Stabilizing Selection Definition
Stabilizing selection is any selective force or forces which push a population toward the average, or median trait. Stabilizing selection is a descriptive term for what happens to an individual trait when the extremes of the trait are selected against. This increases the frequency of the trait in the population, and the alleles and genes which help form it. Many traits which are common across entire groups of species have been formed through the effects of stabilizing selection. Stabilizing selection can be seen in the image below, comparing the three types of selection.
Stabilizing Selection Examples
In this case, the number of eggs in a robin nest has been selected for through a stabilizing selection. Robins are apparently not ably to raise more than 4 chicks with much success. This is probably because of the size of the birds and the amount of food that two adults can provide. By comparison, most penguins can only raise one chick at a time, due to the size of the chick and the amount of food it requires. While they stabilized on different numbers, both are forms of stabilizing selection which maximize the fitness of the species in their environment.
As opposed to the other forms of selection, you can clearly see in the stabilizing selection graph that the population of the median trait increases, while the other populations decrease. In this case, 5 eggs is too many and some would die. On the other hand, 3 is too few. Either the eggs are not viable enough to only rely on 3 eggs, or predation and other forces require more than 3 eggs to overcome them and carry on to another generation.
There is a population of multicolored lemurs on Crazy Island. This particular population of lemurs has been observed by scientists, and they have noticed the following changes in the lemur’s color.
As you can see, the lemurs have obviously undergone a stabilizing selection. The light and dark lemurs have almost disappeared, while the middle brown lemurs have increased. Without further information, it is not clear why this would be the case. It is the job of ecologists and evolutionary biologists to observe the population, noting peculiar aspects of the various forms to understand what may have caused the stabilizing selection. This is no easy question to answer and may have many more than one answer.
In the case if the lemurs, it could be that the darker and lighter lemurs were both easier to spot by predators. If the lemurs only have one predator, this is as easy hypothesis to test. A scientist would simply observe the predator, and see which lemurs it prefers. This would lend evidence to the hypothesis that the stabilizing selection is caused by predation. Further evidence could include the amount of lemurs the predators eat, and models showing how that level of predation could produce the coloration seen.
However, it is much more common that a species has multiple selective pressures, and that each pressure acts on various traits in different ways. For example, the lighter color could be suffering from predation, whereas the darker version could be overheating. (Dark colors absorb more solar heat.) Likewise, predation could be driving both traits, but not influencing them totally. Female lemurs might have a preference for brown lemurs, due to their increased survival. This would be a form of sexual selection, driving a trend of stabilizing selection.
Common Causes of Stabilizing Selection
Stabilizing selection, along with directional selection and disruptive selection, refer to the direction of individual traits. While stabilizing selection pushed the trait towards the average instead of one or both of the extremes, it can be driven by any form of selection. Some of the most common forms of selection are from predation, resource allocation, coloration of the environment, food type, and a wide variety of other forces.
Many traits we don’t talk about regularly have been driven by a variety of causes throughout history. Take the modest insect, for example. All insects have an exoskeleton, a miraculous structure made of chitin and other structural molecules which form a shield around their organs and allow them to maintain a water balance in the harshest of environments. This shield, while it has been modified in to an almost infinite number of forms, was first selected for out of stabilizing selection. The ancestors to insects did not have this adaptation, but once it evolved it was highly favored.
Simply stated, there is no common cause of stabilizing selection, besides the fact that the most average individual is selected for. In that way, like all forms of selection, the cause of stabilizing selection is the increased fitness and reproductive success that the median individuals have. The extreme versions or traits have a disadvantage, in one way or another. This disadvantage, in evolutionary terms, is decreased reproduction. The traits they carry are coded for in part by their DNA, which they can only pass on through reproduction. In stabilizing selection, the increase in the median traits represents their increased success. The other extreme traits are not as successful, possibly causing their owners to die. This increases the resources available to the median animals, further boosting their success. In this way, stabilizing selection is the cause of many traits that entire groups of animals share. These are known as synapomophies.