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Chapter 25 Evolutionary Processes.docx

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McMaster University
Jon Stone

Chapter 25 Evolutionary Processes I) The Hardy Weinberg principle acts as a null hypothesis when researchers want to test whether evolution or non-random mating is occurring at a particular gene a. If the frequencies of alleles A1 and A2 in a population are given by p and q, then the frequencies of genotype A1A1, A1A2, and A2A2 will be given by p^2, 2pq and q^2 for generation after generation b. When alleles are transmitted according to the rules of mendelian inheritance, their frequencies do not change over time, for evolution to occur some other factors must be at play. c. Assumptions (that could be the factor at play if the frequencies are not equal): i. No natural selection. ii. No genetic drift, or random allele frequency changes iii. No gene flow iv. No mutation v. Random Mating II) Each of the four evolutionary mechanisms has different consequences. Only natural selection produces adaptation. Genetic drift cause random fluctuations in allele frequencies. Gene flow equalizes allele frequencies between populations. Mutation introduces new alleles. a. Natural Selection – increases the frequency of certain alleles – the ones that contribute to success in survival or reproduction (directed by the environment) i. Balancing Selection – heterozygous advantage, genetic variation is maintained ii. Directional Selection – the average phenotype changes in one direction purifying selection - when disadvantageous alleles decline in frequency iii. Stabilizing Selection – there is no change in the average value of a trait over time and genetic variation in the population is reduced iv. Disruptive selection – eliminates phenotypes in near the average value and favours the extreme phenotypes – may lead to speciation b. Genetic Drift – causes allele frequencies to change randomly. In some cases, drift may even cause alleles that decrease fitness to increase in frequency. (random) i. Genetic drift is random with respect to fitness ii. Genetic is more pronounced in smaller population because larger sampling error iii. Overtime genetic drift can lead to random loos or fixation of alleles. iv. Causes of Sampling Error: 1. Founder Effect – a change in allelic frequency that occurs when a new population is established 2. Population/genetic Bottleneck- a sudden reduction in the number of alleles of a population c. Gene Flow – occurs when individuals leave one population, join another, and breed. Allele frequencies may change when gene flow occurs, b/c arriving individuals introduce alleles to their new population and departing individuals remove alleles from their old population. (random) i. as an evolutionary mechanism outcome is: equalizes allele frequencies between the source population and the recipient population ii. gene flow can increase genetic diversitygoo
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