On pages 119-120, Coyne talks about how natural selection must provide a direct benefit to its possessor. He claims that there is no "going downhill in the evolution of adaptation." Sickle-Cell Anemia is a genetic, blood disorder that causes "red blood cells to assume an abnormal, rigid, sickle shape." The change in the structure of hemoglobin causes many complications in life, among which is a much shorter life expectancy (even for people in the USA w/ top of the line medical treatment, ave. life expectancy for one who has sickle-cell anemia is around 42-48 years depending on gender) (1). The defect which causes sickle-cell anemia occurs more often in those of tropical, sub-tropical, decent. As many as one-third of people indigenous to the Sub-Saharan region carry the gene for sickle-cell anemia. This mutation in the hemoglobin gene causes the carrier to become resistant to malaria.
If natural selection is only supposed to provide benefit to the organism, why would this degeneration of the red blood cell be an evolutionary advantage? How could this defect have been proliferated if the offspring would have a decreased chance of survival? After all, the individual may be more fit, if the offspring does not survive to reproductive age, it is no different than not having offspring, which means that genes are not passed down.
1: http://en.wikipedia.org/wiki/Sickle-cell_anemia
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Natural selection by definition does not make an organism better or worse as a whole, but instead is seen as the driving force for adaptation. While it is true that an organism with sickle cell anemia has a reduced life span, that is not to say that they do not survive and do not pass on genes. Instead the opposite can be said to be true. Organisms without sickle cell disease are vulnerable to malaria at any age. According to the world health organization infants infected with malaria do not survive to the age of one. This implies that they will never reproduce or pass on genes. Sickle cell disease does not give the organism a decreased chance of survival in an area heavily infected by malaria, but instead boosts their chances of survival by allowing them to reach an average age of 40 years, well pass the age necessary to engage in reproduction. This said it can be clearly seen that in an environment where malaria is prevalent sickle cell anemia is a beneficial adaptation. Instead of the whole population dying out at a young age the population lives till a younger age yet have a developed immunity to a disease.
ReplyDeleteAnother point to note is that if the disease was truly detrimental it would not have been the result of natural selection. The principle behind natural selection is that organisms that are more fit pass on their genes. If sickle cell anemia stopped people from passing on their genes it would not be a problem as it would simply die out of the population. However, this disease remains in the population; a clear indication that it is benefiting malaria stricken populations. When viewed in the context of environments that do not face the problem of malaria sickle cell disease would not have arisen in the population above a nominal amount at most, but, when viewed with malaria added into the environment, it did serve to the better the organisms chances of survival. While this is not the most effective method of survival (like many other parts of humans such as our knees) it serves its purpose.
http://www.universalgiving.org/in_the_news/itemDetail.do?id=35
http://learn.genetics.utah.edu/content/disorders/whataregd/sicklecell/
As mehul mentioned, natural selection doesn't choose "better" organisms, but simply the traits of those that reproduce most get passed on. Sickle cell disease can be very dangerous, even lethal, when in low oxygen environments or extremely dehydrated (http://en.wikipedia.org/wiki/Sickle_cell_anemia), because the sickle-shaped red blood cells are less able to carry oxygen; this could result in hypertension, constricted blood vessels, and even heart failure ("Pulmonary hypertension as a risk factor for death in patients with sickle cell disease"). As a result, sickle cell anemia wouldn't be adaptive for people living in oxygen-poor environments. However, in environments with sufficient oxygen, most with the sickle cell gene can live a relatively symptomless life (besides the shortened lifespan of 42-48 years). The origin of this disorder can be traced to sub-saharan Africa, where oxygen availability isn't a problem; nearly 1/3 of all native people carry a sickle cell allele ( "Mortality in sickle cell disease. Life expectancy and risk factors for early death"). This is because, as Mehul said already, the sickle cell gene makes one more resistant to catching malaria, which is a very common disease in Africa. Since dying of malaria was a much more immediate threat to our Ancestors living in Africa, the sickle cell gene was adaptive to the environment to lower one's chance of catching malaria; even with a shortened lifespan, those with sickle cell anemia are able to reproduce more successfully than those who are at higher risk of catching malaria. Those who are heterozygous for the sickle cell gene are at the greatest advantage, because they are asymptomatic of sickle cell anemia, yet are still resistant to malaria. This trait developed because it was an imperfect adaptation to prevent catching malaria, something that can be used to support the theory of evolution (why would an intelligent creator make an imperfect way to fight disease?). Thus, in environments where malaria isn't necessarily a threat, one would expect a much lower percentage of the population to have the sickle cell trait, and this is what we do see: in the US, only about 0.25% of those of African descent have sickle cell disease, whereas 4% of native West Africans have the disease. This is because the sickle cell gene isn't advantageous at all, and thus will be gradually eliminated by natural selection in the US (http://journals.cambridge.org/action/displayAbstract?aid=1361276).
