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# IV. Selection and Other Factors A. Mutation - PowerPoint PPT Presentation

IV. Selection and Other Factors A. Mutation. IV. Selection and Other Factors A. Mutation - Mutation can maintain a deleterious allele in the population against the effects of selection, such that:. IV. Selection and Other Factors A. Mutation

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## PowerPoint Slideshow about ' IV. Selection and Other Factors A. Mutation' - kura

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A. Mutation

A. Mutation

- Mutation can maintain a deleterious allele in the population against the effects of selection, such that:

A. Mutation

- Mutation can maintain a deleterious allele in the population against the effects of selection, such that:

q(eq) = √(m/s)

A. Mutation

- Mutation can maintain a deleterious allele in the population against the effects of selection, such that:

q(eq) = √(m/s)

- more deleterious alleles are maintained if m increases, or if selection differential declines... this should make sense.

A. Mutation

1. Single Loci

- consider a locus with selection against the heterozygote

peq = t/(s + t) = .25/.75 = 0.33

A. Mutation

1. Single Loci

- consider a locus with selection against the heterozygote

peq = t/(s + t) = .25/.75 = 0.33

A. Mutation

1. Single Loci

- consider a locus with selection against the heterozygote

1.0

0.75

mean fitness

1.0

0

0.33

A. Mutation

1. Single Loci

- suppose there is random movement up the 'wrong' slope?

1.0

0.75

mean fitness

1.0

0

0.33

A. Mutation

1. Single Loci

1.0

- if this is a large pop with no drift, the population will become fixed on the 'suboptimal' peak, (p = 0, q = 1.0, w = 0.75).

0.75

mean fitness

1.0

0

0.33

A. Mutation

1. Single Loci

1.0

- BUT if it is small, then drift may be important... because only DRIFT can randomly BOUNCE the gene freq's to the other slope!

0.75

mean fitness

1.0

0

0.33

A. Mutation

1. Single Loci

1.0

- And then selection can push the pop up the most adaptive slope!

0.75

mean fitness

1.0

0

0.33

A. Mutation

1. Single Loci

1.0

- The more shallow the 'maladaptive valley', (representing weaker selection differentials) the easier it is for drift to cross it...

0.75

mean fitness

1.0

0

0.33

A. Mutation

2. Two Loci - create a 3-D landscape, with "mean fitness" as the 'topographic relief"

Suppose AAbb and aaBB work well, but combinations of the other two do not.

1.0

f(A)

1.0

f(B)

A. Mutation

2. Two Loci - create a 3-D landscape, with "mean fitness" as the 'topographic relief"

Again, strong selection or weak drift will cause mean fitness to move up nearest slopes.

1.0

f(A)

1.0

f(B)

A. Mutation

2. Two Loci - create a 3-D landscape, with "mean fitness" as the 'topographic relief"

Only strong drift or weak selection and some drift (shallow valley) can cause the population to cross the maladaptive valley.

1.0

f(A)

1.0

f(B)

A. Mutation

A. The Complex Phenotype

- traits are often influenced by more than one locus, and their effects are not necessarily independent. In other words, there are often epistatic interactions between loci...the value of an allele may depend on the genotype at other loci

A. The Complex Phenotype

- traits are often influenced by more than one locus, and their effects are not necessarily independent. In other words, there are often epistatic interactions between loci...the value of an allele may depend on the genotype at other loci

- For example, suppose 'A' and 'B' each contribute a unit of growth, and suppose there is selection for intermediate size.

A. The Complex Phenotype

- traits are often influenced by more than one locus, and their effects are not necessarily independent. In other words, there are often epistatic interactions between loci ...the value of an allele may depend on the genotype at other loci

- For example, suppose 'A' and 'B' each contribute a unit of growth, and suppose there is selection for intermediate size.

- There will be three adaptive genotypes: AAbb, AaBb, and aaBB that each have two alleles for growth (0,1,3, and 4 create larger and smaller organisms that are at a selection disadvantage in this example).

