Lecture 10. Week 4 BIOL 108. Evolution PDF

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Cynthia Friedman ...

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Week 4.2 1) Evolution via gene flow •

Transfer of genes (alleles) between populations (migration) o

Example: interbreeding, migration

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Increased variation WITHIN a population

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Decreased variation BETWEEN populations

2) Evolution via genetic drift •

Two main examples: Founder effect and bottleneck effect

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Changes in allele frequency due to chance (regardless of natural selection)

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Allele frequencies “drift” from one generation to the next.

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The impact of drift is greater in small populations

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Essentially due to (random) chance: “small sample size”

a) Founder Effect: •

New population established by a few colonizers

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Small fraction of the total genetic variation compared to the ancestral population & change in allele frequency (evolution)

b) Bottleneck effect

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Only a few individuals survive

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Only these few reproduce in the next generation

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Gene frequency in the next generation is different than previous generation. Rare alleles are more likely be lost due to drift

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B. How to maintain variation (in spite of natural selection)? •

Mutation

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Recombination (crossing-over)

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Independent assortment (of alleles)

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Fertilization (sexual reproduction)

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Disruptive selection (type of natural selection)

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Gene flow (between populations)

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Balancing Selection (works on genotypes)! 1. Negative frequency-dependent selection 2. Heterozygote advantage o

Note: Heterozygote advantage not always the same as “stabilizing selection”.

1) Negative frequency-dependent selection: rare phenotype [morph] & therefore rare genotype [alleles] has advantage 2) Heterozygote advantage:

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Sickle cell disease: problem with hemoglobin protein subunit (carries oxygen) ® sickle-shaped red blood cells

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A = allele for normal hemoglobin; S for allele for abnormal

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Relative fitness: In non-malarial regions: AA > AS SS. Malarial regions: AS > AA or SS

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Body destroys sickle -shaped blood cells, along with the malarial (Plasmodium).

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Heterozygotes have advantage in malarial regions (environment)!

• IV. Evidence for Evolution •

Fossil Record

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Comparative embryology and anatomy

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Biogeography: geographic distribution of related species

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Molecular evidence

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Artificial selection

While the process of natural selection acts on the individuals (causing differential survival and reproduction), the outcome of natural selection, i.e., evolution, is seen at the population level. * Populations evolve, but individual organisms do not *

Topic 3 Problems 3a.1 How did Georges Cuvier account for extinctions in nature? A. Extinctions never occur--there are unexplored parts of the globe where organisms that appear to have gone extinct may still live. B. Extinctions occur when the slow adaptation of organisms over time to their environment is not quick enough to help them respond to changing conditions. C. Extinctions occur at random, they do not reflect God's will. D. Extinctions are due to catastrophic events. E. All of the above. F. The answer is not all of the above, and you cannot choose F. 3a.2 What's the difference between natural selection and sexual selection? A. Sexual selection occurs during sex. B. Natural selection is a type of sexual selection.

C. Sexual selection is a type of natural selection. D. Sexual selection occurs only among large organisms, natural selection does not. E. None of the above. 3a.3 Which of the following is an example of genetic variation? A. Two children have different eye colors. B. One person is older than another. C. One person has a scar, but her friend does not. D. Todd eats meat, but his brother Rod is a vegetarian. E. None of the above 3a.4 Sparrows with average-sized wings survive severe storms better than those with longer or shorter wings, illustrating A. the bottleneck effect. B. disruptive selection. C. neutral variation D. stabilizing selection. E. Negative frequency-dependent selection...