Evolution at Multiple Loci:Quantitative Genetics

I. Rediscovery of Mendel and Challenges to Natural Selection

• Do traits that exhibit continuous variation have a genetic basis?

• If the only traits which have genetic variation are controlled by one or two loci then natural selection not as important as mutation

• Darwin envisioned evolution to be a continuous process of selection acting on limitless genetic variation, with small changes occurring in any one generation, but large changes occurring over long periods.

Why the normal distribution: Central Limit Theorem

Mendelian genetics can explain quantitative traits

Example 1: NILSSON-EHLE: Red and White Kernal Color in Wheat

Example 2:East’s workwith tobacco

Quantitative traits are influenced by the environment as well as genotype

Yarrow plant

1. Fisher’s prediction

Mutation Effect

Prob

abili

ty o

f Fix

atio

n

2. Kimura’s modification

3. Orr’s modification

II. Neo Darwinian SynthesisTheoretical models that support then contend with the Darwinian model

Typical results

Corolla Width (mm)

F2

BC

F1M. micranthus M. guttatus

F1

F2

Fenster & Ritland 1994

Testing the Models:

No filter

Filtered image—“bumblevision”

SegregationOf floral typesDemonstrateGenetic basisOf trait Differences

Convergent evolution??

Yosemite Sam thinks so

1. Fisher’s prediction

Mutation Effect

Prob

abili

ty o

f Fix

atio

n

2. Kimura’s modification

3. Orr’s modification

Theoretical models that support then contend with the Darwinian model

Alleles with a distribution of effect sizes contribute to adaptations

III. Measuring Selection and Response to Selection on Continuous Traits

A. Heritability

58 60 62 64 66 68 70 72 74

Femal eHt

0

0. 05

0. 1Density

90 105 120 135 150 165 180 195 210Femal eWt

0

0. 005

0. 01

0. 015Density

Class Data

Female Wt

Female HT

100 125 150 175 200 225 250 275Mal eWt

0

0. 005

0. 01

0. 015Density

62. 5 65. 0 67. 5 70. 0 72. 5 75. 0 77. 5Mal eHt

0

0. 05

0. 1

0. 15

Density

Male Wt

Male Ht

Model Equat i onFemal eWt = 91. 1457 + 0. 2807 Mot her Wt

100 150 200 250Mot her Wt

100

150

200

FemaleWt

Heritability of Female Wt

Model Equat i onFemal eWt = 75. 0179 + 0. 3094 Fat her Wt

150 200 250 300Fat her Wt

100

150

200

FemaleWt

Heritability of Female Wt

Model Equat i onFemal eWt = 57. 2357 + 0. 4499 Mi dPar ent Wt

150 200 250Mi dPar ent Wt

100

150

200

FemaleWt

Heritability of Female Wt

Model Equat i onFemal eHt = 48. 4108 + 0. 2592 Mot her Ht

55 60 65 70 75Mot her Ht

60

65

70FemaleHt

Heritability of Female Ht

Model Equat i onFemal eHt = 26. 1514 + 0. 5575 Fat her Ht

65 70 75Fat her Ht

60

65

70FemaleHt

Heritability of Female Ht

Model Equat i onFemal eHt = 23. 3220 + 0. 6198 Mi dpar ent Ht

62 64 66 68 70 72Mi dpar ent Ht

60

65

70FemaleHt

Heritability of Female Ht

Model Equat i onMal eWt = 137. 452 + 0. 1867 Mot her Wt

100 150 200Mot her Wt

150

200

250

MaleWt

Heritability of Male Wt

Model Equat i onMal eWt = 107. 950 + 0. 2951 Fat her Wt

150 200 250 300Fat her Wt

150

200

250

MaleWt

Heritability of Male Wt

Model Equat i onMal eWt = 99. 5721 + 0. 3870 Mi dPar ent Wt

150 200 250Mi dPar ent Wt

150

200

250

MaleWt

Heritability of Male Wt

Model Equat i onMal eHt = 29. 1168 + 0. 6420 Mot her Ht

60 65 70Mot her Ht

65

70

75

MaleHt

Heritability of Male Ht

Model Equat i onMal eHt = 31. 2457 + 0. 5623 Fat her Ht

65 70 75Fat her Ht

65

70

75

MaleHt

Heritability of Male Ht

Model Equat i onMal eHt = 14. 7069 + 0. 8275 Mi dpar ent Ht

62 64 66 68 70 72Mi dpar ent Ht

65

70

75

MaleHt

Heritability of Male Ht

Conclusions from class data:

Distributions of Wts and Hts are roughly normal

Distribution indicates that Wts and Hts are likely controlledby many loci, = many loci are segregating alleles that contribute to wt and ht differences among individuals

Heritabilities for Ht >> WT 50% >> 30%

Interpretation for other human traits??