In The Cholesterol Also Rises
chapter, specifically pages 56-57, Dr. Moalem discuss how dark skinned people
are able to make a sufficient amount of Vitamin D even though their skin color
blocks all the ultraviolet rays. The purpose of the ultraviolet rays is to
convert the cholesterol into Vitamin D. Since the darker skin does not allow
the rays to penetrate the skin, there is a major problem from those people with
dark skin. Though it was once seen as a selected advantage-as it was an
adaptation that protected against sunburn-the inability to allow the sun’s rays
into the skin shows how evolution is not perfect.
In order to
compensate for the abundance of melanin in the skin, dark skinned people have
apolipoprotein E( ApoE4) in their gene pool to increase the amount of
cholesterol in your blood stream. This
process relates to Big Idea 2, as humans with dark skin utilize the ApoE4 in
order to maintain homeostasis.
Explain and describe how the ApoE4 allows Dark skinned
people to attain homeostasis. Give two other examples of how evolution is not
always perfect. Explain how light skinned people in areas receiving too little
light would react if they didn’t have the ultraviolet to generate Vitamin D.
Gives specific examples of how the ApoE4 gene can be good or bad.
Rahul Kosgi (rkosgi4@students.d125.org)
Rahul Kosgi (rkosgi4@students.d125.org)
On pg. 54-55, Dr. Moalem asserts that darker skin was a variation selected for in places where humans were highly exposed to ultraviolet rays from the sun—for example, Africa. This variation was selected for because darker skin, with plenty of melanin, protected the body’s stores of folate (folic acid), essential during pregnancy to produce healthy babies. Therefore, people with darker skin were better able to produce healthier babies. However, their darker skin also led to less UV rays penetrating less vitamin D being created—but vitamin D is also essential to the human body. That’s where the ApoE4 protein comes in.
ReplyDeleteThis protein, common in dark-skinned populations, increases the amount of cholesterol flowing through the blood (pg. 57). Because Vitamin D is created by converting cholesterol, “dark-skinned people can maximize the use of whatever sunlight penetrates their skin” (pg. 57) if they have more cholesterol. In this way, they can maintain homeostasis. The ApoE4 protein and its function as well as the relation between skin color and UV rays from the sun relate to Big Idea 2: Biological systems utilize free energy and molecular building blocks to grow, reproduce, and to maintain dynamic homeostasis. Of course, having the gene that codes for the ApoE4 protein also comes with a higher risk of heart disease and stroke because of the increased cholesterol.
But evolution is never a perfect process. Evolution acts only on the variations available in a population, selecting for those that make an organism more likely to survive and reproduce in a given environment—these variations do not make the organism ideal. There are usually some compromises as a result. One example the speed of cheetahs. A variation for longer legs in a cheetah may be favorable because it allows for greater speed and therefore greater ability to catch prey. However, longer legs also lead to more fragile bones and an increased chance of breaking bones (http://evolution.berkeley.edu/evolibrary/article/0_0_0/misconcep_03). A second example is the evolution of the human body and the effects on birth. On pg. 194-195, Dr. Moalem explains that evolution gave rise to skeletal adaptations which allow humans to walk on two feet and humans also evolved bigger brains, which allow for more brain function. However this can lead to complications in birth because skulls must be bigger to contain a bigger brain and the human pelvis now narrows considerably—both of these adaptations, helpful in one case, create difficulties in giving birth.
Lastly, light skinned people in areas with too little light would also have the gene that codes for the ApoE4 protein because they also need to maximize whatever sunlight they receive—Dr. Moalem discusses this on pg. 57. This gene would be selected for because it allows increased Vitamin D production, and the folate stores are already protected because of the limited UV rays.
(Sanika Bhargaw, sbharga4@students.d125.org)