2.5 Directional Selection

Marcos Antonio de Lima Filho, PhD.

Industrial melanism in the British peppered moth (Biston betularia) is one of the most famous pieces of evolution by natural selection. This is largely because of its pedagogical simplicity and copious documentation (Gishlick, 2002). The environmental change in this classic example was brought about by the air pollution around industrial centres in England, which killed the light-coloured lichens that grew profusely on the trunks of forest trees (Zeigler, 2014, p. 90). This environmental change led to dark melanic moths becoming dominant in polluted areas:

In collections made in Britain in the eighteenth century, the form of the moth was always a light, peppered colour. A dark (melanic) form was first recorded in 1848 near Manchester. That melanic form then increased in frequency until it made up more than 90% of the populations in polluted areas in the mid-twentieth century. In unpolluted areas, the light form remained common. Clean air laws were passed in the mid-twentieth century, and the frequency of the melanic form decreased in formerly polluted areas (Ridley, 2004, p. 108).

That is, before the Industrial Revolution, individuals of the commonly called “peppered moth” were predominantly white with black speckles; By the end of the 1800s, they were predominantly charcoal grey (Gishlick, 2002). This process was well documented and led Tutt (1896) to hypothesise that this change resulted from pollution-stained trees’ affecting the camouflage potential of the moths (see Figure 2.5.1). This change was termed “industrial melanism”, causing the peppered moths to stand out clearly against the now-exposed dark bark of the trees, thus making the moths no longer camouflaged from their bird predators (Zeigler, 2014).

In the 1950s, Bernard Kettlewell decided to test the hypothesis that natural selection was working on the differential camouflage of the moths:

In order to do this, he released marked light and dark moths into polluted and non-polluted forests. He found that birds appear to prey selectively on light moths in polluted forests and on dark moths in non-polluted forests and so documented the idea of natural selection of these colour patterns in moths by birds. After anti-pollution laws took effect and the bark lightened, the moth populations in formerly polluted areas returned to previous colour distributions (Gishlick, 2002).

In conclusion, the peppered moth’s melanistic forms were able to survive better than the lighter-winged individuals because they were less visible to birds (Figure 2.5.2). This led to directional selection, which eliminated the lighter-winged peppered moths and favoured uniformly dark individuals (Zeigler, 2014).

Directional selection is a concept in evolutionary biology that refers to the process by which natural selection favours individuals with a particular phenotype or trait that is associated with higher fitness. Evidently, this process is not limited to just colour change but can also extend to other physical characteristics, such as body size. For example, if smaller individuals have a higher fitness than larger individuals, natural selection will favour smaller individuals, and over time, this will lead to a decrease in average body size (Ridley, 2004). Contrarily, directional selection can also produce an evolutionary increase in body size if larger individuals have a higher fitness. Therefore, it is crucial to understand that directional selection is not limited to just one specific trait or characteristic but can apply to a wide range of physical and behavioural traits.