Directional Selection Definition
A directional selection is a force in nature that causes a population to evolve towards one end of a trait spectrum. While some traits are discrete and have specific variations (think eye color), other traits are continuous, and exists as a wide range of nearly infinite values (think height). If a discrete trait goes through a directional selection, only one variety will be selected for (only blue eyes). If a continuous trait is subjected to a directional selection, the highest or lowest value will be selected for (only tall people). Directional selection can be compared to stabilizing selection, in which a middle value or moderate trait is selected fore. Directional selection can also be compared to disruptive selection, or a selection that causes an increase in both extremes of a trait spectrum.
If a directional selection is applied to a population over time, the traits that are selected for will permanently increase, while the traits selected against will be lost. This change in the population over time is known as evolution. Most traits are linked to many genes, which control their appearance, function, structure, etc. Only vary rarely are traits controlled by single genes. Because of this, most traits will tend to be continuous in nature, and contain a wide variety of values for a trait. During directional selection, one side of these values will be selected against, while the other side will increase. Look at the example below of fur color in lemurs.
For whatever reason, nature is not kind to white lemurs. The directional selection could exist in a single source, such as a new predator that only eats white lemurs. It could also be caused by lots of factors that work in conjunction to select against white lemurs. For example, in addition to a new predator, white lemurs could also be subject to increased sunburns, which might decrease their reproductive success. While the causes can be infinite or singular, the selection moves in a decided way against all the lightest colored lemurs. Over time, as seen in the second image, there are no more lightly colored lemurs.
This is just one over-simplified example to give an idea of what kinds of forces can act on a population to create a directional change. The genes and alleles that create phenotypic traits can be complexly related. For example, the white coat color of the lemurs could have been related to some sort of protein deficiency. Therefore, it might not take a large and drastic force to remove the white lemurs completely. On the other hand, white could be almost completely independent of other traits. In that case, it would take a strong directional selection to remove white, as many successful organisms would also be white. Ecologist spend many years studying populations to determine the causes and directions of evolution. While Darwin had no direct evidence of evolution in a population, that phenomena has been documented time and again by scientists in recent times. A few excellent examples are the studies on Darwin’s finches, evolutionary analyses of insects, and many genetic studies that can show evolution happening on the micro level.
Examples of Directional Selection
Also known as Galapagos finches, these little birds where of particular interest to Darwin while he was on his famous discovery expedition. Darwin noticed that the species on different islands where remarkably varied, while undeniably coming from the same source. In recent years, scientist Peter and Rosemary Grant have been studying the finches. In the past thirty years, they have witness all sorts of selection on the finches and the evolution that ensued. In one very drastic example of directional selection the Grants observed as birds with larger beaks were selected for, after only one season of an extremely drastic form of directional selection.
In the Galapagos, certain plant species that produce seeds rely on a very wet rainy season to supply their many seeds with water. In drought years, the plants produce fewer, larger, and tougher seeds. During a drought in 1977, the Grants were actively studying the finches on one of the island. The drought indeed caused the seeds to become larger and stronger. In the population of finches before the drought, beak sizes ranged from very large to very small. Birds with small beaks were apt at handling small seeds, but larger seeds presented a challenge. The drought, and resulting increase in seed size and strength, put a directional selection the finches. Birds with small beaks were no longer able to eat, while birds with large beaks could survive on the large, tough seeds. This directional selection caused the resulting population of finches to have a much larger average beak size than the population before the drought. The Grants have documented many cases such as this in the finches, and have effectively shown evolution happening at a population level, from year to year.
Industrial Revolution as a Directional Selection
While many only know the Industrial Revolution from history books, it became an important part of biology when scientists started studying the changes that humans create in the environment. In 1811, during the earliest years of the industrial revolution, the first black peppered moth was found. Typically, peppered moths were lightly colored. The two varieties can be seen together in the image below, with the lightly colored moth on the right.
However, as the industrial revolution progressed, more and more of the dark moths were seen. The reason for this increase has been traced to a directional selection applied by the industrial revolution. During the industrial revolution, dust, soot, and pollution increased drastically in large cities. In these cities, almost no lightly colored moths were found. In forests far from large cities, the lightly colored moths were predominant. Many experiments on the evolution and ecology of these moths has shown than an increase in the number of dark moths is due to their increased success in avoiding predators. Lightly colored moths on sooty, dirty buildings are easy to spot, and predators quickly eat the lightest moths. The directional selection exists both ways, as industry selects for dark moths in cities and white moths in the woods. Therefore, if the entire population of moths is considered, this could also be an example of disruptive selection. At the level of each city, it is a directional selection, because only the dark variety of moths are selected for.