Friday, April 2, 2010
How we affect natural selection
We know that natural selection favors the organisms which are best adapted therefore also the organisms with mutations which increase their fitness. In the ideal natural selection scenario, an organism with a mutation that lowers their fitness dies, and their genes are not passed on. With humans, however, we artificially keep alive people with negative mutations. We support those with mental diseases or gene mutations, allowing them to survive and possibly reproduce. What are some negative mutations that have occurred in humans, and why did they occur. Finally, how are humans supporting other humans with the negative mutations, and how are their actions affecting natural selection and the advancement of humans?
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There are a plethora of negative mutations around us in human society. On page 219, Coyne mentions a few, such as bad eyesight and bad teeth. Negative mutations can occur for a number of reasons, as we studied in chapter 17. In the case of bad eyesight, that trait is hereditary; the mutated gene was passed down from parent to child. As to why the mutation actually occurred, there are a variety of possibilities. We studied point mutations, which are changes in a single base pair. Other types of mutations are insertions and deletions, which can be disastrous because they throw off the entire sequence of amino acids, and cause the wrong amino acids(and thus the wrong proteins) to be produced. Also, various mutagens like radiation and other chemicals can cause mutations(Campbell 346). It’s hard to tell exactly what type of mutation causes each negative trait humans have. Asking “why did they[negative mutations] occur” is pretty non-specific, as I’m sure negative traits of humans are caused in a variety of ways.
ReplyDeleteTechnology has supported these negative mutations in many ways. For example, the bad eyes gene hasn’t been erased or silenced by natural selection because now we can just put glasses on. A few hundred years ago, people with hemophilia or AIDS would have died much earlier in life than people with those diseases now due huge advances in medicine. As to how these actions affect natural selection, they really decrease the effect of natural selection. Since technology can account for many problems caused by genetic mutations, these mutated changes will be passed down to the offspring of the people with genetic mutations. However, I’m not convinced that our ability to “fix” genetic mutations via technology is stopping the human species’ ability to advance. Is having more people with glasses on in society a hindrance to the evolutionary success of humans? I certainly don’t see why it’s such a bad thing. There is plenty of debate over whether technology has stopped natural selection in humans and whether this is a bad thing. I think the general consensus is that no natural selection on humans is a bad thing. But in some cases, I don’t see why proliferating “bad” genes like the genes of people with bad eyesight is all that bad. This relates to the biological theme of continuity and change. The course outline book description of the theme is as follows: “Changes in gene pools over time can be explained in part by natural selection for the fittest genotypes.” Only in this case, the gene pool isn’t changing as much because bad genes aren’t being deleted or silenced by natural selection. So this relates more to the “continuity” aspect of “continuity and change”.
There is a lot of debate on whether we’re stopping natural selection – Kevin makes a good point there. There was even a fictional novel that some students have read in class that implies that discussion. The novel Running out of Time puts a community under the environmental conditions presented in the 1700’s. This community was under an experiment to try rendering a fit gene pool – those who survived in the community against diseases were deemed fit and would enter present day society, theoretically strengthening the present day society’s gene pool. Of course, the community begins to feel the effects of natural selection, and people begin to die. So yes, in a way, we are hindering evolution, but we are at the same time promoting our survival. And I do believe that the goal in life is to survive and reproduce, not necessarily kill off those who can’t survive to create an elite gene pool of humans.
ReplyDeleteThere are a lot of genetic diseases that we’ve studied in class. One of them would be sickle cell disease, occurring prominently in the black population.
We’ve learned that there are multiple alleles for many genes. For the case of blood, there are alleles that code for hemoglobin A and hemoglobin S. People who have both alleles hemoglobin A and S are resistant to malaria – that’s why Africans have a lower chance of getting the disease.
However, if someone has two alleles for hemoglobin S, then that person will have sickle cell disease, a disease that doesn’t let hemoglobin be able to carry oxygen well (http://sickle.bwh.harvard.edu/malaria_sickle.html). Sickle cell disease is recessively inherited – the hemoglobin A allele is dominant to the hemoglobin S allele (Campbell 278).
I remember I saw a few months ago how they found a cure to sickle cell disease, but I don’t know how the procedure goes. However, other treatments involve a bone marrow transplant (MayoClinic). As we learned in class, red blood cells originate in bone marrow (Campbell 912-4). A bone marrow transplant insures that there would be healthy red blood cells for a certain amount of time
With regards to natural selection, I suppose we are stopping the process from doing what it is suppose to. We are letting people who have bad genes survive with people who have stronger genes. However, I wouldn’t call it a bad thing. This is analogous to what the scientists were trying to do in Running out of Time . Though we are stopping natural selection, we are surviving and reproducing, finding loopholes in the system with our intellect.
