Microevolution - Mechanisms of Microevolution...

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Mechanisms of Microevolution Reading:Freeman, Chapter 24
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Microevolution vs. Macroevolution The term “ microevolution ” applies to evolutionary change within a lineage It occurs continuously. Depending upon the organism and the circumstances, it can transform a lineage. dramatically over time. Alternately, a lineage may appear to remain the same over time-this is called stasis . Macroevolution is the origin and extinction of lineages. It can happen gradually, or slowly. Both processes are essential to evolution. Microevolution is probably better understood, and better documented, because in some organisms it takes place on timescales we can study directly by experiment and direct observation. Ironically, in “On the Origin of Species” Darwin lays out a theory of microevolution…he assumed macroevolution would inevitably result from microevolution. It would be 100 years later that Ernst Mayr, and others, would develop a scientific theory of speciation. The replacement of one species by another (as opposed to the replacement of one allele by another), by the way, is an ecological process. .it is not evolution in the usual sense, though this phenomenon usually leads to extinction of some species and diversification of others.
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The Population is the Basic Unit of Microevolutionary Change The genotype of an individual is, essentially, fixed at birth. The population is the smallest unit where evolutionary change is possible. Unlike individuals, populations permit the origin of new alleles through mutation, and the change in the frequency of alleles through selection, genetic drift, etc. . Individuals do not evolve, populations and species evolve.
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Population Genetics Population genetics refers to the study of evolution via the observation and modeling of allele frequencies and genetic change in populations of organisms. There are three parameters to keep in mind: allele frequency: the proportion of a specific allele at a given locus, considering that the population may contain from one to many alleles at that locus. genotype frequency: the proportion of a specific genotype at a given locus, considering that many different genotypes may be possible. phenotype frequency: the proportion of individuals in a population that exhibit a given phenotype.
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Consider a population of N organisms. Two phenotpyes, yellow and tan. Suppose that they are diploid and reproduce sexually. Consider one gene with two alleles, A and a . The possible genotypes are therefore: AA, Aa, and aa.
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Phenotype Frequencies To calculate the frequency of a phenotype, count the number of individuals with that phenotype, and divide by the total. Therefore, the frequency of the yellow phenotype in the population below is 4/10=.40
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Genotype Frequencies- To calculate the frequency of a genotype in the population, find the total number of individuals in the population with that genotype, and divide by the population size, N. f(AA)= #(AA)/N
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This note was uploaded on 10/28/2010 for the course BIO 32345 taught by Professor Molumby during the Spring '10 term at Loyola Chicago.

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Microevolution - Mechanisms of Microevolution...

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