"'Bad' mutations such as the sickle cell and G6PD mutations and the irreversible fossilization of genes can be favored in meeting the imperatives imposed by conditions of selection. It is all a matter of the immediate benefits outweighing the immediate costs, if only by a slim margin," (Carroll 186).
Of the many examples Carroll provides, one which is most intriguing is that of the sickle-cell disease that can provide greater protection from malaria. Malaria has had a great effect as a driving force upon the genetics of human populations. Carroll cites the evolution of the sickle cell gene as "the classic textbook example of natural selection in humans," (176). Although the sickle-cell gene may provide an advantage in protection from malaria, it provides a disadvantage as to the amount of oxygen a red blood cell (RBC) can carry (on the hemoglobin molecule). With such a disease, humans who carry/suffer from the disease have chronic pain and fatigue*. With such inadvertent effects, are the benefits with the development of the disease worth the costs (protection from malaria)? Applying this ideology to other diseases that could possibly develop or changes in the genome, are the benefits greater, or are the costs greater? If natural selection favors humans who carry the sickle-cell gene, could natural selection at one point favor the return of normal hemoglobin and RBCs? Under what conditions would this occur? Relate all answers to mutations and themes of genetic disorders, evolution/natural selection, mutations, and inheritance.
*Please take time to review this website as to inform yourself of sickle-cell anemia: http://www.nhlbi.nih.gov/health/dci/Diseases/Sca/SCA_WhatIs.html
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Well, Danny Baby, if the sickle cell anemia did not increase the chance of survival than it would never have evolved in the populations that it did evolve in. So, because the benefits outweighed the detriments, the chance of survival into reproductive years was increased, and over the many generations that this trait was passed along, human populations evolved. Regarding your second question, an evolved trait will only exist if the benefits outweight the detriments, even if the benefit only slightly increases fitness. Dr. Carroll specificially states that this is the fact of the matter. In my opinion, natural selection will never select normal RBC's in the native African populations that originally possessed the trait, the reason being that haveing sickle cells only causes severe complications after a long period of time, usually after reproductive years. However, there is a chance that (because people today are having children at a much later age) normal RBC's will be selected by evolution simply because our society has changed the age of reproduction such that sickle cells may decrease life span enough to decrease sexual fitness. Also, it is possible that because malaria is not as large of a threat in the United States as it is in Africa, sickle cell anemia may just die out in African Americans.
ReplyDeleteExpounding on what Sam has said, I believe that selection will start to favor the return to normal red blood cells in the malaria infected areas of the world through human use of technology. By using pesticides and malaria medication like quinine, humans can protect against the deadly disease and effectively eliminate the selective pressure of malaria. That being said, it is definitely evident that with malaria being absent from the environment, natural selection will favor humans with the genes for normal erythrocytes. This is because humans with normal erythrocytes will be able to transport more oxygen more efficiently through their cardiovascular systems, which gives them an advantage in being able to create more ATP for energy, enhancing survival chances and reproductive chances.
ReplyDeleteThe one flaw in Sam's argument is that the are where the sickle-cell gene is prevalent are rather impoverished and also have high birthrates for younger ages, meaning that the problems sickle cell causes won't manifest until after reproductive years. The main trend among most of America is that the reproductive age is rising, but in these impoverished areas of Africa, malaria is prevalent and the reproductive age is still quite low so the sickle cell gene may be still be selected for. Not until charities such as the Gates Foundation as well as work by the United Nations effectively eliminate malaria will sickle cell anemia be selected against in these areas.