This further raises the question whether our intellect proves to be a selective advantage to our survival by helping us develop medicine to aid those who can’t help themselves. Personally, I think it is. We have the selective advantage of intelligence – intelligence has helped us survive and reproduce better than other organisms. Not only that, but our intelligence is constantly increasing. Coyne states that “brain size in the human lineage increased on average about 0.001 percent per generation,” concrete proof that our brains are getting bigger on average per generation, leading to the intelligence that helps us survive today (Coyne 134).
Sickle cell disease is an example of continuity and change. Even though the sickle cell allele is suppose to help people survive malaria, over a period of time, this allele may be paired up with another, changing the gene from a helpful gene to a harmful one.
Many gene mutations today are hereditary. As Kevin stated previously, some genetic mutations include bad eyes and bad teeth. However, these mutations aren't very serious, and can be medically corrected (i.e. glasses for bad eyesight). There are mutations, though, that are very serious. These mutations, such as Down Syndrome and Hemophilia, are much more serious. Down Syndrome occurs when there is trisomy of the 21st chromosome. Hemophilia is a sex linked mutation that doesn't allow blood in the body to clot. Recall that hemophilia occurs from an enzyme that fails to function properly in the clotting enzyme cascade (Campbell 913). Hemophilia can result from any of these enzymes failing to function properly.
ReplyDeleteVarious technological and medical treatments help people cope with Down Syndrome and hemophilia, although these diseases cannot be beaten. For example, people with Down Syndrome can get plastic surgery on their face in order to reduce the social stigma associated with the disease. Also, hemophiliacs can take medicine that clots their blood for them. The down side of these treatments is that they cost a lot of money. In the past, plastic surgery wasn't a viable surgery. Also, knowledge of the mechanics of blood clotting and a medicine to help the blood clot weren't available until recently. In this case, technology has been counteracting natural selection.
Keeping hemophiliacs alive alters with natural selection in a major way. In the past, hemophiliacs did not live an age at which they could reproduce. Even today, only male hemophiliacs are the only ones able to reproduce. Female hemophiliacs can live to puberty, but die after their first menstruation. Since hemophilia is a sex linked disease, male hemophiliacs can produce normal children, whereas in the past hemophiliacs didn't even live to sexual maturity. However, female offspring of males with hemophilia are guaranteed to be at least a carrier of the disease, which results in the proliferation of the hemophilia genes. This proliferation of diseased genes could be a bad thing for the advancement of humans in terms of evolution. Bad genes are showing up more and more in society because natural selection is being undermined by technology. The mutation of genes that result in hemophilia relates to the theme of science, technology, and society because technology is undermining natural selection.
Outside source: http://humangenetics.suite101.com/article.cfm/introduction_to_gene_mutations
Henry brings up an interesting point in the cure to sickle cell anemia. In the past, people with sickle cell anemia were forced to endure the pain of the disease. I recall watching a show on ESPN about Make-A-Wish granting a wish to a boy named Charlie Pena, who had sickle cell anemia. When Charlie got pains from sickle cell anemia, he said the pain was so bad he wished he was dead. However, with current cures as Henry mentioned, those who are born with sickle cell anemia can cure it, and lead a normal lifestyle. Another example of technology undermining natural selection.
ReplyDeleteThere are plenty of “negative” genes in our genome, such as the ones Kevin pointed out, but I believe that “negative” is a rather subjective term. As we know, the same DNA sequence can be used to express many different genes, depending on which sets of codons are selected as exons during mRNA processing. Therefore, the genes we perceive as “negative” may have effects of which we are not aware; if, for example, a virus from an alien planet made its way to earth, a gene resistant to the virus might be found only in the genomes of those who are near-sighted. So far, mutations to our genomes, including errors during DNA transcription, occur mainly, if not completely, by chance. To artificially mutate the genome of every single one of the trillions of cells in the human body is most certainly a daunting task. Often times, critical genes such as the CCR5-Δ32 gene, which “provides its carriers with strong protection against infection with the AIDS virus” (Coyne, 2009, p. 219), are the results of rare mutations. Therefore, while the fact that we humans, through our hospitals, doctors, medicine, and pharmacists, are keeping alive those who carry “negative” genes seems to inhibit our ability to adapt, we are really increasing our ability to adapt by maintaining a high level of genetic diversity within our species and increasing the probability that at least one individual will have the gene to adapt to any given threat.
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