ComputerEx2_2010_KEY_evolution in finite pops at 1 & 2 loci

ComputerEx2_2010_KEY_evolution in finite pops at 1 & 2 loci

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C. Baer, PCB 4674 Fall 2010 Computer Exercise #2 ANSWER KEY : Advanced Evolutionary Dynamics Answers are given in BLUE; questions for which no answers are given have a wide range of allowable answers and/or depend on your specific values generated with Populus. The assignment includes N questions directly based on the simulations, of which 15 will be graded , worth N points each. The assignment is worth 25 total points. Questions are presented in BOLD TEXT, some of which are followed by a number in brackets [1] and are highlighted in yellow background. Only the highlighted questions will be graded, so please INCLUDE ANSWERS ONLY TO THE HIGHLIGHTED QUESTIONS ON YOUR ANSWER SHEET . However, you will need to answer all of the questions to be able to interpret the for-grade questions. When you turn in your assignment you should label your answers correspondingly, e.g., 5 [1], 11 [2], etc. . I. Evolution in a Finite Population - Natural Selection at One Locus In Assignment 1, you learned that the outcome of directional selection in an infinite population was deterministic: the frequency of the favored allele increased until it became fixed in the population. Moreover, you learned that in small populations, the change in allele frequencies across generations was unpredictable and the fixation of one allele or the other occurred at random. Now we will investigate the interaction between natural selection and genetic drift in a finite population. Recall that selection is a predictable evolutionary force only when a population is sufficiently large or selection is sufficiently strong. The outcome of selection can be predicted by the relationship between the selection coefficient (s), a measure of the strength of selection against a genotype, and the effective population size. If: s > 1/N then natural selection will determine the outcome of allele frequencies s < 1/N then genetic drift will determine the outcome of allele frequencies This is a profound result. This result means that if an allele confers only a very small advantage in fitness when contrasted against the other alleles at a locus, selection will be able to increase its frequency only if the population is very large. Thus not all favorable alleles will be spread through a population or species by natural selection, and good mutations of small effect may well be lost through genetic drift. Think about this from the other direction: in a small population only alleles with large effects on fitness will be spread through natural selection. On the other hand, in large populations, even alleles with very small benefits will increase in frequency through natural selection. The purpose of this assignment is to convince you of this result and allow you to explore these dynamics for yourself. A. Strong Directional Selection in Finite Populations
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This note was uploaded on 06/08/2011 for the course PCB 4674 taught by Professor Baer during the Fall '08 term at University of Florida.

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ComputerEx2_2010_KEY_evolution in finite pops at 1 & 2 loci

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