announcements 1.make up exam: friday nov 9, 2-4pm, room bv363 (see blackboard for details) 2. video...

Announcements1. Make up exam:

Friday Nov 9, 2-4pm, Room BV363 (see blackboard for details)

2. Video 2 is posted on weboption: ‘Evolution: The Eternal Arms Race’

3.Office hours this weekTODAY SW551 11am to noon.

Friday SW551 & Virtual. 1pm – 2pm

4. Slide correction: Cystic Fibrosis example on p.214-218

5. End of HIV lecture will be provided as an audio file linked to powerpoint by Monday.

Tropical Tropical Biodiversity Biodiversity Field CourseField Course(BIOC51 Spring)(BIOC51 Spring)

Costa RicaCosta Rica

• Semester-long course with lectures at UTSC and a Semester-long course with lectures at UTSC and a 8 day field trip to Costa Rica (during Feb break)8 day field trip to Costa Rica (during Feb break)

• Focus on biodiversity of tropical freshwater Focus on biodiversity of tropical freshwater ecosystems ecosystems

• Learn field sampling techniquesLearn field sampling techniques• Participate in biological survey of the Costa Rica Participate in biological survey of the Costa Rica

fish faunafish fauna• See amazing animals, plants, and habitatsSee amazing animals, plants, and habitats

The COURSE :The COURSE :

• Lectures: Spring, Tuesdays 11am-noonLectures: Spring, Tuesdays 11am-noon• Field trip: Feb 16Field trip: Feb 16thth to Feb 24 to Feb 24thth

• Cost: $1800 + tuition for 0.5 FCECost: $1800 + tuition for 0.5 FCE• Contact N. Lovejoy or M. Kolmann as soon as Contact N. Lovejoy or M. Kolmann as soon as

possible to reserve a spot (deadline November 15possible to reserve a spot (deadline November 15 thth))• For more information, contact Prof Nathan For more information, contact Prof Nathan

Lovejoy, [email protected], [email protected]• Or TA Matt Kolmann, Or TA Matt Kolmann,

[email protected]@mail.utoronto.ca

The DETAILS:The DETAILS:

• Quantitative traits

• Heritability & Evolution

Lec 17: Quantitative Genetics & Heritability

Recall: from Population Genetics:

Simplification: Assume phenotypes fall into discretecategories, determined strictly by genotypes

We can now assign some relativelifetime reproductive success to each genotype

& use Hardy-Weinberg analyses

rr RR Rr

e.g.,

GG, Gg gge.g.,

genes

protein

phenotype

•e.g.,•amino acids•enzymes

•e.g., •other proteins•ph

protein

Biotic & abiotic factors

genes

BUT: internal & external environment importantfor development of most traits

Quantitative Genetics

Quantitative trait = a characteristic for which phenotypes show continuous variation among individuals (e.g., height)

Most traits

• traits determined by the environment and many mendelian genes (at many loci)with small effects

focus on:• phenotypes & heritability• statistical properties of population traits (e.g., mean, variance)

e.g., Quantitative Traits

Sprint speed

Colour

IntelligenceStem length

Genotypes

Plant form (height, #stems) varies with:

1. Environment(elevation)

clones

height (cm)

2. Genotype

AND

1. High elevation

2. Medium elevation

3. Low elevation

e.g., Yarrow (Achillea)

Take plants with exactly the same genotypes, rear in three

different environments

Genotypes

Plant form (height, #stems) varies with:

1. Environment(elevation)

clones


Take plants with exactly the same genotypes, rear in three

different environments

height (cm)

2. Genotype

AND

1. High elevation

2. Medium elevation

3. Low elevation

Most phenotypes are affected by BOTH

genes and the environment…

This is particularly true for QUANTITATIVE

TRAITS

e.g., Quantitative Traits

5.0 5.2 5.4 5.6 5.8 5.10 6.0 6.2 6.4 6.6

# students

Height (feet.inches)

20181614121086420

Typical distribution:normal distribution

(bell curve)

Quantitative Genetics

Tools for measuring:

•Heritable variation in traits (heritability)•Differences in lifetime reproductive success

(intensity of selection)

Tools for predicting:•Effect of selection on phenotypic trait value

(Evolutionary response to selection)

Problem: we usually do not know which locicontrol quantitative traits

How do we study evolution of these traits?

Common mistake ALERT !!

This is NOT your grandma’s

‘heritability’

‘HERITABLE’ has a specific meaning in evolutionary

biology and has little to do with

the common usage of the words ‘heritable’

or ‘inherited’

Heritability

Heritability? = Is this student 5’ tall due to genes or environment?

•Due to her genes

•Due to the environment

•Due to combined effect of genes and environment

Always true

Heritability

Heritability: Why is the shortest student shorter than the tallest student?

•Differences in their genes

•Differences in their environments

•Differences in genes and environment

Heritability = extent to which phenotypic differences are due to differences in genes underlying those traits

= fraction of total variation in trait that is due to variation in genes

Heritability

Population level:What fraction of the variation in height is due to

variation in genes (& what fraction is due to variation in environment)?

