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Biol 121- 2010.03.17- Evolution- Variation in Populations and Mechanisms for Selection (Ch. 25).docx

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BIOL 121
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Biol 121 225 Freeman 508-511 (Ch. 25) Mar. 17, 10 Variation in Populations and Mechanisms for Selection Why is variation good? -variation is usually good because if there is an environmental change, there needs to be natural selection (which requires genetic variation) so that the species does not die out Genetic variation -refers to the number and relative frequency of alleles that are present in a particular population How is variation maintained in a 1) Sexual Reproduction population? (4) 2) Diploidy 3) Balanced polymorphism – heterozygote advantage 4) Frequency-dependent selection 1) 1) Sexual reproduction -in meiosis, independent assortment as well as crossing over occur, which contributes to variation in individuals 2) 2) Diploidy – 2 alleles for every gene -recessive alleles “hide” and are not expressed when there is a dominant allele -both parents contribute an allele, and both must give a recessive allele for there to be a recessive phenotype -this means that having a recessive phenotype is less likely to occur which is a good thing if the recessive allele is ‘bad’ 3) 3) Balanced polymorphism – heterozygote advantage -polymorphism is when there are more than two phenotypes present (can be due to multiple alleles, incomplete dominance and/or codominance) -heterozygote advantage means that individuals that are heterozygous have higher fitness than homozygous individuals (results in balancing selection) e.g. a point mutation in DNA can lead to sickle cell anemia (glutamic acid becomes valine). Sickle cell anemia disease – high mortality rate, the affected rarely reproduce so why is it still in the population? -malaria is the most chronic health problem worldwide caused by parasite Plasmodium carried by mosquito Anoholes -infects red blood cells and causes anemia -however, people with sickle cell anemia have sickle cells that actually due to their shape inhibit entry of the Plasmodium parasite -the heterozygote for the gene will have both normal RBC and sickle cell RBC -therefore, in terms of sickle cell anemia, he will not be sick and furthermore he can resist Plasmodium -heterozygote advantage in this case is a trade-off -this example of heterozygote advantage is codominance – both phenotypes present (have both sickle cells and normal cells) 4) 4) Frequency-dependent selection -is an evolutionary process where the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population -in positive frequency dependent selection, fitness of a phenotype increases as it becomes more common e.g. -the host (e.g. human cells) have multiple receptors on the cell surface -parasites recognize and binds to one, and entry into the host is facilitated -host responds so that an enhanced immune response results -new antigen appears on surface of cell, T cell attaches and kills that cell before new parasites have been produced inside Biol 121 225 Freeman 508-511 (Ch. 25) Mar. 17, 10 -some parasites are able to recognize different host receptor, becoming more effective in invading -host responds again “Evolutionary arms race” -this is frequency-dependent because parasites recognize host cells, then parasites increase in number -the greater the # parasites with that phenotype, the greater the pressure on the host and the increased likelihood that the host population with the more enhanced immune response will be selected for -alleles alternatively are selected for and against -this example was specifically negative frequency dependent selection, where fitness of the phenotype increases as it becomes less common – this is also a particular mechanism of balancing selection Types of Selection -there are different types of natural selection -natural selection occurs when individuals with certain phenotypes produce more offspring than individuals with other phenotypes do -if certain alleles are associate
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