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Directional selection is a type of natural selection in which the observable characteristics or phenotype of the species tend towards one extreme rather than the average phenotype or the opposite extreme phenotype . Directional selection is one of the three most studied types of natural selection, aside from stabilizing selection and disruptive selection . In stabilizing selection, the extreme phenotypes are gradually reduced in number in favor of the average phenotype, while in disruptive selection the average phenotype is reduced in favor of the extremes in either direction.
Conditions Leading to Directional Selection
The phenomenon of directional selection is generally observed in environments that have changed over time. Changes in weather, climate, or food availability can lead to directional selection. A pertinent example primarily related to climate change is what has been recently observed in Alaska with sockeye salmon and the change in spawning time, probably due to increased water temperatures.
In a statistical analysis of natural selection, directional selection shows a population bell curve for a particular trait shifting to the left or right. However, unlike in the stabilizer selection, the height of the bell curve does not change in the directional selection. There are far fewer “average” individuals in a population that has undergone directional selection.
Human interaction can also speed up directional selection. For example, fishermen or hunters who capture or pursue prey often target the largest individuals in the population, either for their meat, skin, or other large ornamental or useful parts. Over time, this causes the population curve to skew towards smaller individuals. A directional selection bell curve for size will show a shift to the left in this directional selection example. Animal predators can also create directional selection. Because slower individuals in a prey population are more likely to be hunted down and eaten by predators, directional selection will gradually bias the population toward faster individuals.
Directional Selection Examples
Directional selection is one of the forms of natural selection, so there are plenty of examples that have been studied and documented. Let’s see some known cases:
- Charles Darwin (1809-1882), the British naturalist who developed the theory of evolution based on natural selection, while on the Galapagos Islands studied what would later be called directional selection. He observed that the beak length of Galapagos finches changed over time due to the adaptation of their beaks to available food sources. When insects were lacking to eat, the finches with larger beaks survived because the structure of the beak was useful for breaking seeds. Over time, as insects became more abundant, directional selection began to favor finches with smaller and longer beaks, which were more useful for catching insects.
- Fossil records reveal that black bears in Europe decreased in size during continental interglacial periods, that is, during the less cold periods in the ice age; however, they increased in size during periods of ice expansion, when the cold was most severe. This was probably because larger individuals have advantages in extremely cold conditions and when food supply conditions are limited.
- In 18th and 19th century England, peppered moths that had been predominantly white, thus blending in with light-colored trees, began to evolve into a predominantly dark species that allowed them to blend in with surroundings that were increasingly covered in soot from factories during the Industrial Revolution.
