The idea of natural selection is that organisms in a species that have characteristics favoring survival are most likely to survive and produce offspring with the same characteristics. Because the survival of organisms with particular characteristics is favored over the survival of other organisms in the same species that lack these characteristics, future generations of the species are likely to include more organisms with the favorable characteristics.

One of the most thoroughly analyzed examples of natural selection in operation is the change in color that has occurred in certain populations of the peppered moth, Biston betularia, in industrial regions of Europe during the past 100 years. Originally moths were uniformly pale gray or whitish in color; dark-colored (melanic) individuals were rare and made up less than 2 percent of the population. Over a period of decades, dark-colored forms became an increasingly large fraction of some populations and eventually came to dominate peppered moth populations in certain areas —especially those of extreme industrialization such as the Ruhr Valley of Germany and the Midlands of England. Coal from industry released large amounts of black soot into the environment, but the increase of the dark-colored forms was not due to genetic mutations caused by industrial pollution. For example, caterpillars that feed on soot-covered leaves did not give rise to dark- colored adults. Rather, pollution promoted the survival of dark forms on soot-covered trees. Melanics were normally quickly eliminated in nonindustrial areas by adverse selection; birds spotted them easily. This phenomenon, an increase in the frequency of dark-colored mutants in polluted areas, is known as industrial melanism. The North American equivalent of this story is another moth, the swettaria form of Biston cognataria, first noticed in industrialized areas such as Chicago and New York City in the early 1900s. By 1961 it constituted over 90 percent of the population in parts of Michigan.

The idea that natural selection was responsible for the changing ratio of dark- to light-colored peppered moths was developed in the 1950s by H. B. D. Kettlewell of Oxford University. If natural selection was the explanation, then there should be different survival rates for dark- and light- colored moths. To determine whether this was true, Kettlewell released thousands of light and dark moths (each marked with a paint spot) into rural and industrialized areas. In the nonindustrial area of Dorset, he recaptured 14.6 percent of the pale forms but only 4.7 percent of the dark forms. In the industrial area of Birmingham, the situation was reversed: 13 percent of pale forms but 27.5 percent of dark forms were recaptured.

Clearly some environmental factor was responsible for the greater survival rates of dark moths. Birds were predators of peppered moths. Kettlewell hypothesized that the normal pale forms are difficult to see when resting on lichen-covered trees, whereas dark forms are conspicuous. In industrialized areas, lichens are destroyed by pollution, tree barks become darker, and dark moths are the ones birds have difficulty detecting. As a test, Kettlewell set up hidden observation positions and watched birds voraciously eat moths placed |on tree trunks of a contrasting color. The action of natural selection in producing a small but highly significant step of evolution was seemingly demonstrated, with birds as the selecting force.

Not every researcher has been convinced that natural selection by birds is the only explanation of the observed frequencies of dark and light peppered moths. More recent data, however, provide additional support for Kettleweir's ideas about natural selection. The light-colored form of the peppered moth is making a strong comeback. In Britain, a Clean Air Act was passed in 1965. Sir Cyril Clarke has been trapping moths at his home in Liverpool, Merseyside, since 1959. Before about 1975, 90 percent of the moths were dark, but since then there has been a steep decline in melanic forms, and in 1989 only 29.6 percent of the moths caught were melanic. The mean concentration of sulphur dioxide pollution fell from about 300 micrograms per cubic meter in 1970 to less than 50 micrograms per cubic meter in 1975 and has remained fairly constant since then. If the spread of the light-colored form of the moth continues at the same speed as the melanic form spread .in the last century, soon the melanic form will again be only an occasional resident of the Liverpool area.

