Tuesday, March 9, 2010
Genetic Drift
"Random change in the frequency of genes over time is called genetic drift," (Coyne, 123). The line between genetic drift evolution and natural selection evolution to me seems very blurred. How can gene interactions be labeled random when the people mixing their genes to create a child may still exist because they have traits that helped them in natural selection? How can genes become "fixed" in a population to the point where there is 100 percent frequency of the gene, isn't there too much variation in the world to make that happen? What information can be found that verifies that genetic drift is a random process; that there is no possible reason that it happens? Also, genetic drift is said that it cannot cause adaptations then why is it considered evolution if evolution is a process that leads to adaptation? The book says that "What drift can do is cause the evolution of features that are neither useful or harmful to the organism," (Coyne, 123). This is saying that genetic drift does not help or hurt the organism but on the next page it contradicts itself by saying that it can "Raise the frequency of harmful genes even though selection is working in the opposite direction," (Coyne, 124). If genetic drift can raise the frequency of harmful genes than it is hurting the organism correct? Overall, what is the point of genetic drift and where is the line between genetic drift and natural selection?
Subscribe to:
Post Comments (Atom)
It is imperative not to equate natural selection with evolution. According to the Darwinian theory of evolution “natural selection is based on differential success in survival and reproduction…” (Campbell & Reece, 475).
ReplyDeleteAlthough natural selection may result in the amplification of an adaptation in subsequent generations and cause what we know as evolution, evolutionary change is not necessarily contingent upon whether a certain allele frequency is beneficial or maladaptive to a species. Coyne emphasizes that drift is “a random process” which may “…change the frequencies of alleles regardless of how useful they are…” (p.123)
This concept is clearly illustrated by the “extinction vortex” in small populations. Populations have a “minimum viable population” at which they may still survive and proliferate. (Richmond)
Genetic drift is the random variation of allele frequency in a population. Since the individuals of a population will only produce a certain number of offspring, the variation is limited. By random chance, it is possible for genetic variation to increase the frequency of a certain allele in any given generation and inbreeding – the mating of individuals in a population that share considerable portions of the genetic code - may therefore exacerbate the occurrence of deleterious alleles in small populations and eventually result in the extinction of the population. Therefore, although the allele in question may be detrimental to population survival and reproduction, complete genetic change, and thus evolution, may result over time.
Such genetic drift has been observed in human populations and often correlates with the occurrence of certain adverse alleles. According to Andrew J Bonohak of the San Diego University “the high frequency of achromatopsia on Pingelap atoll in Micronesia provides an…example of how extreme bottlenecks can affect the frequency of certain deleterious alleles”. (“Genetic drift in Human populations”). Similarly, Coyne identifies “gaucher’s disease in northern Swedes, Tay-sachs in the Cajuns of Louisiana and retinitis pigmentosa in…Tristan da Cunha” (p.124) as examples of genetic drift being the cause of harmful heterogeneity in the gene pool of a small population. Other species, such as Tympanuchus cupido, similarly exhibit harmful effects of genetic drift. It was found in 1993, that the small populations of prairie birds as a result of agricultural expansion caused the loss of nine alleles present in museum specimens from the 1960s. Thus, genetic drift resulted in a loss of genetic variation. (Campbell & Reece, 477)
The “bottleneck effect” is an effect often seen when an environmental disturbance reduces the number of individuals in a population. (Campbell & Reece, 476). According to Encyclopedia Brittanica, “allelic frequencies may have been considerably altered and thereby affect the future evolution of the species”. In this case, random chance may reduce genetic variability in surviving individuals and continual inbreeding may increase the frequency of harmful alleles. It seems important to realize that the overrepresentation of a given allele may counteract the effects of natural selection since reduced genetic variability will hamper the selection of “favorable” traits if these are virtually non-existent in a population. Thus, Coyne mentions that genetic drift “can actually overpower natural selection” (p.124). It is easy for neutral alleles to become fixed since these have no effect on a population, whereas natural selection will tend to not select for those traits that are harmful to the individuals of a population.
Sources:
evolution. (2010). In Encyclopædia Britannica. Retrieved March 20, 2010, from Encyclopædia Britannica Online School Edition: http://www.school.eb.com/eb/article-49866
Andrew J Bohonak, San Diego State University, San Diego, California, USA. “Genetic Drift in human populations”.
Campbell, Neil A. Reece, Jane B.. Biology, AP Edition 8e. 2008.
Endangered Species, Measuring. Elliot Richmond.Mathematics.
Beatriz is right when she says that one can't draw a direct correlation between natural selection and evolution; although natural selection is the driving force for the vast majority of all evolution, species can evolve without developing adaptations. If a species is no longer able to breed successfully with its original species, then evolution has occurred. Thus, genetic drift may contribute to evolution, but not adaptive evolution. Genetic drift could, for example, give humans genes that make them more likely to have heart disease wouldn't harm one's ability to reproduce, and would stay in a population (Process and Pattern in Evolution). While it may be a little blurry as to what traits are a result of selection or genetic drift, it is clear as to the definition of what is genetic drift. Genetic drift doesn't produce 'adaptations', because this word insists that there was a specific change due to environmental pressures. Instead, genetic drift may cause changes in traits of populations due to the random increase in allele frequency. So, to ask what the 'point' of genetic drift is isn't a valid question, because genetic drift simply exists as a result of the random occurrence of certain genes. Species don't consciously try to gain better genes, but rather genes appear either through the increased fitness of certain individuals or the appearance (or disappearance) of certain genes randomly through genetic drift (Futuyma, Douglas, "Evolutionary Biology").
ReplyDelete