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Study guide for Friday's exam

by: Saida Muktar

Study guide for Friday's exam Biolgoy 142

Saida Muktar

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Review notes for Friday's exam. Dr. Leips
Biology 142
Dr. Leips
Study Guide
biology 142
50 ?




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This 3 page Study Guide was uploaded by Saida Muktar on Thursday April 14, 2016. The Study Guide belongs to Biolgoy 142 at University of Maryland - Baltimore County taught by Dr. Leips in Spring 2016. Since its upload, it has received 22 views. For similar materials see Biology 142 in Biomedical Sciences at University of Maryland - Baltimore County.


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Date Created: 04/14/16
Chapter 26 Natural selection is only one of four processes that can shift allele frequencies in populations overtime, causing evolution: 1. Natural Selection - Increases the frequency of certain allels – the ones that contribute to reproductive success in a particular environment. Natural selection is the only one of the four processes that leads to adaptation. 2. Genetic drift - Causes allele frequencies to change randomly. In some cases, drift may cause allels that decrease fitness to increase in frequency 3. Gene flow - Occurs when individuals leave one population, join another and breed. Allele frequencies may change when gene flow occurs, because arriving individuals introduce alleles to their new population and departing individuals remove alleles from their old population 4. Mutation - Modifies allele frequencies by continually introducing new alleles. The alleles created by mutation may be beneficial or detrimental or have no effect on fitness. Natural selection is not the only agent responsible for evolution, and each of the four evolutionary processes has different consequences for genetic variation and fitness. Hardy-Weingberg principle – serves as a mathematical null hypothesis for the study of evolutionary processes. 26.1 Analyzing Change in Allele Frequencies: The Hardy-Weinberg Principle Population - a group of individuals from the same species that live in the same are at the same ime and can interbreed and that vary in the traits they posses. - All of the allees from all the gametes produced in each generation go into a single group called the gene pool and then combine at random to form offsprint. - The resulting Hardy-Weinberg principle makes two fundamental claims: 1. If the frequencies of alleles A a1d A in 2 population are given by p and q, then the frequenceies of genotypes A A , A A , 2 2 1 1 1 2 and A 2 2ill be given by p , pq, q respectively for generation after generation , That is : Allele frequencies : p + q = 1 2 2 Genotype frequencies : p , pq, q 2. When alleles are transmitted via meiosis and random combinations of gametes, their frequencies do not change over time. For evolution to occur, some other factors must come to play. - The Hardy-Weingberg model is based on important assuptions about how populations and alleles behave: 1. Random Mating - This condition was enforce by picking gametes from the gene pool at random. Individuals were not allowed to choose a mate 2. No natural selection - The model assumed that all members of the parental generation survived and contributed equal numbers of gametes to the gene pool, no matter what thier genotype 3. No genetic drift( random allele frequency changes) - The model assumed that alleles were picked in their exact frequencies p and q and not at some different values caused by change – that is, the model behaved as though the population was indefinitely large 4. No gene flow - No new alleles were added by immigration or lost through emigration. As a result, all of the alleles in the offspring population came from the original population’s gene pool. 5. No mutation - The model didn’t consider that new A s or1A s or2other, new alleles might be introduced into the gene pool 26.3 Natural Selection - Evolution by natural selection occurs when heritable variation leads to differential success in survival and reproduction. - Natural selection occurs when individuals with creating phenotypes produce more surviving offspring than individuals with other phenotypes do. - If certain alleles are associated with favored phenotypes, they increase in frequency while other alleles decrease in frequency. This result is evolution – a violation of the assumptions of the Hardy-Weinberg model. - Genetic variation – the number and relative frequency of alleles that one present in a particular population - Selection can occur only if heritable variation exists in a population - Natural selection occurs in a wide variety of patterns, or models, each with different consequences to genetic variation: o Directional selection changes the average value of a trait  Tends to reduce the genetic diversity of populations o Stabilizing selection reduces variation in a trait  There is no change in the average value of a trait over time and genetic variation in the population is reduced. o Disruptive selection increases variation in a trait  Sometimes plays a role in speciation o Balancing selection maintains variation in a trait  Occurs when no single allele has a distinct advantage  Balancing selection occurs when:  Heterozygous individuals have higher fitness than homozygous individuals do, a pattern called heterozygote advantage  The environment varies over time meaning that certain alleles are favored by selection at different times. As a result overall genetic variation in the population is maintained  Certain alleles are favored when they are rare, but not when they are common – a pattern known as frequency dependent selection


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