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Download IB Biology Year 2 / IHS ALTERING ALLELE FREQUENCIES KEY
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IB Biology Year 2 / IHS ALTERING ALLELE FREQUENCIES KEY Directions: use Neil 9e sections 23.3 & 23.4 to complete this chart Process Description and, if appropriate, names of different types Predicted effect on a population’s genetic structure Example(s) Mutation Mutation, the replacement of one allele with another, is too infrequent to change p, q Non-random mating Non-random mating, the choosing of partners by particular phenotypes, does not change p and q, only genotype frequencies. Natural selection Differential success in survival and reproduction. The result, usually, is that NS matches organisms to their environments. Directional selection: pop. mean shifts one direction or the other. If this occurs in several populations beginning from one gene pool, it is known as ADAPTIVE RADIATION. Disruptive selection: acquisition of bimodal population. May change p, q Stabilizing selection: the most common phenotype is the most selected for; at work all the time, everywhere. May change p, q Sexual selection: Female choice (usually) selects for an exaggerated trait in males. (Why? Probably association with health!) May change p, q Founder Effect: colonization, when a small part of the population breaks off from the rest; likely the colonists’ gene pool is different from that of the source population. May change p, q or even lose alleles due to chance Bottleneck: when a population experiences a natural disaster (unrelated to pop. members' genetics) or other event that reduces its size at some point. May change p, q or even lose alleles due to chance, thus reducing genetic variation; in subsequent pop’s; can also fix alleles. Remember, NS won’t cull all unfavorable alleles: it doesn’t “see” recessive alleles in a heterozygote, sometimes heterozygotes are most fit, which maintains recessive alleles in the pop. Sometimes NS is frequency dependent, so the fitness of a phenotype depends on how common it is. Genetic drift = Chance events causing p & q to fluctuate unpredictably, even sometimes losing genetic variation May change p, q Galapagos finch beaks as a result of drought affecting type of seed available; Drosophila resistance to insecticides like DDT Patchy environments, like gold mice on sand and black mice on lava flows; or seed cracker finches in Cameroon Human birth weight: very small and very large babies experience increased mortality relative to “average” sized babies. A male peacock’s bright feathers. NOTE: his big, gaudy feathers do not confer a survival advantage but if peahens choose him to mate, that’s what counts! RP in human pop. on Tristan de Cunha, a British island in the Atlantic. All people descended from original group of 15, one of which carried RP allele Greater prairie chicken pop’s in Illinois, modern vs. museum specimens, and vs. KS and NE flocks – fewer alleles per locus. Gene flow Transfer of alleles in/out of a population due to movements of fertile individuals or their gametes Increases genetic variation within a pop, but reduces it between pop’s. May be the result of incomplete reproductive isolation. Two songbird pop’s on Dutch island of Vlieland; the central population has gene flow with mainland pop, bringing untested alleles. Eastern population has less gene flow and is therefore more adapted to island conditions.