The nearby city, however, slowly became industrialized. Smokestacks and foundries in the town puffed out soot and smoke into the air. In a fairly short time, the soot settled on everything, including the trees, and killed much of the lichen. As a result, the appearance of the trees became nearly black in color. Suddenly the pepper-color moths were obvious against the dark tree trunks, while the black moths that had been easy to spot now blended in against the trees. Over the course of years, residents of the town noticed that the population of the moths changed. Whereas about 90 percent of the moths used to be light, after the trees became black, the moth population became increasingly black.

When the trees were lighter in color, natural selection favored the pepper-color moths because those moths were more difficult for predators to spot. As a result, the pepper-color moths lived to reproduce and had pepper-color offspring, while far fewer of the black moths lived to produce black offspring. When the industry in the town killed off the lichen and covered the trees in soot, however, the selection pressure switched. Suddenly the black moths were more likely to survive and have offspring. In each generation, more black moths survived and had offspring, while fewer lighter moths survived to have offspring. Over time, the population as a whole evolved from mostly white in color to mostly black in color.

 

 

    In a normal population without selection pressure, individual traits, such as height, vary in the population. Most individuals are of an average height, while fewer are extremely short or extremely tall. The distribution of height falls into a bell curve.

    Natural selection can operate on this population in three basic ways. Stabilizing selection eliminates extreme individuals. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures act to favor plants of medium height.

    Directional selection selects against one extreme. In the familiar example of giraffe necks, there was a selection pressure against short necks, since individuals with short necks could not reach as many leaves on which to feed. As a result, the distribution of neck length shifted to favor individuals with long necks.

    Disruptive selection eliminates intermediate individuals. For example, imagine a plant of extremely variable height that is pollinated by three different pollinator insects: one that was attracted to short plants, another that preferred plants of medium height, and a third that visited only the tallest plants. If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against, and the population would tend toward both short and tall plants, but not plants of medium height.