2. 1. Review of natural selection
2. What is stabilizing selection?
3. How does stabilizing selection work?
4. Examples of stabilizing selection
5. What is directional selection?
6. Reasons for directional selection
7. Examples of directional selection
8. What is disruptive selection?
9. Causes of disruptive selection
10.Examples of disruptive selection
3. 1. Variation [3]
2. Inheritance [3]
3. High rate of
population growth [3]
4. Differential survival
and reproduction [3]
4.
5.
6.
7. •prevent divergence of
form & function [2]
• visible polygenic
characters/traits [2] [4]
• results in differential
reproduction [4]
20. oClam size – small and large
clams are favoured over
medium sized clams [11]
21.
22.
23. 1. "Stabilizing selection - Wikipedia, the free encyclopedia." Wikipedia, the free
encyclopedia. N.p., n.d. Web. 16 Apr. 2012.
<http://en.wikipedia.org/wiki/Stabilizing_selection>.
2. "Types of Natural Selection - Stabilizing Selection." About.com. N.p., n.d. Web. 16 Apr.
2012. < http://evolution.about.com/od/NaturalSelection/g/Types-Of-Natural-Selection-
Stabilizing-Selection.htm>.
3. " Evolution and Natural Selection ." The Global Change Program at the University of
Michigan. N.p., n.d. Web. 16 Apr. 2012.
<http://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selectio
n.html>.
4. "Research Stabilizing Selection by | BookRags.com." BookRags.com | Study
Guides, Lesson Plans, Book Summaries and more. N.p., n.d. Web. 17 Apr. 2012.
<http://www.bookrags.com/research/stabilizing-selection-wog/>.
5. "natural selection: stabilizing." SASindex. N.p., n.d. Web. 17 Apr. 2012.
<http://www.starsandseas.com/SAS%20Evolution/SAS%20natselection/natselec_stabilizi
ng.htm>.
24. 6. "Stabilizing selection and a stable environment | darwinbookcats." darwinbookcats |
a blog about evolution, books, and cats. N.p., n.d. Web. 17 Apr. 2012.
<http://darwinbookcats.wordpress.com/2011/06/22/stabilizing-selection-and-a-stable-
environment/>.
7. "natural selection: directional." SASindex. N.p., n.d. Web. 18 Apr. 2012.
<http://www.starsandseas.com/SAS%20Evolution/SAS%20natselection/natselec_directi
onal.htm>.
8. Scoville, Heather. "Directional Selection - Types of Natural Selection." Evolution -
Natural Selection, History of Life on Earth, Darwinism, Lesson Plans and More!. N.p., n.d.
Web. 18 Apr. 2012. <http://evolution.about.com/od/NaturalSelection/g/Types-Of-
Natural-Selection-Directional-Selection.htm>.
9. Scoville, Heather. “Disruptive Selection – Types of Natural Selection. “Evolution –
Natural Selection, History of Life on Earth, Darwinism, Lesson Plans and More!. N.
p., n.d. Web. 19 Apr. 2012 < http://evolution.about.com/od/NaturalSelection/g/Types-
Of-Natural-Selection-Disruptive-Selection.htm>
10. "natural selection: disruptive." SASindex. N.p., n.d. Web. 19 Apr. 2012.
<http://www.starsandseas.com/SAS%20Evolution/SAS%20natselection/natselec_disrup
tive.htm>.
11. definition. "Research Disruptive Selection by | BookRags.com." BookRags.com |
Study Guides, Lesson Plans, Book Summaries and more. N.p., n.d. Web. 19 Apr. 2012.
<http://www.bookrags.com/research/disruptive-selection-wog/>.
Editor's Notes
Before we start, we first need to review Darwin’s theory of the process of natural selection consists of 4 components. The first one is variation, which is when organisms exhibit different variation in appearance and behaviour, such as body size, hair colour, voice properties, number of offspring, etc. However, some traits show little to no variation among individuals, for example, number of eyes. The second one is inheritance, which is when some traits are consistently passed on from parent to offspring. Some traits are heritable, whereas other traits are strongly influenced by environmental conditions and show weak heritability. The third is the rate of population growth. Most populations have more offspring each year than local resources can support leading to a struggle for resources. Each generation experiences substantial mortality. And the last one is differential survival and reproduction. Individuals possessing traits well suited for the struggle for local resources will contribute more offspring to the next generation. [3]
Stabilizing selection can also be referred to as normalizing selection, optimizing selection, as well as ambidirectional selection. This type of selection commonly uses negative (or purifying) selection. In terms of natural selection, negative selection or purifying selection is the selective removal of alleles that are deleterious (causing harm or damage). This can result in stabilizing selection through the purging of deleterious variations that arise. 1In other words, it is the selecting against extreme values of a trait/character. 1
So why did I talk about natural selection and what exactlyis stabilizing selection? Well, stabilizing selection is a type of natural selection, where genetic diversity decreases as the population stabilizes on a particular trait value. The point of stabilizing selection is to reduce phenotypic variation (which you guys learned in the first unit, physical traits one can see) to maintains the status quo by preventing divergence of form or function. It is also probably the most common type of selection under natural selection. In essence, stabilizing selection favours the norm, the common, and average traits in a population.[5]
Stabilizing selection specifically acts on visible characters/traits, or traits that are polygenic, which means that more than one gene controls the phenotype and there is a wide range of possible outcomes. Over time, some of the genes that control the characteristic can be “turned off” or “masked” by other genes, depending on where the favourable adaptations are coded. This results in difference in reproductive success (differential reproduction). This is a reason why the anatomy of some organisms, such as sharks and ferns, has remained largely unchanged for millions of years.
So what does stabilizing selection statistically look like? Well, suppose that each population of an organism can be portrayed as a frequency distribution for some sort of trait.[3] Then, stabilizing selection will look like a modified bell curve that is narrower and taller than the norm on a graph.[2] And variation in a trait is so important because it is the critical raw material for evolution to occur.[3]
After taking in all this information, let’s apply it to real life examples. The most classic example of stabilizing selection is human birth weight. Babies that are born with a lower than average weight tend to lost heat more quickly.[1] This leaves them in poor health, and they easily get infectious diseases.[1] On the other hand, babies with a higher than average weight are more difficult to deliver through the pelvis during birth.[1] Thus, the mortality rate for babies born with a more medium weight is much lower than larger and smaller babies.[1] In addition, interesting enough, human birth weight is not only a polygenic trait, but it is also controlled by environmental factors.[2]
Another examples would be Siberian Huskies, a bred of dogs well designed for working in the snow.[5] The Siberian Husky is a medium dog, with males weighing about 35-60 lbs. They have strong pectoral and leg muscles, allowing them to move through the dense snow.[5] But these dogs did not just magically obtained these suitable bodies overnight; it was through stabilizing selection. If a Siberian Husky had heavier muscles, it would sink deeper into the snow, which would mean they would move slower, exposing themselves to predators or never catching prey, and even get stuck in the snow.[5] And yet, on the opposite end, if the Siberian Husky had lighter/weaker muscles, it would not be strong enough to pull sleds and equipment.[5] This would mean that the Siberian Husky would have little value as a working dog, and may be ceremonially sacrificed in a tribe because it has little value now.[5] Thus, huskies who are medium built have more chance of survival, which is why stabilizing selection has taken its toll on too strong or too weak huskies.
