Download 16-2 Evolution As Genetic Change

Evolution of Populations
Other Mechanisms of Genetic Change
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Gene Flow
Movement of Alleles
From One Population
To Another
• Increases Genetic Variation Of The Receiving
Population
• Less Gene Flow – More Differences Between
Populations
– Increases Probability of Each Population
Becoming Separate Species.
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Genetic Drift
• Natural Selection Is Not The Only
Source of Evolutionary Change
• In Small Populations
– An Allele Can Become More Or Less
Common By Chance
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Genetic Drift
• Smaller Populations
– Can Have Shifts In The Relative Frequency
Of Alleles Much Faster Than Larger
Populations
– Their Gene Pool Is Smaller
Genetic Drift Is
The Random Change
In Allele Frequency
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Genetic Drift
Key Concept:
In Small Populations,
Individuals That Carry A Particular Allele
May Leave More Descendents Than Other
Individuals, Just By Chance.
Over Time, A Series Of Chance Occurrences
Of This Type Can Cause An Allele To
Become Common In A Population
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Genetic Drift
• Circumstances That May Result In
Genetic Drift:
–Founder Effect
• Colonization Of A New Habitat By A
Small Number of Individuals
–Hawaiian Fruit Flies
–Darwin’s Finches
–Hawaiian Honey Creepers
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Founder Effect
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Genetic Drift
The Bottleneck Effect
• Occurs When Man Made, or Natural
Disasters Destroy Most Of A
Population.
• The Remaining Members Usually
Possess A Severely Diminished
Gene Pool
Foothill High School Science Department
Foothill High School Science Department
Hardy-Weinberg Principle (1908)
Key Concept:
Five Conditions Are Required To Maintain
Genetic Equilibrium From Generation To
Generation:
1. There Must Be Random Mating
2. The Population Must Be Very Large
3. There Can Be No Movement In or Out Of
The Population
4. No Mutations
5. No Natural Selection
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Random Mating
All Members Of The
Population
Must have An Equal
Opportunity
To Produce Offspring
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Large Population
Genetic Drift
Has Less Effect
On Large Populations
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No Movement Into or Out Of The Population
• New Alleles Must Not Enter
• Rare Alleles Must Not
Be Allowed To Leave
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No Mutations
Mutations Introduce New
Alleles Causing A Change In
Allele Frequencies
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No Natural Selection
All Genotypes
Must Have Equal Opportunity
To Survive & Reproduce
No Phenotype
Can Have An Advantage
Over Another
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