which one type of animal within a species develops well because of certain characteristics that make it more likely to live than others in its group. The history of the peppered moth(桦尺蛾) is an example of the natural selection process.
In nineteenth-century England, certain types of peppered moths were able to better blend (融合) into their surroundings. During that time period, great changes were happening in Great Britain. The Industrial Revolution was part of this change, and with it came air pollution. Natural selection usually takes hundreds or even thousands of years to occur. For the peppered moth, this process occurred relatively quickly .
At the beginning of the Industrial Age, most peppered moths in England were light-coloured and covered with black markings, although a few moths had dark-coloured wings. Because the light-coloured moths blended into the light-coloured bar k on the trees, they could not be easily seen by birds that would eat them. As the air grew more polluted, however, tree trunks (树干) became covered with soot (煤烟) and became darker. The light-coloured moths became easy for birds to see against the dark tree trunks. Since the dark-coloured moths now had the advantage, their numbers grew. Within 50 years, the peppered moth went from being mostly light-coloured to being mostly dark-coloured.
In the twentieth century, the air cleared up, and the peppered moth population changed again. As tree trunks lightened due to less soot in the air, light-coloured moths once again had an advantage. Their numbers increased as soot levels declined. Depending on their environment, the colouration of the moths helped them to be "naturally selected" to survive.
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