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16-1 Genes and Variation
16-1 Genes and Variation
Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises
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2. How Common Is Genetic Variation?
Many genes have at least two alleles.
All organisms have genetic variation.
An individual organism is heterozygous for many genes.
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Variation and Gene Pools
Population – group of individuals of the same species that interbreed.
Gene pool – all genes present in a population.
Relative frequency – The number of times the allele occurs in a gene pool (usually a %).
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4. Variation and Gene Pools
Gene Pool for Fur Color in Mice
Sample Population
Frequency of Alleles
allele for brown fur
allele for black fur
When scientists determine whether a population is evolving, they may look at the sum of the population’s alleles, or its gene pool. This diagram shows the gene pool for fur color in a population of mice. 
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5. Variation and Gene Pools
Evolution is any change in the relative frequency of alleles in a population.
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6. Sources of Genetic Variation
Two main sources of genetic variation-
mutations
genetic shuffling that results from sexual reproduction.
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7. Sources of Genetic Variation
Mutations
Any change in a sequence of DNA.
Occur because of mistakes in DNA replication or as a result of radiation or chemicals in the environment.
Mutations do not always affect an organism’s phenotype.
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8. Sources of Genetic Variation
Gene Shuffling
Crossing-over increases the number of genotypes that can appear in offspring.
Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population.
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9. Single-Gene and Polygenic Traits
REVIEW
Single-gene trait – controlled by one gene that has two alleles. Variation in this gene leads to only two possible phenotypes.
In humans, a single gene with two alleles controls whether a person has a widow’s peak (left) or does not have a widow’s peak (right). As a result, only two phenotypes are possible. The number of phenotypes a given trait has is determined by how many genes control the trait.
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10. Single-Gene and Polygenic Trait
REVIEW
Polygenic traits – controlled by two or more genes.
One polygenic trait can have many possible genotypes and phenotypes.
Examples-height, skin color, hair
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11. 16-2 Evolution as Genetic Change
Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises
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12. 16-2 Evolution as Genetic Change
REVIEW
Natural selection affects which individuals survive and reproduce and which do not.
Evolution is any change over time in the relative frequencies of alleles in a population!
Populations, not individual organisms, can evolve over time!
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13. Natural Selection on Single-Gene Traits
Natural selection on single-gene traits can lead to changes in allele frequencies and then to evolution.
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14. Natural Selection on Single-Gene Traits
Example-
Organisms of one color may produce fewer offspring than organisms of other colors.
For example, a lizard population is normally brown, but has mutations that produce red and black forms.
Red lizards are more visible to predators, so they will be less likely to survive and reproduce. Therefore, the allele for red color will become rare.
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15. Natural Selection on Single-Gene Traits
Black lizards may warm up faster on cold days. This may give them energy to avoid predators. In turn, they may produce more offspring.
The allele for black color will increase in relative frequency.
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16. Natural Selection on Single-Gene Traits
Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Organisms of one color, for example, may produce fewer offspring than organisms of other colors.
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17. Natural Selection on Polygenic Traits- Occurs 3 ways-
Directional Selection
Stabilizing Selection
Disruptive Selection
In your book, draw these 3 graphs and a
brief description of each!!!
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18. Evolution Versus Genetic Equilibrium
Will evolution ever NOT occur?
Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.
When allele frequencies remain constant it is called genetic equilibrium.
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There must be random mating!
The population must be large!
There can be no movement into or out of the population!
There may be no mutations!
There is no natural selection!
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20. 16-3 The Process of Speciation
Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises
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21. 16-3 The Process of Speciation
Natural selection and chance events can change the relative frequencies of alleles in a population and lead to speciation.
Speciation: formation of new species.
Species: a group of organisms that breed with one another and produce fertile offspring.
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Reproductive Isolation: when members of 2 populations cannot interbreed.
Can develop by:
Behavioral isolation (different rituals)
Geographic isolation (separated by lakes/rivers)
Temporal isolation (reproduction at different times)
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23. Speciation in Darwin's Finches
Speciation in the Galápagos finches occurred by:
founding of a new population
geographic isolation
changes in new population's gene pool
reproductive isolation
ecological competition
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24. Speciation in Darwin's Finches
Founders Arrive 
A few finches—species A—travel from South America to one of the Galápagos Islands.
There, they survive and reproduce.
Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species.
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25. Speciation in Darwin's Finches
Geographic Isolation
Some birds from species A cross to a second island.
The two populations no longer share a gene pool.
Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species.
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26. Speciation in Darwin's Finches
Changes in the Gene Pool
Seed sizes on the second island favor birds with large beaks.
The population on the second island evolves into population B, with larger beaks.
Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species.
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27. Speciation in Darwin's Finches
Reproductive Isolation
If population B birds cross back to the first island, they will not mate with birds from population A.
Populations A and B are separate species.
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28. Speciation in Darwin's Finches
Ecological Competition
As species A and B compete for available seeds on the first island, they continue to evolve in a way that increases the differences between them.
A new species—C—may evolve.
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29. Speciation in Darwin's Finches
Continued Evolution
13 different finch species eventually evolved to today!!
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