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Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh Edition Solomon Berg Martin Chapter 18 Evolutionary Change in Populations.

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Presentation on theme: "Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh Edition Solomon Berg Martin Chapter 18 Evolutionary Change in Populations."— Presentation transcript:

1 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh Edition Solomon Berg Martin Chapter 18 Evolutionary Change in Populations

2 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations A populations gene pool Includes all the alleles for all the loci present in the population –Diploid organisms have a maximum of two different alleles at each genetic locus –Typically, a single individual therefore has only a small fraction of the alleles present

3 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Evolution of populations is best understood in terms of frequencies: Genotype Phenotype Allele

4 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Genotype frequencies for all 1000 individuals of a hypothetical population

5 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Phenotype frequencies for all 1000 individuals of a hypothetical population

6 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Allele frequencies for all 1000 individuals of a hypothetical population

7 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Hardy-Weinberg Principle Explains stability of successive generations in populations at genetic equilibrium Essential to understanding mechanisms of evolutionary change

8 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Genetic equilibrium requires Random mating No net mutations Large population size No migration No natural selection

9 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Hardy-Weinberg principle Shows that if population is large, process of inheritance alone does not cause changes in allele frequencies Explains why dominant alleles are not necessarily more common than recessive alleles

10 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Hardy-Weinberg equation p = frequency of dominant allele q = frequency of the recessive allele: p + q = 1

11 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations The genotype frequencies of a population are described by the relationship p 2 + 2pq + q 2 = 1 p 2 is frequency of homozygous dominant genotype 2pq is frequency of heterozygous genotype q 2 is frequency of homozygous recessive genotype

12 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations (a) Genotype and allele frequencies

13 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations (b) Segregation of alleles and random fertilization

14 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Microevolution Intergenerational changes in allele or genotype frequencies within a population Often involves relatively small or minor changes, usually over a few generations

15 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Changes in allele frequencies of a population caused by microevolutionary processes: Nonrandom mating Mutation Genetic drift Gene flow Natural selection

16 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Nonrandom mating Inbreeding –Inbreeding depression Assortative mating Both of these increase frequency of homozygous genotypes

17 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Mutation Source of new alleles Increases genetic variability acted on by natural selection

18 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Genetic drift Random change in allele frequencies of a small population Decreases genetic variation within a population Changes it causes are usually not adaptive

19 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Genetic drift Bottleneck is a sudden decrease in population size caused by adverse environmental factors Founder effect is genetic drift occurring when a small population colonizes a new area

20 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Gene flow Movement of alleles caused by migration of individuals between populations Causes changes in allele frequencies

21 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Natural selection Causes changes in allele frequencies leading to adaptation Operates on an organisms phenotype Changes genetic composition of a population favorably for a particular environment

22 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Modes of selection Stabilizing –Favors the mean Directional –Favors one phenotypic extreme Disruptive –Favors two or more phenotypic extremes

23 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Modes of selection (a) No selection (b) Stabilizing selection

24 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Modes of selection (c) Directional selection (d) Disruptive selection

25 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Genetic variation in populations caused by Mutations Sexual reproduction –Allows new phenotypes

26 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Methods of evaluating genetic variation Diploidy – to have two alleles ; hides genetic variation from selection in heterozygote's but maintains diversity Genetic polymorphism (when two or more morphs are present in noticeable freq.) –Balanced polymorphism (ability to maintain diversity in a population) –Heterozygous advantage (sickle-cell anemia – Ss – resistant to malaria Neutral variation – no selective advantage/disadvantage –Little impact on reproductive success Geographic variation

27 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations

28 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Balanced polymorphism: two or more alleles persist in a population over many generations Heterozygote advantage Frequency-dependent selection

29 Copyright © 2005 Brooks/Cole Thomson Learning Biology, Seventh EditionCHAPTER 18 Evolutionary Change in Populations Clinal variation in yarrow (Achillea millefolium)


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