‘broad sense heritability’

Heritability

Is the trait heritable?

Loosely: “Do offspring tend to resemble theirparents?”

(when we control for variation due to environment)

The greater the relative effect of GENES on a trait, the stronger the evolutionary response to selection

Why do we care about heritability?

size

parents

Need to know heritability to predict evolutionary effects of selection

selection

IF stronggenetic effect

on trait

High heritability

IF strongenvironmentaleffect on trait

Lowheritability

offspring

offspringNO Evolutionary change…

Evolutionary change…

HeritabilityPopulation level:

What fraction of the variation in height is due to variation in genes, what fraction is

due to variation in environment?

VP = phenotypic variation = total variation in trait

VP = VG + VE

VG = genetic variation = variation among individuals due to variation in their genes

VE = environmental variation = variation among individuals due to variation in their environment

VG = VA + VD

Different types of genetic variation

VP = VG + VE

Additive genetic variation

Dominance genetic variation

Differences among individuals due to additive effects of genes • contribution of each allele to phenotype isindependent of other alleles

Differences among individuals due to interactive effects of genes • e.g.,dominance, epistasis

Heritability depends on

additive genetic

variation underlying variation in

traits

Heritability depends on additive genetic variation underlying variation in traits

Loosely: additive genetic variation is the type of genetic variation that leads to offspring looking like ~

‘average of their parents’

Defined to allow us to predict effect of selection on phenotypic distributions

Different types of genetic variation• simplified (qualitative) consideration of problem:• two alleles, R and r

genotype

phenotype

rr Rr RR

every allele contributed by parent changes the phenotypeof offspring by one ‘unit’ (additive, predictable response to selection)

1. Additive genetic variance (no dominance)VA

Each ‘R’ allele contributes one ‘unit’ of Red, each ‘r’ allele

one ‘unit’ of white

Different types of genetic variation• simplified consideration of problem:• two alleles, R and r

genotype

phenotype

rr Rr RR


Imagine:

Strong selection

for red


Different types of genetic variation• simplified consideration of problem:• two alleles, R and r


genotype

ph

eno

typ

e

rr Rr RR

Parents:

RR x RR

Next generation:

All red



2. VD Dominance genetic variance

effect of alleles contributed by parent on phenotype of offspring depends on other alleles (response to selection not predictable)

ph

eno

typ

e

rr Rr RR

‘R’ allele is dominant, ‘r’ allele is recessive

Compare to:


ph

eno

typ

e

rr Rr RR

Imagine:

Strong selection

for red



Compare to:



ph

eno

typ

e

rr Rr RR

Parents:

RR & Rr

Next generation:

Red & White


Heritability

h2 = VA = VA

VP VA + VD + VE

Narrow-sense heritability ( h2)

= fraction of total variation in trait that is due to additive genetic variation

= h2 (0 < h2 < 1)

This is what we mean when we say ‘heritability’ in evolutionary studies

All genesAll environment

Measuring HeritabilityA. Parent-offspring regression:

how much do offspring resemble their parents?(when we control for variation due to environment)

Measure of the amount of variation in parental trait due to

additive genetic variation

Mid-offspring height (inches)

Mid-parent height (inches)

Slope = 0.84

h2 = slope of parent-offspring regression

riserun


how much do offspring resemble their parents?

Most traits

Measuring Heritability

0 10 0 10 0 10

10

0

Mid-offspring trait

Mid-parent trait

Heritability 0 Heritability 1.0Heritability 0.5

A. Parent-offspring regression: how much do offspring resemble their parents?


BUT: Parents may resemble offspring due to:•similar genotype•similar environment

To measure heritability accurately, must controlfor environmental effects

A. Parent-offspring regression:


Mid-offspring height (inches)

Mid-parent height (inches)

h2 = 0.84

Good dietPoor diet

Poor diet

Good diet

Problem: similarities may be due to similar environments...

?


Problem: similarities may be due to similar environments...

Control for environment = isolate genetic effects

Strategies

1) Ensure offspring and parents havedifferent environmentse.g., cross-fostering / adoption (animals) orreciprocal transplant (plants) experiments

2) Ensure all offspring have sameenvironmente.g., ‘common garden’ experiments

A. Parent-offspring regression:


1. Cross-fostering experiments

Heritability of beak depth

Song sparrow



Offspring are reared by non-genetic

parents

(foster parents)



Measure beak depths of adult offspring, genetic parents & foster parents

Measuring HeritabilityA. Parent-offspring regression (1. cross fostering)

(different environment, any similarity due to genes) Accurate estimate of h2

Reared in nest of ‘foster parent’


Similarity is due to underlying genetic similarity rather than environmentally-induced similarities

Genetic parents & cross-fostered offsprings

Foster parents & foster-offspring (same environment, different genes)

Compare to


2. Common-garden experiment

Common Lab Environment

Measuring heritability

(i) monozygotictwins reared

apart

•identical genotype•different environment

Jack Yuferaised Jewish inCarribbean

Oskar Stohrraised Catholic in

Nazi Germany

B) Twin studies

how similarare they as

adults?


