evolutionary change in populations
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Evolutionary Change in Populations. A population’s 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 - PowerPoint PPT PresentationTRANSCRIPT
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Biology, Seventh Edition Solomon • Berg • Martin
Chapter 18Evolutionary Change Evolutionary Change
in Populationsin Populations
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• A population’s 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
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• Evolution of populations is best understood in terms of frequencies:• Genotype• Phenotype• Allele
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
Genotype frequencies for all 1000 individuals of a hypothetical population
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
Phenotype frequencies for all 1000 individuals of a hypothetical population
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
Allele frequencies for all 1000 individuals of a hypothetical population
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition CHAPTER 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
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• Genetic equilibrium requires• Random mating• No net mutations• Large population size• No migration• No natural selection
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition CHAPTER 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
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• Hardy-Weinberg equation• p = frequency of dominant allele• q = frequency of the recessive
allele:p + q = 1
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• The genotype frequencies of a population are described by the relationship p2 + 2pq
+ q2 = 1• p2 is frequency of homozygous
dominant genotype• 2pq is frequency of heterozygous
genotype• q2 is frequency of homozygous
recessive genotype
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
(a) Genotype and allele frequencies
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
(b) Segregation of alleles and random fertilization
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Biology, Seventh Edition CHAPTER 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
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Biology, Seventh Edition CHAPTER 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
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• Nonrandom mating• Inbreeding
–Inbreeding depression• Assortative mating• Both of these increase frequency
of homozygous genotypes
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• Mutation• Source of new alleles• Increases genetic variability
acted on by natural selection
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Biology, Seventh Edition CHAPTER 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
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Biology, Seventh Edition CHAPTER 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
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• Gene flow• Movement of alleles caused by
migration of individuals between populations
• Causes changes in allele frequencies
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• Natural selection• Causes changes in allele
frequencies leading to adaptation• Operates on an organism’s
phenotype• Changes genetic composition of
a population favorably for a particular environment
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• Modes of selection• Stabilizing
–Favors the mean• Directional
–Favors one phenotypic extreme• Disruptive
–Favors two or more phenotypic extremes
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
Modes of selection
(a) No selection (b) Stabilizing selection
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
Modes of selection
(c) Directional selection (d) Disruptive selection
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• Genetic variation in populations caused by• Mutation• Sexual reproduction
–Allows new phenotypes
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• Methods of evaluating genetic variation• Genetic polymorphism
–Balanced polymorphism• Neutral variation• Geographic variation
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
• Balanced polymorphism: two or more alleles persist in a population over many generations• Heterozygote advantage• Frequency-dependent selection
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Biology, Seventh Edition CHAPTER 18 Evolutionary Change in Populations
Clinal variation in yarrow (Achillea millefolium)