Chapter 24

Chapter 24 - Chapter 24: Evolutionary Processes Even though...

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Chapter 24: Evolutionary Processes Even though natural selection acts on individuals, evolutionary change occurs in populations A population is a group of individuals from the same species that live in the same area and regularly interbreed Natural selection is not the only process that causes evolutionary change There are actually four mechanisms that shift allele frequencies in populations o Natural selection increases the frequency of certain alleles—the ones that contribute to improved reproductive success o Genetic drift causes allele frequencies to change randomly. In some cases, drift may even cause alleles that decrease fitness to increase in frequency o Gene flow occurs when individuals immigrate into (enter) or emigrate from (leave) a population. Allele frequencies may change when gene flows occurs, because arriving individuals introduce alleles to their new population and emigrating individuals remove alleles from their old populations o 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 evolution, and each of the four evolutionary processes has different consequences Natural selection is the only mechanism that consistently results in adaptation 24.1 ANALYZING CHANGE IN ALLELE FREQUENCIES: THE HARDY-WEINBURG PRINCIPLE To study how the four evolutionary processes affect populations, biologists take a three- pronged approach First they create mathematical models that track the fate of alleles over time Then they collect data to test predictions made by the models’ equations Finally, they apply the results to solve problems in conservation biology or human genetics This research began with work published in 1908 by G. H. Hardy and Wilhelm Weinberg As the time, it was commonly believed that changes in allele frequency occur simply as a result of sexual reproduction—meiosis followed by the random fusion of gametes (egg and sperm) to form offspring Some biologists claimed that dominant alleles would inevitably increase in frequency, while others predicted that two alleles of the same gene would eventually reach a frequency of 0.5 To test the hypothesis they analyzed what happens to the frequencies of alleles when any individuals in a population mate and produce offspring They wanted to know what happened in an entire population, when all of the individuals —and thus all possible genotypes—bred To analyze the consequences of mating among all the individuals in a population, they imagined that all of the gametes produced in each generation go into a single bin called the gene pool
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They then had to simply calculate what happened when two gametes were plucked at random our of the gene pool, many timesa, and each of these gamete pairs was then combined to form offspring These calculations would predict the genotypes of the offspring that would be produced,
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This note was uploaded on 04/17/2008 for the course BIOL 202 taught by Professor Kopeny during the Spring '08 term at UVA.

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Chapter 24 - Chapter 24: Evolutionary Processes Even though...

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