自然选择的思想是：一个物种里拥有有利于生存的特征的个体更容易生存下来并产生相同特征的后代。因为在同一个物种内部，拥有某种特定特征的生物会比没有这种特征的生物的存活更容易。所以这一物种的下一代里，拥有这种特征的个体会变多。 这里有一个被全面分析过的自然选择的相关例子，那就是某几种桦尺蠖颜色的改变。这个案例发生在过去100年间欧洲的工业地区。在一开始，桦尺蠖都是灰色或者白色的；黑色的桦尺蠖很少，也就占百分之二的数量。但是几十年过去，黑色桦尺蠖在某些桦尺蠖种群中所占的比例越来越大，最终在某些地区成为优势物种——尤其是重度工业化的地区例如德国的Ruhr Valley和英国的Midlands。工业生产需要的煤块会产生大量的黑色煤烟，排放进环境中，但是黑色桦尺蠖数量的增加并不是因为工业污染导致的基因突变。例如，毛毛虫形态的幼年桦尺蠖并不会因为吃了覆盖了煤烟的树叶而长成黑色的成年桦尺蠖。然而，工业污染促进了被黑色煤烟覆盖的叶子上的黑色桦尺蠖的存活。在非工业地区，黑色桦尺蠖一般来说很快就会被不利的自然选择淘汰了；因为鸟类很容易（从绿色树叶上）发现（黑色的）它们。这种工业污染地区黑色桦尺蠖数量增长的现象被称为工业黑变病。北美的相似例子是另外一种蛾子，它是swettaria形态的Biston cognataria。它们首先是在20世纪早期于芝加哥以及纽约的工业地区被人发现的。到1961年的时候，它已经占据了密歇根地区它们种族百分之九十的数量。 在20世纪50年代，牛津大学的H. B. D. Kettlewell提出，自然选择可以解释桦尺蠖种群里黑色桦尺蠖和浅色桦尺蠖数量比例的变化。如果这个理论是正解，那么黑色桦尺蠖和浅色桦尺蠖的存活率应该有区别。为了确定这个理论是不是对的，Kettlewell在乡村和工业地区放生了上千只浅色和黑色的桦尺蠖（每一只都做好了标记）。在多赛特的非工业地区，他回收了百分之14.6的浅色桦尺蠖，但是仅仅回收了百分之4.7的黑色桦尺蠖。在伯明翰的工业地区，情况翻转了：他回收了百分之13的浅色桦尺蠖和百分之27.5的黑色桦尺蠖。 显然，一些环境因素导致了黑色桦尺蠖更高的存活率。鸟类是桦尺蠖的捕食者。Kettlewell假设，正常的浅色桦尺蠖栖息在地衣覆盖的树叶上的时候是很难被鸟类发现的。但是黑色桦尺蠖就比较容易被发现。在工业地区，地衣已经被工业污染破坏了，树干被染成了黑色，这样黑色桦尺蠖就变成了难以被发现的种类。Kettlewell在一个测试中躲在隐秘的观察点，观察到鸟类大吃特吃与树干颜色形成鲜明对比的桦尺蠖。这种自然选择使得桦尺蠖出现了一步步微小但重要的进化。这个把鸟类视为桦尺蠖进化动力的理论似乎说得通。 并不是每个研究者都相信自然选择是黑色和浅色桦尺蠖数量变化的唯一解释。然而，更近期的数据为Kettlewell的自然选择学说提供了更多的支持。浅色桦尺蠖卷土重来了。在英国，1965年通过了一部净化空气法案。自从1959年起，Cyril Clarke爵士就在他的家乡利物浦的默西塞德捕获桦尺蠖。在1975年以前，他捕获的桦尺蠖里百分之九十都是黑色的，但是从1975年开始，黑色桦尺蠖的数量突然急剧下跌。到1989年，只有百分之29.6的桦尺蠖是黑色的了。这片地区的平均二氧化硫的浓度也从1970年的每立方米300微克降到了1975年的每立方米50微克，并且稳定在这个水平。如果浅色桦尺蠖数量增长的速度和黑色桦尺蠖当时增长的速度一样，那么在最近的世纪，很快黑色桦尺蠖在利物浦地区就会重新变得稀少。