Jack Yufe Oskar Stohr

despite 47 years apart they both “like sweet liqueurs, …store rubber bands on their wrists, read magazines back to front, dip buttered toast in their coffee and have highly similar personalities”

Holden . 1980. Science. 207:1323-1328


B) Twin studies

monozygotic dizygotic

(ii) Compare similarity of

monozygotictwins to

similarity of dizygotic twins

Clones

100% genetic similarity

Siblings

50% genetic similarity

monozygotic dizygoticMonozygotic:•Same genes•Same environment

Dizygotic: •different genes•same environment

Heritability high

Heritability low

>

=

Heritability: a common error

Heritability is NOT A FIXEDcharacteristic of a trait

Heritability is specific to a particular population in a particular environment

Why??

Heritability

h2 = VA = VA

VP VG +VE

What happens to h2 if VE changes?

h2 will change

•Even if VA underlying trait remains the SAME

Heritability is not a fixed value: hypothetical example

e.g., What is the heritability of skin colour amongCaucasians in Vancouver?

e.g., What is the heritability of skin colour amongCaucasians in Vancouver?

Facts: sun exposure is the largest environmental determinant of skin colour• Vancouver: winter (hardly any sun, rains all the time); summer is quite sunny

Assume: random variation in how much time people spend outdoors (tanning)

when it’s sunny (VE)

Heritability is not a fixed value: hypothetical example

Heritability = VA / (VG + VE)

Prediction: h2 is low

Winter

Summer

VE is low

VE is high

Prediction: h2 is high

Mid-parent skin tone

Mid-offspringskin tone

light dark

dark

light

Mid-parent skin tone

Mid-offspringskin tone

light dark

dark

light

Heritability: common errors 1

Error 1. If all members of a population have the same trait value, that trait is highly

heritable

What is the heritability of

having a nose in humans?


Error 1. If all members of a population have the same trait value, that trait is highly

heritable

Correction:If there is no variation in a trait, the trait is

NOT heritable

Heritability = VA / VP

= 0/0


Error 2. Heritability is a fixed value for a particular trait

Correction:Heritability if NOT a fixed characteristic of a

trait

Heritability of a trait is specific to a particular population in a particular

environment


Error 3. ‘Heritability tells us whether differences between populations are due to

differences in genes or the environment’

Heritability

Hypothetical example: you observe that people from Vancouver have fairer skin than people from

Los Angeles on average

VancouverLos Angeles

Light Dark

Does heritability tell us something aboutthe source of differences between populations?

Does this mean that the difference in average skin colour of people from Vancouver versus California is due to genetic differences between the populations?

Mid-parent

Mid-offspring

Mid-parent

Mid-offspring

h2 highh2 high

Winter, Vancouver Winter, Los Angeles

You also find: High heritability for skintone within each population


Jan – March in Los Angeles

Jan – March in Vancouver

Is this difference in skin colour due to a genetic difference?


NO

One likely hypothesis:It is sunnier in Los Angeles than in Vancouver, thus Los

Angeleans are darker skinned on average

e.g., in this case, difference between populationsmainly due to VE despite high heritability within each

population


Error 3. ‘Heritability tells us whether differences between populations are due to

differences in genes or the environment’

CORRECTION:Heritability does not tell you anything

about the cause of differences BETWEENpopulations

Genotypes

1. High elevation

3. Low elevation

2. Medium elevation


High heritability WITHIN each

population

height (cm)

Why is there variation in plant

form between populations?

reminder

SO:•Heritability is NOT a fixed characteristic of a trait(varies with environment & across populations)

•Is NOT useful for identifying the source of differences BETWEEN populations

Why are you messing with my mind????Did you just waste this entire lecture on nonsense???

SO:•Heritability is NOT a fixed characteristic of a trait(varies with environment & across populations)

•Is NOT useful for identifying the source of differences BETWEEN populations

Allows prediction of whether selection on a trait in a given population will cause that

trait to evolve

Why bother to measure heritability?

R = h2S

Measuring the response to selection

Evolutionary response to selection

Magnitude & direction of change in trait in offspring

2. Selection differential

•magnitude & direction of selection

1. Heritability

• extent to which parental phenotype predicts offspring phenotype

Depends on:


Trait h2

Gestation period 0.3Milk yield 0.3Height 0.6

Domestic cattle

Which trait

would you select

for if you want to

increase

profitability of

your herd?


Trait h2

Litter size 0.1Back fat thickness 0.64

Domestic pigs

Which trait would

you select for to

increase

profitability of your

drift (of pigs)?

NEXT LECTURES

The (abuse and) Misuse of Heritability

announcements 1.make up exam: friday nov 9, 2-4pm, room bv363 (see blackboard for details) 2. video...

Documents

quantitative traitse

effect of genes

yarrow achilleatake

day field trip

continuous variation

tallest student

phenotypic differences

low elevationmost phenotypes