i. i.microevolution d. d.genetic drift 1. 1.bottleneck effect ex: elevated frequency of tay-sachs...
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I. Microevolution
D. Genetic Drift1. Bottleneck effect
• Ex: Elevated frequency of Tay-Sachs Disease in Ashkenazi Jews
• Ex: Genetic homogeneity in populations of African cheetahs
2. Founder effect• Allele frequencies in small populations may reflect
genotypes of founding individuals• Common in isolated populations• Ex: Finns descended from small group of people
~4000 years ago; genetically distinct from other Europeans
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I. Microevolution
E. Gene Flow• Movement of fertile individuals or gametes
among populations• Tends to
• Increase diversity within populations• Decrease diversity among populations
• Elevated gene flow can amalgamate separate populations into a single population
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Fig. 23.12
• 43% of central & 13% of eastern first-time-breeding females immigrated from mainland• Mainland females survive and reproduce poorly• Gene flow from mainland reduces fitness of central vs. eastern females
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II. Genetic Variation
• Provides raw material for natural selection
• Homogeneous population – little opportunity for differential fitness
• Sources1) Mutation
2) Crossing over
3) Independent assortment (Meiosis)
4) Random fertilization
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II. Genetic Variation
A. Within Populations• Variation in
• Discrete characters• Ex: Color in some flowers (pink or white)
• Quantitative characters• Ex: Skin color in humans
1. Polymorphism• Two or more alleles at a single locus• Extensive in most populations• Phenotypic – Different morphs (body forms)• Genotypic – May not produce discrete phenotypes• Measurement
• Drosophila – 14% heterozygosity, ~1% nucleotide variability
• Homo sapiens – ~0.1% nucleotide variability
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II. Genetic Variation
B. Between Populations1. Geographic Variation
• Differences among genetically distinct populations within a species• Differences may be due to random variation
• Differences may occur over a geographic range• Cline – Graded variation in phenotype and genotype
over a geographic range• Common among species with continuous ranges
over large areas• Higher latitudes: Smaller individuals (plants)• Higher latitudes: Larger individuals (animals)• Why?
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II. Genetic Variation
C. Natural Selection• Can alter frequency distribution of heritable traits1. Directional selection
• Environmental change over time favors phenotypes at one extreme
• Possible only if population contains multiple alleles, at least one of which is favored
• Ex: Black bears in Europe larger during glacial periods, smaller during interglacials
2. Disruptive selection• Favors extremes at expense of mean• Also called diversifying selection• Ex: During a drought, Galápagos finches with long beaks
able to open cactus fruits, birds with wide beaks stripped off tree bark to expose insects, intermediate beaks less useful
3. Stabilizing selection• Favors mean at expense of extremes• Reduces variation• Ex: Birth weight in humans
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Fig. 23.13
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II. Genetic Variation
C. Natural Selection• Can alter frequency distribution of heritable traits1. Directional selection
• Environmental change over time favors phenotypes at one extreme
• Possible only if population contains multiple alleles, at least one of which is favored
• Ex: Black bears in Europe larger during glacial periods, smaller during interglacials
2. Disruptive selection• Favors extremes at expense of mean• Also called diversifying selection• Ex: During a drought, Galápagos finches with long beaks
able to open cactus fruits, birds with wide beaks stripped off tree bark to expose insects, intermediate beaks less useful
3. Stabilizing selection• Favors mean at expense of extremes• Reduces variation• Ex: Birth weight in humans
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Fig. 23.13
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II. Genetic Variation
C. Natural Selection• Can alter frequency distribution of heritable traits1. Directional selection
• Environmental change over time favors phenotypes at one extreme
• Possible only if population contains multiple alleles, at least one of which is favored
• Ex: Black bears in Europe larger during glacial periods, smaller during interglacials
2. Disruptive selection• Favors extremes at expense of mean• Also called diversifying selection• Ex: During a drought, Galápagos finches with long beaks
able to open cactus fruits, birds with wide beaks stripped off tree bark to expose insects, intermediate beaks less useful
3. Stabilizing selection• Favors mean at expense of extremes• Reduces variation• Ex: Birth weight in humans
![Page 13: I. I.Microevolution D. D.Genetic Drift 1. 1.Bottleneck effect Ex: Elevated frequency of Tay-Sachs Disease in Ashkenazi Jews Ex: Genetic homogeneity in](https://reader035.vdocuments.net/reader035/viewer/2022062409/56649f4f5503460f94c71939/html5/thumbnails/13.jpg)
Fig. 23.13
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Stabilizing Selection
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Fig. 23.13
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II. Genetic Variation
D. Preservation of Variation• Why aren’t we all homozygous for the most
favorable alleles?• Balancing selection occurs when natural
selection maintains two or more phenotypes in a population = balanced polymorphism
1. Heterozygote advantage• Heterozygotes more fit than homozygotes• Ex: Sickle-cell disease
2. Frequency-dependent selection• Phenotypic fitness depends on rarity in population• Ex: Non-selective predation
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Fig. 23.17
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http://www.cdc.gov/malaria/about/biology/sickle_cell.html
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II. Genetic Variation
D. Preservation of Variation• Why aren’t we all homozygous for the most
favorable alleles?• Balancing selection occurs when natural
selection maintains two or more phenotypes in a population = balanced polymorphism
1. Heterozygote advantage• Heterozygotes more fit than homozygotes• Ex: Sickle-cell disease
2. Frequency-dependent selection• Phenotypic fitness depends on rarity in population• Ex: Non-selective predation
![Page 20: I. I.Microevolution D. D.Genetic Drift 1. 1.Bottleneck effect Ex: Elevated frequency of Tay-Sachs Disease in Ashkenazi Jews Ex: Genetic homogeneity in](https://reader035.vdocuments.net/reader035/viewer/2022062409/56649f4f5503460f94c71939/html5/thumbnails/20.jpg)
III. Development of New Species
A. Anagenesis (Phyletic Evolution)• Accumulated changes transform one species
into another• Same number of species at beginning and
end
B. Cladogenesis (Branching Evolution)• Formation of new species, with parental
species continuing to exist (potentially altered)• Increased number of species
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III. Development of New Species
• Biological Species Concept• Developed by Ernst Mayr• “Population or group of populations whose
members have the potential to interbreed in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species”
• Why don’t individuals from different species interbreed?
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IV. Reproductive Isolation
A. Prezygotic barriers• Prevent fertilization
B. Postzygotic barriers• Act after fertilization has occurred
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Fig. 24.3
Time of Day
Time of Year
Courtship
Sounds/Songs
Flowers
Snails
Plants
Broadcast Spawners
Bullfrog x
Leopard Frog
Horse (2n=64) x
Donkey (2n=62) Mule (2n=63)
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IV. Reproductive Isolation
C. Limitations of Biological Species Concept• Mayr’s definition emphasizes reproductive
isolation; may not work in all situations• Ex: Classifying fossil organisms• Ex: Species that reproduce asexually [prokaryotes,
some protists, fungi, plants (e.g. bananas), animals (e.g. fishes, lizards)]
• Ex: Multiple species are inter-fertile but remain distinct (e.g. orchids)