ReplyDeleteEvolutionary advantages are not always only beneficial to the organism. These advantages are considered advantages if they help an organism reach reproductive age and reproduce, passing along their genes. As Coyne said, “a gene will actually be favored if it helps you reproduce in your youth but kills you in your old age.” This is the case for sickle cell anemia because it helps people survive until they can pass along their genes. Since sickle cell anemia aids in the reproduction of people afflicted with malaria, then a dangerous disease in a first world country, where there are only negative effects, can be a positive evolutionary advantage in a mosquito ridden African country. This is shown by the fact that in four separate places in Africa and Middle East, sickle cell anemia evolved because it was a selective advantage in the local populations.
ReplyDeleteSickle cell anemia prevents the spread of malaria because sickle cell anemia changes the structure of the hemoglobin, which in turn deforms the blood cell that the malaria parasite is living in, killing it and allowing the host to avoid a crippling disease. Offspring with sickle cell anemia are resistant to malaria infection, which will kill much faster than sickle cell. Coyne writes, “a gene that knocks you off after reproductive age incurs no evolutionary disadvantage.” (120) This tradeoff of having a slow killing disease that prevents a quick killing and widespread disease is beneficial to the humans that have it. Thus, their progeny that inherits sickle cell because they actually have a higher chance of survival, at least until reproductive age, which is what really counts. While some of the offspring won’t survive, more offspring that have sickle cell anemia will survive than those without it; this is what contributed to the eventual selection of sickle cell infected people.
Humans have other problems that plague them during the post reproductive ages including heart disease, prostate enlargement (in males) and menopause (in females). These are all genes that stay in the gene pool because they do not adversely affect reproduction.
Generally, benefits that come from natural selection are mostly good, but natural selection only selects for genes that ensure the passing of genes to the next generation. If natural selection favored genes that only selected for survival, that species will go extinct from a lack of new births.
http://en.wikipedia.org/wiki/Sickle-cell_disease
http://en.wikipedia.org/wiki/Malaria
pgs. 119-120
Jerry Coyne describes evolutionary advantages as those traits and characteristics that allow an organism to reproduce. If an organism reaches the age where it can reproduce then that trait may be selected for even if it does not help later in life (in old age, why we see wrinkles and inhibited function of organs). Thus, sickle cell anemia would not be done away with by evolution as those infected live around 42-48 years (past the reproductive years so those people still have ample chance to pass on their genes). A stronger reason for why a trait for sickle cell anemia may have been selected for is its ability to protect against malaria. Sickle cell anemia is a mutation in the shape of the hemoglobin gene causing it to resemble a sickle. This shape stunts the effectiveness of malaria so it cannot infect that person and it cannot spread. The most advantageous combination would be in the person that has only one of the two alleles carrying the sickle cell trait. If both alleles carried the sickle cell trait, that person would suffer from sickle cell anemia and have a shorter life expectency. If that person had two normal homoglobin alleles then he or she would not be protected from malaria. However, a selective advantage only requires that the trait increase the liklihood of reproduction (reaching that reproducing age). So sickle cell anemia defends against malaria thus, increasing people's life span so they may reach reproducing age. Even if they die young (in their forties), they were able to reproduce.
ReplyDeleteIt would still be beneficial for a person with sickle cell anemia to reproduce and pass his or her genes, especially in countries with high exposure to malaria. This trait is extremely helpful in protection from this disease and would therefore, be beneficial for next generations.
Why Evolution is True
http://sickle.bwh.harvard.edu/malaria_sickle.html
http://en.wikipedia.org/wiki/Sickle-cell_disease