Gene Flow, Mutation-Selection Balance, and Population Structure
Evolution in Structured Populations
Gene flow
Gene flow occurs when alleles move between populations through migration and interbreeding. It can introduce beneficial alleles, reduce local adaptation, and make populations more genetically similar.
Population structure
Real populations are often subdivided. When dispersal is limited, local allele frequencies can differ because selection, drift, and migration vary across space. Structure matters in conservation, disease spread, and speciation.
Important concepts
Problem-solving clue
If you see a trait that varies by location, ask whether the pattern is caused by local selection, limited migration, or both.
What is a likely effect of strong gene flow between two populations experiencing different selection pressures?
Gene flow tends to make populations more similar, although selection may counteract this.
Correct answer: It can reduce divergence by homogenizing allele frequencies
How can gene flow both help and hinder adaptation?
Migration is not inherently good or bad; its effect depends on context.
Correct answer: It can bring in beneficial alleles and increase variation, but it can also swamp locally adaptive alleles with less favorable ones from elsewhere.
Forces affecting population structure
| Force | Typical effect on differentiation |
|---|---|
| Migration | Decreases differentiation |
| Genetic drift | Increases differentiation |
| Divergent selection | Increases differentiation |
| Assortative mating | Can increase differentiation |
Why is mutation-selection balance important?
Deleterious alleles can recur by mutation even while selection removes them.
Correct answer: It explains why harmful alleles may persist despite selection
Interpreting a geographic trait pattern
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1
Map the trait or allele frequency across locations.
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2
Compare locations with barriers to dispersal.
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3
Ask whether environmental differences match the trait pattern.
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4
Consider migration, selection, and drift together rather than separately.