Deviations from Null Models
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© 1997
David H.A. Fitch
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Deviations of natural populations from the null hypotheses

I.  At a single locus, how and why might natural populations deviate from a HW equilibrium of genotype frequencies?

A.  Two main results in terms of the distributions of alleles in genotypes:
1.  Excess of heterozygotes
2.  Deficiency of heterozygotes (common)

B.  Possible causes:
1.  Nonrandom mating (e.g., inbreeding is expected to result in heterozygote deficiency), affects genotype frequencies (but alone does not affect allele frequencies)
2.  Population is small (finite, not vast) and allele frequencies "drift" away from equilibrium due to sampling (thus affecting genotype frequencies)
3.  Gene flow occurs, changing allele frequencies (and thus genotype frequencies)
4.  Selection occurs (e.g., heterozygote advantage could result in heterozygote excess) and could also affect allele frequencies

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II.  At 2 loci, how and why might natural populations deviate from linkage equilibrium?

A.  Population has just been formed by mixis between two different populations (or gene flow) and equilibrium has not yet been reached

B.  Tight linkage between loci will result in a much longer time for linkage equilibrium to occur

C.  Mechanisms that restrict recombination
1.  Paracentric inversions
2.  Nonrandom mating structure (e.g., selfing), cloning, or asexual reproduction

D.  Small population size will result in genetic drift (change in allele frequencies) which in turn will disrupt any equilibrium

E.  Selection may maintain nonrandom associations among particular alleles at different loci wherever there is "epistasis for fitness" (i.e., where interactions among particular alleles at different loci have an effect on fitness; fitnesses associated with particular allele combinations are higher than other combinations)
Examples:
1.  Heterostyly in primrose
2.  Batesian mimicry in Papilo dardanus (also an excellent example of "frequency-dependent selection"
3.  MHC locus

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 Nonrandom
Mating  Genetic
Drift  Selection:
One Locus  Selection:
Many Loci
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[Nonrandom Mating] [Genetic Drift] [Selection: One Locus] [Selection: Many Loci]

[Evolutionary Genetics]

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