A. The Complex Phenotype

- traits are often influenced by more than one locus, and their effects are not necessarily independent. In other words, there are often epistatic interactions between loci... the value of an allele may depend on the genotype at other loci

- For example, suppose 'A' and 'B' each contribute a unit of growth, and suppose there is selection for intermediate size.

- There will be three adaptive genotypes: AAbb, AaBb, and aaBB that each have two alleles for growth (0,1,3, and 4 create larger and smaller organisms that are at a selection disadvantage in this example).

- However, a population full of heterozygotes is impossible, the population will either move to "fixation" at AAbb OR aaBB.

A. The Complex Phenotype

- traits are often influenced by more than one locus, and their effects are not necessarily independent. In other words, there are often epistatic interactions between loci... the value of an allele may depend on the genotype at other loci

- For example, suppose 'A' and 'B' each contribute a unit of growth, and suppose there is selection for intermediate size.

- There will be three adaptive genotypes: AAbb, AaBb, and aaBB that each have two alleles for growth (0,1,3, and 4 create larger and smaller organisms that are at a selection disadvantage in this example).

- However, a population full of heterozygotes is impossible, the population will either move to "fixation" at AAbb OR aaBB.

- So, is an "aa" genotype good or bad? Well, it depends on what's happening at "B".

A. The Complex Phenotype

A. The Complex Phenotype

1. A two-Locus Equilibrium Model

- Suppose: f(A) = p1 and f(a) = q1 f(B) = p2 and f(b) = q2

A. The Complex Phenotype

1. A two-Locus Equilibrium Model

- Suppose: f(A) = p1 and f(a) = q1 f(B) = p2 and f(b) = q2

- Expected frequency of AB haplotype (gamete) = a = p1*p2

A. The Complex Phenotype

1. A two-Locus Equilibrium Model

- Suppose: f(A) = p1 and f(a) = q1 f(B) = p2 and f(b) = q2

- Expected frequency of AB haplotype (gamete) = a = p1*p2

f(Ab) = b = p1q2

f(aB) = c = q1p2

f(ab) = d = q2q2

A. The Complex Phenotype

1. A two-Locus Equilibrium Model

- In essence, this becomes the equilibrium model for two loci; when a population reaches linkage equilibrium, the frequency of haplotypes will be equal to the product of the independent gene frequencies….

A. The Complex Phenotype

1. A two-Locus Equilibrium Model

- In essence, this becomes our equilibrium model for two loci.

- TWO LOCI, EVEN IF THEY ARE CLOSELY LINKED, WILL CROSS-OVER OCCASIONALLY. OVER TIME, EVEN WITH CLOSELY LINKED GENES, THE FREQUENCIES OF HAPLOTYPES WILL REACH THE ‘EQUILIBRIUM’ VALUES (ACTING LIKE THEY ASSORT INDEPENDENTLY) IF PANMIXIA OCCURS.

A. The Complex Phenotype

1. A two-Locus Equilibrium Model

- In essence, this becomes our HWE model for two loci.

- TWO LOCI, EVEN IF THEY ARE CLOSELY LINKED, WILL CROSS-OVER OCCASIONALLY. OVER TIME, EVEN WITH CLOSELY LINKED GENES, THE FREQUENCIES OF HAPLOTYPES WILL REACH EQUILIBRIUM IF PANMIXIA OCCURS.

- So, recombination drives a population towards equilibrium. The rate depends on the distance between loci. (Neighboring loci take longer to equilibrate because less crossing over occurs each generation).

A. The Complex Phenotype

1. A two-Locus Equilibrium Model

A. The Complex Phenotype

1. A two-Locus Equilibrium Model

- Linkage DISequilibrium is measured as "D"

If D = 0, the pop is in equilibrium.

If D is not zero, then D becomes a measure of deviation from the equilibrium.

1. A two-Locus Equilibrium Model

1. A two-Locus Equilibrium Model

- Linkage!! without sufficient time to equilibrate haplotype frequencies

1. A two-Locus Equilibrium Model

- Linkage!!! without sufficient time to equilibrate haplotype frequencies.

- Selection!!! Some haplotypes occur more frequently than random because these combos confer a selective advantage.

1. A two-Locus Equilibrium Model

- Linkage!!! without sufficient time to equilibrate haplotype frequencies.

- Selection!!! Some haplotypes occur more frequently than random because these combos confer a selective advantage.

Curiously, selection will only create linkage disequilibrium if the alleles act "epistatically".

1. A two-Locus Equilibrium Model

- Linkage!!! without sufficient time to equilibrate haplotype frequencies.

- Selection!!! Some haplotypes occur more frequently than random because these combos confer a selective advantage.

Curiously, selection will only create linkage disequilibrium if the alleles act "epistatically".

Papilio memnon - females mimic different model species.

1. A two-Locus Equilibrium Model

- Linkage!!! without sufficient time to equilibrate haplotype frequencies.

- Selection: Some haplotypes occur more frequently than random because these combos confer a selective advantage.

Curiously, selection will only create linkage disequilibrium if the alleles act "epistatically".

Papilio memnon - females mimic different model species.

In the simplest terms:

T (swallowTAIL) > t (no tail) and

C >c, where CC, Cc is coloration for a tailed model and

cc is coloration for a tailless model.

1. A two-Locus Equilibrium Model

- Linkage!!! without sufficient time to equilibrate haplotype frequencies.

- Selection: Some haplotypes occur more frequently than random because these combos confer a selective advantage.

Curiously, selection will only create linkage disequilibrium if the alleles act "epistatically".

Papilio memnon - females mimic different model species.

In the simplest terms:

T (swallowTAIL) > t (no tail) and

C >c, where CC, Cc is coloration for a tailed model and

cc is coloration for a tailess model.

So, fitness at t locus depends on the fitness at the C locus. If CC is advantageous, then TT is, too. If cc is advantageous, then TT is NOT. So, fitness at T depends on fitness at C.

1. A two-Locus Equilibrium Model

- Linkage!!! without sufficient time to equilibrate haplotype frequencies.

- Selection: Some haplotypes occur more frequently than random because these combos confer a selective advantage.

Curiously, selection will only create linkage disequilibrium if the alleles act "epistatically".

Papilio memnon - females mimic different model species.

In the simplest terms:

T (swallowTAIL) > t (no tail) and

C >c, where CC, Cc is coloration for a tailed model and

cc is coloration for a tailess model.

So, fitness at t locus depends on the fitness at the C locus. If CC is advantageous, then TT is, too. If cc is advantageous, then TT is NOT. So, fitness at T depends on fitness at C.

So, TC haplotype and tc haplotypes will dominate in a constant disequilibrium.

Papilio memnon (palatable) females

ttcc

TTCC

ttcc

Toxic Models On Left

Papilio memnon male

T (swallowTAIL) > t (no tail) and

C >c, where CC, Cc is coloration for a tailed model and

cc is coloration for a tailless model.

So, fitness at t locus depends on the fitness at the C locus. If CC is advantageous, then TT is, too. If cc is advantageous, then TT is NOT. So, fitness at T depends on fitness at C.

So, TC haplotype and tc haplotypes will dominate in a constant disequilibrium.

f(T) = p1 = .5

f(t) = q1 = .5

f(C) = p2 = .5

f(c) = q2 = .5

D = ad - bc = .25 - 0 = 0.25.... not in equilibrium

1. A two-Locus Equilibrium Model

- Linkage!!! without sufficient time to equilibrate haplotype frequencies.

- Selection: Some haplotypes occur more frequently than random because these combos confer a selective advantage.

Curiously, selection will only create linkage disequilibrium if the alleles act "epistatically".

If allelic effects are multiplicative or additive (like the size example), then the system will proceed to equilibrium.

1. A two-Locus Equilibrium Model

- Linkage!!! without sufficient time to equilibrate haplotype frequencies.

- Selection: Some haplotypes occur more frequently than random because these combos confer a selective advantage.

Curiously, selection will only create linkage disequilibrium if the alleles act "epistatically".

If allelic effects are multiplicative or additive (like the size example), then the system will proceed to equilibrium.

