Over a period of a half of a century, the dark form became the most common. Biologists hypothesized that this was an adaptive shift in response to a changing environment. They thought increased burning of coal during the Industrial Revolution had led to such bad air pollution that the pale-colored lichens growing on tree trunks were killed. As a result, tree trunks, which previously appeared to be a light, scaly color from the lichens, became very dark. Tree trunk color is important to the survival of moths.
Moths are active at night and rest on tree trunks during the day.
Birds, who are their primary enemies, mostly eat varieties that are easy to find. Thus the light form of this moth became rare. To test this hypothesis, scientists raised moths in the laboratory and released them in both wooded areas that were and were not affected by air pollution. After a period of time during which the moths would have reproduced, moths were sampled in one of the two wooded areas at night using lights. If the gene frequencies in the surviving adults had shifted to .4B and .6b, predict the frequencies on the genotypes in the next generation.