Monday, March 8, 2010
Genetic Drift
Coyne brings up another way that species evolve, without natural selection, on pgs. 122-24. This is called genetic drift. Genetic drift happens when a relatively small population of organisms reproduces with each other. Eventually, because of how the small population breeds together, variation is lost and it is possible to get alleles in weird ratios or even completely loose alleles. The allele that gets expressed more often is random and environmental pressures do not affect it. Coyne uses the example of weird blood type frequencies in the Amish and Dunker religious communities. What other traits in populations of plants and animals might arise from genetic drift? Are these traits beneficial or hurtful? If they are harmful, what can we do to help populations get rid of the harmful gene (or can we do anything at all)?
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ReplyDeleteA great example of a trait that arises from genetic drift is an increase in disorders that are caused by recessive genes. According to Genetic Drift and the Founder Effect, a small group of Amish people in Pennsylvania who exhibit higher concentrations of dwarfism and other rare inherited disorders show genetic drift at work. This is due to the small size of the group leading to the genes of the few people becoming disproportional in society. When people from the group marry others in the group, there is a greater chance that recessive genes from the founders (who are the ancestors of all people in the group) will show up more often than if people who are not related in any way were to marry and have kids. The founders of the group have been traced back to one couple, who went on to have offspring that have descendants in the community. People in the community who have similar genes because they had similar ancestors are more likely to have offspring with disorders that are carried in the genes. The disorders caused by genes will show up proportionally higher in this small group of people who share common genes than it would in a larger population. This trait due to genetic drift is not beneficial because it cause disorders like dwarfism to appear in more of the population. The way to solve this problem is to have people from this small community mate with people from other areas in order to increase the gene pool and decrease the chance that disorders will be passed on to their offspring.
ReplyDeleteSources:
http://www.pbs.org/wgbh/evolution/library/06/3/l_063_03.html
Genetic drift is often easiest to spot within the human species, since we are so fascinated with ourselves and have decrypted the entire human genome. One condition proposed to have been caused by genetic drift is the usually-fatal disease Tay-Sachs disease. The disease is prevalent in Ashkenazi Jews, as well as Louisiana Cajuns. The reason that the affliction is believed to have its roots in genetic drift is because it seldom appears outside of these two populations. Both populations have a strong tendency for inbreeding, due to cultural pressures. This is one example of gene-culture coëvolution. Jews, throughout history, have been cut off from breeding with non-Jews, so once they developed a mutation in the HEXA gene, responsible for the production of one of the subunits of a lysozomal enzyme, it propagated itself through the culture rather quickly. The very same thing can be said about the Cajuns. A language barrier has kept them separate from other populations, so the genetic mutation (which appears to have arisen a second time, rather than being transferred from an Ashkenazi Jew) continues to be passed down. It is autosomal-recessive, which means that it doesn’t often show itself, meaning that people who carry only one copy of the malformed gene are unaffected. After generations of breeding within these populations, eventually a copy of the errant gene is spread to far more people than in outside populations, which means that eventually, recessive-recessive pairings are inevitable, and children will start being afflicted with Tay-Sachs. Another famous example of genetic drift is an example that we talked about earlier in the year, the Blue People of Appalachia. These people gained a genetic variation over time, which caused them to display the syndrome methemoglobinemia, a disease otherwise extremely rare. The quantity of inbreeding that took place among this single family continued to propagate the disorder, and it appeared in ratios much higher than in any other population in the world. Thus, it was an example of genetic drift, because the inbreeding changed the likelihood of receiving a certain gene. Eventually, as the world became more interconnected, these people began to leave their comfy Appalachian abodes and traveled to other places, where normal genetic crosses took place, mostly eliminating the errant gene. In addition, doctors can now treat methemoglobinemia, which means that having the disease doesn’t affect one’s ability to reproduce, slowing the process of human natural-selection. In the case of the two traits that I just mentioned, the genetic drift has been harmful. Both these disorders can lead to death, which means no reproduction. Death is also deadly, so no survival will be going on either. However, genetic drift can also be neutral, if the mutations in the genes don’t affect any protein making processes (i.e. they are in introns). Genetic drift is rarely beneficial, unless a population is extremely secluded from the rest of its species, because if it was beneficial, the beneficial gene would spread throughout the entire species, leading to natural selection, rather than pure genetic drift. Getting rid of genetic drift is simple to do, although there are moral issues that go along with it. The easiest way to remove genetic drift would be to force a population to mate with others outside of the population, watering down the frequency of the drifted gene. However, humans cannot be forced to mate without breaking a multitude of laws, so, this cannot really be done. Another way would be to sterilize the carriers of the errant gene, but, this too is frowned upon (I wonder why?), so it isn’t of value to humans.
ReplyDeleteSources:
http://en.wikipedia.org/wiki/Blue_Fugates#Carriers
http://en.wikipedia.org/wiki/Tay-sachs
http://judaism.about.com/od/health/a/geneticdisorder.htm