Transcript lecture 12 - quantitative traits I - Cal State LA

Trait evolution
Up until now, we focused on microevolution – the forces that
change allele and genotype frequencies in a population
This portion of the class will focus on the evolution of traits
- morphology (features of the physical body)
- behavior (mate preference; habitat choice;
nocturnal vs diurnal activity)
- life history (age at 1st reproduction; life span; # of offspring)
Quantitative traits and heritable variation
Some traits are discrete, meaning they can fall into certain
categories but not have in-between values
- eye color can be blue, brown, or hazel, but not a blend
- you either have cystic fibrosis, or you don’t
What about traits controlled by multilocus genotypes?..
Quantitative traits and heritable variation
Quantitative traits show continuous variation among individuals
- height: you can be 4’6” tall, or 6’9” tall, or any height in
between (there is a continuum of heights in a population)
- quantitative traits are determined by multiple genes, and are
also affected by the individual’s growing environment
Quantitative traits and heritable variation
Most quantitative traits reflect contributions both of genes and
the environment you grew up in
For instance, tall parents tend to have tall kids
However, children of first generation immigrants are usually
taller than either parent, because of better childhood nutrition
Thus, both genes and environment determine your adult height
Quantitative traits and heritable variation
Quantitative traits usually have a normal (bell-curve)
distribution of values
What makes the tallest
person so tall?
Is it my genes?
Or the environment I
grew up in?
Or how much of each?
How heritable is a quantitative trait?
Take a pair of identical twins from short parents
Raise one under the conditions in which the
shortest individual grew up (environment #1)
4’ 6”
Raise his twin under the conditions in which
the tallest individual grew up (environment #2)
4’ 6”
If his twin grows up to be just as
short, then all the variation in
height must be genetic (heritable)
How heritable is a quantitative trait?
Take a pair of identical twins from short parents
Raise one under the conditions in which the
shortest individual grew up (environment #1)
4’ 6”
Raise his twin under the conditions in which
the tallest individual grew up (environment #2)
6’ 9”
If his twin grows up to be very
tall, then all the variation in height
must be due to the environment
he grew up in
How heritable is a quantitative trait?
Take a pair of identical twins from short parents
Raise one under the conditions in which the
shortest individual grew up (environment #1)
4’ 6”
Raise his twin under the conditions in which
the tallest individual grew up (environment #2)
5’ 11”
If his twin grows up to be in the
middle, then the variation in height
must be due to a combination of
genetics and environment
mean
a
population
Parental Generation ---
- individuals from extremes
of the trait distribution are
mated with their own kind
- offspring raised in a common,
controlled environment
- if the means of offspring differ,
the trait is heritable
heritable
mean
Parental Generation ---
- individuals from extremes
of the trait distribution are
mated with their own kind
- offspring raised in a common,
controlled environment
- if the means of offspring differ,
the trait is heritable
not
heritable
Sources of phenotypic variation
The total variation in a trait is the phenotypic variation, VP
- subtract the height of the smallest person from the tallest
person; this will give you the range in heights, VP
Variation among individuals due to differences in their genes
is genetic variation, VG
Variation among individuals due to differences in their
environment is environmental variation, VE
VP
=
VG
+ VE
Quantitative traits and heritable variation
The fraction of the total variation in a trait that is due to variation
in the genes is termed the heritability of that trait
broad heritability
=
=
= VG =
VG
VP
VG + VE
genetic variation, VG
total variation, VP
genetic variation, VG
genetic variation + environmental variation, VE
This is termed H2, the broad-sense heritability
How do you measure heritable variation?
If variation is due to genes, offspring will resemble their parents
average height of offspring
Plot the average value for 2 parents against the average for their
offspring, for many sets of parents & kids
1) Scatterplot of values
for all families
Family #2:
tall parents,
tall kids
Family #1:
short parents,
short kids
midparent height (average height of mother & father)
How do you measure heritable variation?
If variation is due to genes, offspring will resemble their parents
average height of offspring
Compute the best-fit line through the points for all families, using
least-squares linear regression
2) Best-fit line through
all the points
The slope of this line estimates
how much variance in parents
is due to variance in their genes
midparent height (average height of mother & father)
How do you measure heritable variation?
average height of offspring
The slope of the best-fit line is used to directly measure
the heritability of a trait
midparent height (average height of mother & father)
Slope of the line, h2, is termed the narrow-sense heritability
Sources of genetic variation
Total genetic variation, VG , comes from two sources:
VG = VA + VD
(1) additive genetic variation is variation between
individuals due to the combined effects of many
genes working together in each individual
(2) dominance genetic variation is the variation due to
gene interactions like dominance and epistasis,
where an allele of one gene can “over-rule”...
- another allele of the same gene (dominance)
- any allele of a different gene (epistasis)
Narrow-sense vs. broad-sense heritability
Slope of the line, h2, is termed the narrow-sense heritability
h2 = VA
VP
=
VA
VG + V E
VA
VA + VD + VE
Narrow-sense heritability is the fraction of variation between
individuals that is due to additive genetic variation only
h2 =
additive genetic variance
variance from genes + variance from environment
Narrow-sense vs. broad-sense heritability
Broad-sense heritability = all variation due to genetic differences
total phenotypic variance
Narrow-sense heritability = variance due to the effects of many
alleles all added together
total phenotypic variance
My height is due in part to many alleles of different genes each
contributing a certain amount; this is additive genetic variation
My height may also result from dominance of one allele over a
recessive allele at a particular locus; that’s dominance genetic
variation, and we aren’t so interested in that
Narrow-sense vs. broad-sense heritability
Slope of the line, h2, is termed the narrow-sense heritability
h2 = VA
VP
=
VA
VA
VG + VE
VA + VD + VE
Why do we care about all this?
Understanding narrow-sense heritability lets us:
(1) measure the heritability of a trait in a population
(2) determine if the mean value of a trait is likely to
change in response to selection
Genes vs. environment for offspring + parents
Parents often raise their offspring under similar environments
to those in which the parents themselves grew up
- how then do you distinguish whether offspring are like their
parents because of shared genes, or shared environments?
1) Reared-apart experiments: offspring of same parents raised
under different conditions (esp. useful with identical twins)
2) Common garden experiments: offspring of different parents
raised under identical conditions
3) Random distribution of young: scatter offspring so they are not
raised by their own parents; environment is a random variable
Heritability of beak depth in song sparrows
Eggs taken from many
nests, placed in other
nests with foster
parents
Result: chicks grew up to
resemble their biological parents
and not their foster parents
Heritability, h2 = 0.98
Strength of selection
To predict if a trait will evolve in response to selection, you need
to know two things:
(1) the heritability of the trait, since only inherited traits can
change in response to selection
(2) the strength of selection, or how much of a reproductive
advantage a trait confers on parents
- if parents with a high mid-parent value for a certain trait are
more likely to reproduce, then that trait will respond more
strongly to selection

Strength of selection
The strength of selection can be calculated as the mean value
among parents that successfully reproduce, compared with the
mean value of the whole population
This value, S, is termed the
selection differential
mean of successful
parents
– mean of all
parents
Predicting the response to selection
The response to selection is given as: R = h2S
Response = (heritability) x (selection differential)
Can be expressed as h2 = R
S
Summary: heritability and strength of selection
Heritability can be estimated by comparing the similarity of
relatives
The strength of selection can be measured as the relationship
between phenotype and fitness-- how a trait plays out as a
reproductive advantage
Response to selection tells you how much a trait will change
over a generation, based on how much of the trait is genetic
and how much that trait contributes to fitness