An organism has specific traits that are expressed, and these can be influenced by the environment. Recall that the collective expression of traits is referred to as the phenotype of the organism. A change in a trait (e.g., flower color) results in an alternative phenotype. It follows then that any change in a population's phenotypic frequency may be a reflection of a changing environment. The peppered moths described earlier represent a good example of this phenomenon. When air quality declined, the darker moths became more common (i.e., the bark on trees became darker with soot and light moths were more visible on the bark when compared to dark moths). Now that air quality has improved, the mottled and lighter phenotype is more common.
Based upon this example, you may conclude that natural selection always shows a clear increase, over time, of one trait (and a decrease of another). Indeed this does occur (as in the moth example), but different selective pressures can give rise to alternative phenotypic distributions. These situations are described by three major modes of natural selection – stabilizing, directional, and diversifying.
Stabilizing selection favors intermediate phenotypic variants by acting against extreme phenotypes. Organisms that display stabilizing selection may show ancestral character traits if the environment has also remained unchanged. View the animation below and click on stabilizing, then the play button to see how stabilizing selection favors an intermediate phenotype.
One condition that favors stabilizing selection is a stable environment. Many terrestrial, aquatic, and marine environments don't change much from year to year. Stabilizing selection can also occur when extremes are a disadvantage. Birth weights in humans are an excellent example. If a baby is born too small, it may not survive. Conversely, if it is born too big, it may not get out of the womb safely or it may endanger the mother (and/or itself), also reducing survival.
The horseshoe crab has many stable character traits that display stabilizing selection. Horseshoe crabs represent a group of ancient arthropods that have inhabited shallow ocean waters for a long period of time. These organisms are very well adapted to scavenging on sandy bottoms. They are a good example of stabilizing selection because when living horseshoe crabs are compared to fossils, many of their features have not changed in the past 250 million years.
Directional selection favors individuals with phenotypes that are at one end of the phenotypic range; this type of selection is often seen during periods of environmental change. Directional selection favors the variants at one extreme by selecting against the other extreme; thus, resulting in a shift of the phenotype frequency curve. View the animation below, click on directional, then the play button to see how directional selection favors variants at one extreme of the phenotypic range.
For example, changes in weather patterns can cause directional selection. Let's say that a group of relatively tall plants exists in an area without much wind. The weather patterns change and the plants are now exposed to greater wind forces. We might expect the phenotype frequency curve to shift toward shorter plants that can withstand this change in the environment, a selective pressure. The peppered moth mentioned earlier is another example of directional selection.
Diversifying selection (also referred to as disruptive selection) favors individuals at both extremes of the phenotypic range; this is also seen during periods of environmental change, but is less common than directional selection. Open this animation, click on diversifying, then the play button to see how diversifying selection selects variants at both extremes of a phenotypic range. Imagine a population in which the main food supply has decreased. Because less of the preferred food is available, the variants of both extremes may be able to utilize different food supplies, whereas intermediates may not be able to use either food source as well. Selection against intermediates would result, due to the decrease in available preferred food. For example, black-bellied seedcracker finches with small beaks crack soft seeds more easily than those with larger beaks. Finches with large beaks crack hard seeds more easily than those with smaller beaks. Finches with intermediate-sized beaks do not crack either type of seed as well. Therefore, there is a shift in the population to both end of the distribution.
View this animation to review the three modes of selection:
Genetics Selection and Speciation Part 1 VoiceThread Transcript