Download Notes Outline: Natural Selection (9

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Notes Outline: Natural Selection (10.3)
“Has natural selection affected your life directly? Yes, because your body has been shaped by natural selection. For example, the
ability of your eyes to focus, the way your hands grip objects, your upright posture, your large brain, the color of your skin, and
numerous other characteristics are all results of evolution by natural selection.”
I.
How Natural Selection Causes Evolution
A. Introduction
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The key factor in natural selection is the environment.
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The environment “selects” which organisms will survive and reproduce.
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Traits by successful organisms at survival and reproduction are more likely to be transmitted to the next generation
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B. The Process of Natural Selection
II.
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Variation is the raw material for natural selection.
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Living things face a constant struggle for existence.
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Only some individuals survive and reproduce.
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Natural selection causes genetic change.
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Species adapt to their environment.
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The Peppered Moth: Natural Selection in Action
A. Introduction
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Over many generations natural selection gradually changes a species in response to its environment.
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Adaptation: Process by which a species becomes better suited to its environment
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(Before 1850s) Dark gray peppered moths were rare, almost all were cream color
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(After 1850s) Dark peppered moths started to become more common, usually in heavy industrialized areas
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(By 1950s) Peppered moth population near industrial centers consisted almost entirely of dark individuals
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III.
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Darwin’s theory of natural selection suggests an explanation
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White tree trunks were blackened by heavy pollution from factories
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This made white moths vulnerable to predators
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The Puzzle of Sickle Cell Anemia
A. Introduction
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Sickle cell anemia is a hereditary disease that affects hemoglobin molecules
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Persons homozygous for the defective sickle cell allele have sickle cell anemia and often die at an early age
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Heterozygous individuals, carry the allele but are healthy
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In Africa- one of 100 African American is homozygous
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In U.S.- one of 500 African American is homozygous
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Disease is almost unknown in other races
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Why is this potentially fatal disease very common in the African-American population?
B. The sickle cell allele confers an unexpected advantage in Africa
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The defective allele is common in central Africa b/c people who are heterozygous for the sickle cell allele are much less
susceptible to malaria
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In central Africa, the number of deaths from sickle cell anemia is far lower than would occur from malaria if heterozygous
individuals were not malaria resistant.
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Natural selection has favored the sickle cell allele in central Africa b/c the payoff in survival of heterozygotes is greater
than the price in death of homozygotes
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Balancing Selection: Situation in which natural selection for & against an allele balance out, so allele is maintained
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C. Selection does not favor the sickle cell allele in the United States
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Malaria is a disease that does not occur in North America today
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The protection against malaria that is associated with being heterozygous for the sickle cell allele offers no real advantage
in the United States
IV.
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Result = allele for sickle cell has become far less common in the U.S.
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Directional Selection: _______________________________________________________________________________
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How Species Form
A. Introduction
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Species: ___________________________________________________________________________________________
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Populations of a species are found in several different kinds of environments
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Selections will act to make each population suit its particular environment
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Separate populations can eventually become quite distinct
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Ecological Races: ___________________________________________________________________________________
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B. Ecological Races Form New Species
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Ecological races often become increasingly different
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Divergence: _______________________________________________________________________________________
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Eventually, races can accumulate so many differences that biologists consider them separate species
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C. Does Evolution Occur in Spurts
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Gradualism: ______________________________________________________________________________________
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Punctuated Equillibrium: ___________________________________________________________________________
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