bild 3 lecture 7

bild 3 lecture 7 - The graphs below show the results In the...

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The graphs below show the results. In the cloudy parts of the lake (a, left), red and blue males are mixed together at a shallow depth and the color differences between the fish are not marked. In the clear parts, blue males are found in shallow depths and red males at greater depths. Steepest Light loss gradient Shallowest The opsin alleles show a similar split in the clear parts of the lake. They are mostly blue-sensitive alleles in the cloudy parts of the lake, but the population is split into groups carrying blue-sensitive and red-sensitive alleles in the regions where the water is clear. These observations demonstrate that disruptive selection can push gene pools apart when there is a range of environments that the fish can inhabit, but that disruptive selection breaks down when the fish are crowded into a uniform
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environment. Disruptive selection has aided the process of speciation in these fish when the water is clear, but recent human activities have made parts of the lake cloudy and have reversed the speciation process. Balancing Selection That Maintains Polymorphism Advantageous alleles that survive the first few generations after their introduction by mutation will increase rapidly in frequency as a result of directional selection. If a population is sampled just as a new advantageous allele is in the process of replacing the old one at a locus, the sample will show that the population is polymorphic at that locus. The polymorphism will soon be lost, however, when the new allele becomes fixed. This kind of polymorphism is called a transient polymorphism , because it lasts for a relatively small number of generations before the allele that confers an advantage replaces the disadvantageous one. There are, however, situations in which natural selection will maintain genetic polymorphisms, sometimes for long periods. Such polymorphisms are called balanced polymorphisms, because they are maintained by a balance of selective pressures. Balanced polymorphisms play an extremely important role in evolution. They are responsible for the ability of populations of organisms to change rapidly in response to environmental change. Several mechanisms are important in the maintenance of genetic polymorphism. Heterozygote Advantage J. B. S. Haldane suggested in 1949 that the sickle cell anemia allele persists at high frequency in some human subpopulations in Africa and the Middle East as a result of heterozygote advantage , a situation in which the heterozygote at the locus has a higher Darwinian fitness than either the mutant or the wild type homozygote. We now know that heterozygotes have an advantage not because they are more resistant to malaria than other members of the population, but because they are least likely to suffer harmful effects from malaria infections. Such heterozygote advantage tends to retain sickle cell mutant alleles at high
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bild 3 lecture 7 - The graphs below show the results In the...

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