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I. Evolution by Natural Selection
Variation – Members of a population differ from one another
Inheritance – Differences between individuals are heritable genetic differences
Differential adaptedness – some differences affect how an organism is adapted to its environment
Differential reproduction – better adapted individuals are more likely to reproduce
A. Natural Selection
Selection maintains a well adapted condition by eliminating deviations from it
B. Directional Selection
1. Favors one extreme form over others
2. Leads to a change in the population phenotype
Giraffe’s necks
Peppered Moths
Antibiotic and pesticide resistance
C. Stabilizing Selection
1. Maintains already well adapted condition
2. Eliminates the extreme phenotypes
3. As long as environments remains the same, the fittest organisms remain unchanged
D. Disruptive Selection
1. Occurs when two extreme phenotypes are favored
2. Leads to more than one distinct form-Speciation
II. Variations Maintained
A. Some alleles maintained in a population
B. Diploidy and heterozygosity
1. Repository of rare recessive alleles
2. May provide "genetic insurance" should the environment change
C. Sickle Cell Anemia
1. Fatal disease resulting from homozygous recessive alleles
2. Codes for a defective subunit for hemoglobin
3. Red cells collapse forming odd shapes including sickles
4. Decreased oxygen delivery to tissues
5. Single nucleotide substitution
GAA coding for glutamic acid is replaced by GUA coding for valine
6. Despite lethality, allele frequencies as high as 40% exist in some parts of Africa.
DARWIN’S THEORY OF EVOLUTION: NATURAL SELECTION
The fossil record shows that over time, species change, species become extinct, and new species appear. Charles
Darwin offered an explanation, known as Natural Selection, for how these changes could occur.
NATURAL SELECTION
Main Points of Natural Selection
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Trait:
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Adaptation:
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Results in: