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Quantitative Genetics of Natural Variation: some questions. Do most adaptations involve the fixation of major genes?. micromutationist view : adaptations arise by allelic substitution of slight effect at many (innumerable) loci, and no single substitution constitutes a major

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Quantitative genetics of natural variation some questions
Quantitative Genetics of Natural Variation: some questions

Do most adaptations involve the fixation of major genes?

micromutationist view: adaptations arise by allelic substitution of slight effectat many (innumerable) loci, and no single substitution constitutes a major

portion of an adaptation (Darwin, Fisher)

macromutationist views:

1. single “systemic” mutations produce complex adaptations in essentially perfect form (Goldschmidt)

2. adaptation often involves one or a few alleles having large effects

• Of 8 studies, only 3 consistent with changes involving > 5 loci (Orr and Coyne 1992)


Quantitative genetics of natural variation some questions1
Quantitative Genetics of Natural Variation: some questions

• How many loci contribute to naturally occurring phenotypic

variation, and what are the magnitudes of their effects?

• What sorts of genes —and changes in these genes—are responsible for trait variation within populations (e.g., transcription factors, structural genes, metabolic genes)

• Do the same genes that contribute to variation within species also contribute to variation between species?

• What genes underlie evolutionary novelties?

• What are the genetic bases for evolutionary novelties?

• How do pleiotropic effects of genes evolve?

Answers require a mechanistic approach towards identifying the relevant loci and how genetic differences are translated into phenotypic differences


Quantitative traits depend on multiple underlying loci
Quantitative traits depend on multiple underlying loci

one locus +

environment

two loci +

environment

one locus

four loci +

environment

many loci +

environment


Phenotypic value and population means

A2A2

A1A2

A1A1

genotype

– a

0

d

+ a

genotypic value

Phenotypic Value and Population Means

P = G + E

Phenotypic value = Genotypic value + Environmental Deviation

Genotype Freq Value Freq x Val

A1A1 p2 +a p2a

A1A2 2pq d 2pqd

A2A2 q2 -a -q2a

Sum = Pop Mean = a(p-q) + 2dpq


Timing of Metamorphosis

The majority of organisms on planet earth have complex life cycles

Predictable

Larval Habitat

Hatching

Metamorphosis

Predictable

Ephemeral Pond

Time


Thyroid Hormone Receptors as Candidate Genes for

Variation in Metamorphic Timing

Hypothalamus

TRH

Pituitary

TSH

Thyroid

TH

Target cells

T4

deiodionation

T3

TRs

transcription

An extreme difference in

metamorphic timing


Thyroid Hormone Receptors : A Hypothetical Example

Thyroid Hormone Receptor

Alpha Genotype

A1A1

A1A2

A2A2

Timing of

Metamorphosis

(Days)

200

160

150

d

-15

-a

a

-25

25

0

Homozygote

Midpoint

(175)


Genotype Freq Value Freq x Val

A1A1 p2 25 p2(25)

A1A2 2pq -15 2pq(-15)

A2A2 q2 -25 -q2(25)

Sum = Pop Mean = 25(p-q) + 2(-15)pq

(adds time)

(reduces time)

p = f(A1)

q = f(A2)

A1A1 A1A2 A2A2

Mean

0 0 -25

2.25 -6.3 -12.25

6.25 -7.5 -6.25

12.25 -6.3 -2.25

25 0 0

0.0

0.3

0.5

0.7

1.0

1.0

0.7

0.5

0.3

0.0

-25 (150)

-16.3 (158.7)

-7.5 (167.5)

3.7 (178.7)

25 (200)


Let’s Consider a Second Locus

Thyroid Hormone Receptor

Alpha Genotype

A1A1

A1A2

A2A2

Timing of

Metamorphosis

(Days)

200

160

150

0

Thyroid Hormone Receptor

Beta Genotype

A1A1

A1A2

A2A2

Timing of

Metamorphosis

(Days)

200

140

0

-a

a

-30

30

Homozygote

Midpoint

(170)


Genotype Freq Value Freq x Val

A1A1 p2 30 p2(30)

A1A2 2pq 0 2pq(0)

A2A2 q2 -30 -q2(30)

Sum = Pop Mean = 30(p-q) + 2(0)pq

(adds time)

(reduces time)

P = f(A1)

Q = f(A2)

A1A1 A1A2 A2A2

Mean

-30 (140)

-12 (158)

0 (170)

12 (182)

30 (200)

0 0 -30

2.7 0 -14.7

0 0 0

14.7 0 -2.7

30 0 0

0.0

0.3

0.5

0.7

1.0

1.0

0.7

0.5

0.3

0.0


Consider the joint effect of both TH Loci

Total Range = 2Sa=110

Tha A1A1

Thb A1A1

Tha A2A2

Thb A2A2

Timing of

Metamorphosis

(Days)

227.5

117.5

0

-a

a

55

55

Average

Homozygote

Midpoint

(172.5)

Overall

Mean

=

Sa(p-q) + S2dpq


Genotypic value is not transferred from parent to

offspring; genes are.

Need a value that reflects the genes that an

individual carries and passes on to it’s offspring

Breeding Value

Empirically: An individual’s value based on the mean deviation of its progeny

from the population mean.

Theoretically: An individual’s value based on the sum of the average effects

of the alleles/genes it carries.


.

a1 = pa + qd - [ a (p – q) + 2dpq ]

population mean

f (A1)

f (A2)

Average Effect of an Allele

Type of Values and Freq Mean value Population Average

gamete of gametes of genotypes mean effect of

gene

A1A1 A1A2 A2A2

a d -a

A1 p q pa + qd -a(p-q) + 2dpq q[a+d(q-p)]

A2 p q -qa + pd -a(p-q) + 2dpq -p[a+d(q-p)]

average effect of An:

an = mean deviation from the population mean of individuals that received An from one parent, if the other parent’s allele chosen randomly

a1 = q [ a + d (q – p)]

a2 = –p [ a + d (q – p)]


When there are only two alleles at a locus

Average effect of a gene substitution

A1A1

A1A2

A2A2

+a

d

-a

(a - d)

(d + a)

p(a - d) + q(d + a)

a = a + d(q - p)

average

effect of

A1

average

effect of

A2

a

pa


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