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Biol 121- 2010.03.15- Evolution- Allele Frequencies in Populations (Ch. 25).docx

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BIOL 121
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Biol 121 225 Freeman 503-508, 511-520 Mar 15, 10 Allele Frequencies in Populations Population -a population is a group of individuals from the same species that live in the same area at the same time (and interbreed) Population genetics -population genetics looks at: -contribution of alleles to a population -quantification of genetic variation in a population -measurement of changes in a population over time = evolution Four mechanisms that shift allele 1) Natural selection increases frequency of certain alleles – the ones that frequencies in populations contribute to success in survival and reproduction 2) Genetic drift causes allele frequencies to change randomly, and in some cases drift may even cause alleles that decrease fitness to increase in fq. 3) Gene flow occurs when individuals leave the population and join another and breed. Allele frequencies may change when gene flow occurs b/c arriving individuals introduce alleles while departing individuals remove alleles 4) Mutation modifies allele frequencies by continually introducing new alleles – the alleles created by mutation may be beneficial, detrimental, or have no effect on fitness -natural selection is not the only agent responsible for evol -each of these four processes have diff consequences -natural selection is the only mechanism that acting alone can result in adaptation -the other three (mutation, gene flow, drift) do not favour certain alleles over others -mutation and drift introduce a nonadaptive component into evol Agrostis tenuis growing on heavy -A. tenuis is a plant species and some of its individuals can grow on heavy metals on an abandoned mine in metals such as in an abandoned mine site in Wales – the population has Wales. adapted and evolution has occurred -therefore, a change in allele frequency has occurred Gene pool -all the gametes of a single generation in a population -can put all the genes together in a ‘pool’ and calculate the expected frequency of particular alleles if pulled out gametes at random p and q p = frequency of dominant allele q = frequency of recessive allele q² = frequency of homozygous recessive, 2pq = fq of heterozygous p + q = 1 p^2 + 2pq + q^2 = 1 Hardy-Weinberg (HW) principle or -HW describes a non-evolving population equilibrium -ie. The frequency of alleles and genotypes in the gene pool remain constant -sexual recombination through meiosis and random fertilization do not change the population’s genetic structure Assumptions of HW theorem (5) 1) No natural selection at the gene in question (no advantage of an allele) 2) No genetic drift (random change in allele frequency) 3) No gene flow (immigration = emigration = 0) 4) No mutation (no new alleles) 5) Random mating (no sexual selection) Limitations of HW theorem (2) 1) Assumptions (above 5) are rarely true in a natural population 2) Usually, the allele frequency changes from one generation to the next Applications of HW theorem (2) 1) Modeling – being able to predict the allele frequencies 2) Null model – works as null hypothesis – if not met, then evol is occurring 1 Biol 121 225 Freeman 503-508, 511-520 Mar 15, 10 Non-Directional (Random) Changes in Allele Frequencies Genetic Drift -Genetic drift causes allele frequencies to change randomly, and in some cases drift may even cause alleles that decrease fitness to increase in fq. -there are two types of this -genetic drift is random with respect to fitness – the allele fq changes are not adaptive; and genetic drift is most pronounced in small pop -genetic drift tends to decrease genetic diversity over time, as alleles are randomly lost or fixed Two types of Genetic Drift 1) Founder Effect 2) Bottleneck Effect 1) 1) Founder Effect -when a group of individuals emigrate to a new geographic area and establishes a new population -if the group is small enough, allele frequencies in the new population are almost guaranteed to be diff from those in the old, due to sampling error -a change in allele frequencies that occurs when a new pop is established is called a founder effect -e.g. Hurricanes swept through the Carribean region in 1995, and a large raft floated onto Anguilla with green iguana and they had different allele frequencies than the original pop because only 15 individuals arrived 2) 2) Bottleneck effect -if a large pop experiences a sudden reduction in size, population bottleneck is said to occur – survivors are random -caused by disease outbreaks, floods, fires, storms, etc -a genetic bottleneck is a sudden reduction in the number of alleles in a
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