Types of Selection: Disruptive, Directional, Stabilizing, and Artificial.
Disruptive selection occurs when extreme values of a trait succeed over intermediate values of the same trait, in a given population. It can be influence by humans. In disruptive selection, the normal curve hits extremes and bypasses levels of a trait in the middle. It is the rarest form of selection.
Scoville, H. (2013). Types of natural selection. Retrieved from: http://evolution.about.com/od/NaturalSelection/g/Types-Of-Natural-Selection-Disruptive-Selection.htm
Sinervo, B. (1997). Disruptive Selection in Adaptation and Selection.
Directional selection occurs when environmental conditions cause a change in phenotype, where an extreme phenotype is favored over a less extreme one. And example of this can be seen in peppered moths. With the introduction of smog, the darker phenotype of coloring was favored for camouflage purposes.
Groot, A.T., Horovitz, J.L., Hamilton, J., Santangelo, R.G., Schal, C. Gould, F. (2006). Experimental evidence for interspecific directional selection on moth pheromone communication. Proceedings of the National Academy of Sciences of the United States of America 103(15): 5858-5863. DOI:10.1073/pnas.0508609103
Sabeti P.C., et. al. (2006). Positive Natural Selection in the Human Lineage. Science 312 (5780): 1614–1620. doi:10.1126/science.1124309
Stabilizing selecton occurs when genetic diversity declines and as a result, the population mean value stabilizes for a specific trait. The intermediate value is being favored over the extreme values for a trait. In this type of selection, species are usually stay the same and show little change. Human birth rates could be seen as an example of stabilizing selection.
Scoville, H. (2013). Stabilizing selection. Retrieved from: http://evolution.about.com/od/NaturalSelection/g/Types-Of-Natural-Selection-Stabilizing-Selection.htm
Stabilizing Selection: Retrieved from: http://www.evotutor.org/Selection/Sl5A.html
Artificial selection is selection that does not occur naturally. This can be seen in captive breeding programs. Humans find a gene or trait that is most desirable and breed the animals to have those traits. It is like survival of the fittest but manipulated by humans to have a certain end result.
Theunissen, B. (2012). Darwin and his pigeons: The analogy between artificial and natural selection revisited. Journal of History of Biology 45:179-212