There are different types of selection processes which are a result of environmental pressures.
In directional selection, phenotypic variation shifts consistently in response to the environmental conditions. The study on the Peppered moth from the Industrial Revolution is one example of this type of selection. As the environment changed, one color became more favorable than the other until it was considered to be the most common trait. The darker colored phenotype becomes the most suitable trait to hide the moth against the darker trees. (Gardener,2005).
Stabilizing selection occurs when intermediate forms of a trait within a population are favored. Extreme forms are wiped out. This counters gene flow, genetic drift and mutations while preserving the most common phenotypes. An example of this is the gallmaking fly. This insect’s larvae burrow into the stems of goldenrod to feed. The tissues of the plant encase the larvae into a tumorous mass. A parasitic wasp can penetrate the smaller masses, attacking the smallest of larvae. Meanwhile a downy woodpecker can penetrate the larger masses, attacking the biggest larvae. Through this predator based selection process, larvae which are intermediate in size are most favorable for survival (Kerkhoff, 2010).
Disruptive selection is formed at opposite variation ranges. Intermediate forms of a trait are not favored. For example, the black-bellied seed cracker, an African finch, has only two beak size variations. Some have large beaks, while others have small beaks. There are no sizes in between. The beak size is unrelated to gender or geography and appear about equally within a population. Small beaks are favored for soft seeds while large beaks are favored for hard seeds. Both types of seeds are available when the finches reproduce. All the birds tend to prefer softer seeds, however when the dry season peaks, soft seeds and other food items dwindle. Finches with small beaks, and young birds in general, compete more. This leads to a higher mortality rate of birds with small beaks. Small beaks may be better suited for the favored food items, but large beaks are better suited for year round survival. Large beaks are more versatile (Starr and Taggart, 2004).
Kerkhoff. (2010). Agents of selection: Predation by birds on Gall-making Flies. http://biology.kenyon.edu/courses/biol229/galls.pdf
Starr,C. and Taggart, R. (2004). Biology: The Unity and Diversity of Life, 10th ed, pg 282-285.
Gardener, L. (2005). Peppered Moths: An Example of Natural Selection. http://www.windows2universe.org/cool_stuff/tour_evolution_8.html&lang=en