23_Lecture_post - The Evolution of Populations Overview:...

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The Evolution of Populations Overview: The Smallest Unit of Evolution One misconception is that organisms evolve during their lifetimes Natural selection acts on individuals, but only populations evolve Consider, for example, a population of medium ground finches on Daphne Major Island During a drought, large-beaked birds were more likely to crack large seeds and survive The finch population evolved by natural selection
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1976 (similar to the prior 3 years) 1978 (after drought) Average beak depth (mm) 10 9 8 0
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Microevolution is a change in allele frequencies in a population over generations Three mechanisms cause allele frequency change: Natural selection Genetic drift Gene flow Only natural selection causes adaptive evolution
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Genetic variation makes evolution possible Variation in heritable traits is a prerequisite for evolution Mendel’s work on pea plants provided evidence of discrete heritable units (genes) Genetic variation among individuals is caused by differences in genes or other DNA segments Phenotype is the product of inherited genotype and environmental influences Natural selection can only act on variation with a genetic component
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Figure 23.3 (a) (b)
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Variation Within a Population Both discrete and quantitative characters contribute to variation within a population Discrete characters can be classified on an either-or basis Quantitative characters vary along a continuum within a population Genetic variation can be measured as gene variability or nucleotide variability For gene variability, average heterozygosity measures the average percent of loci that are heterozygous in a population Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals
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Variation Between Populations Most species exhibit geographic variation , differences between gene pools of separate populations For example, Madeira is home to several isolated populations of mice Chromosomal variation among populations is due to drift, not natural selection
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Figure 23.4 1 2.4 8.11 9.12 10.16 3.14 13.17 5.18 19 6 XX 7.15 1 2.19 9.10 11.12 13.17 3.8 15.18 4.16 5.14 XX 6.7
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Some examples of geographic variation occur as a cline , which is a graded change in a trait along a geographic axis For example, mummichog fish vary in a cold- adaptive allele along a temperature gradient This variation results from natural selection
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Figure 23.5 1.0 0.8 0.6 0.4 0.2 0 46 44 42 40 38 36 34 32 Maine Cold (6 ° C) Latitude (ºN) Georgia Warm (21ºC) Ldh-B b allele frequency 30
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Sources of Genetic Variation New genes and alleles can arise by mutation or gene duplication Formation of New Allelles A mutation is a change in nucleotide sequence of DNA Only mutations in cells that produce gametes can be passed to offspring A point mutation is a change in one base in a gene
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The effects of point mutations can vary: Mutations in noncoding regions of DNA are often
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This note was uploaded on 04/30/2011 for the course BIOL 1362 taught by Professor Loeblich during the Spring '08 term at University of Houston.

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23_Lecture_post - The Evolution of Populations Overview:...

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