In the case of size, it goes to fixation for AAbb or aaBB, each of which is ad - bc = 0 = linkage equilibrium. So selection, in this case, does NOT create a disequilibrium.

In the case of size, if goes to fixation for AAbb or aaBB, each of which is ad - bc = 0 = linkage equilibrium. So selection, in this case, does NOT create a disequilibrium.

For example, if the population fixes at AAbb:

f(A) = p1 = 1.0

f(a) = q1 = 0.0

f(B) = p2 = 0.0

f(b) = q2 = 1.0

D = ad - bc = 0 - 0 = 0.... in equilibrium

B. "Linkage" Equilibrium each of which is ad - bc = 0 = linkage equilibrium. So selection, in this case, does NOT create a disequilibrium.

1. A two-Locus Equilibrium Model

- Linkage!!! without sufficient time to equilibrate haplotype frequencies.

- Selection: Some haplotypes occur more frequently than random because these combos confer a selective advantage.

- Drift:

B. "Linkage" Equilibrium each of which is ad - bc = 0 = linkage equilibrium. So selection, in this case, does NOT create a disequilibrium.

1. A two-Locus Equilibrium Model

- Linkage!!! without sufficient time to equilibrate haplotype frequencies.

- Selection: Some haplotypes occur more frequently than random because these combos confer a selective advantage.

- Drift: Random fluctuation in the relative frequencies of haplotypes can cause disequilibrium.

B. "Linkage" Equilibrium each of which is ad - bc = 0 = linkage equilibrium. So selection, in this case, does NOT create a disequilibrium.

1. A two-Locus Equilibrium Model

- Linkage!!! without sufficient time to equilibrate haplotype frequencies.

- Selection: Some haplotypes occur more frequently than random because these combos confer a selective advantage.

- Drift: Random fluctuation in the relative frequencies of haplotypes can cause disequilibrium.

- Non-random mating:

B. "Linkage" Equilibrium each of which is ad - bc = 0 = linkage equilibrium. So selection, in this case, does NOT create a disequilibrium.

1. A two-Locus Equilibrium Model

- Linkage!!! without sufficient time to equilibrate haplotype frequencies.

- Selection: Some haplotypes occur more frequently than random because these combos confer a selective advantage.

- Drift: Random fluctuation in the relative frequencies of haplotypes can cause disequilibrium.

- Non-random mating: If A individuals preferentially mate with B individuals, then AB will be more frequent than expected by chance.

B. "Linkage" Equilibrium each of which is ad - bc = 0 = linkage equilibrium. So selection, in this case, does NOT create a disequilibrium.

1. A two-Locus Equilibrium Model

4. Effects

B. "Linkage" Equilibrium each of which is ad - bc = 0 = linkage equilibrium. So selection, in this case, does NOT create a disequilibrium.

1. A two-Locus Equilibrium Model

4. Effects

- Hitchhiking - selection at one locus can drive changes in linked genes before recombination eliminates disequilibrium. This can work to prevent the acquisition of advantageous alleles at a locus if it is closely linked to a deleterious allele at a second locus. However, this effect declines as the distance between genes (and thus the rate of recombination) increases.

B. "Linkage" Equilibrium each of which is ad - bc = 0 = linkage equilibrium. So selection, in this case, does NOT create a disequilibrium.

1. A two-Locus Equilibrium Model

4. Effects

- Hitchhiking - selection at one locus can drive changes in linked genes before recombination eliminates disequilibrium. This can work to prevent the acquisition of advantageous alleles at a locus if it is closely linked to a deleterious allele at a second locus. However, this effect declines as the distance between genes (and thus the rate of recombination) increases.

B. "Linkage" Equilibrium each of which is ad - bc = 0 = linkage equilibrium. So selection, in this case, does NOT create a disequilibrium.

1. A two-Locus Equilibrium Model

4. Effects

5. The Utility of Linkage Disequilibrium

B. "Linkage" Equilibrium each of which is ad - bc = 0 = linkage equilibrium. So selection, in this case, does NOT create a disequilibrium.

1. A two-Locus Equilibrium Model