Download Selection on quantitative characters

February 5th, 2010
Bioe 109
Winter 2010
Lecture 13
Selection on quantitative traits
Selection on quantitative traits
- From Darwin's time onward, it has been widely recognized that natural populations harbor a
considerably degree of genetic variation.
- Darwin came to this conclusion from the experiences of animal and plant breeders of his day
and relied on it heavily in developing his theory of evolution by natural selection.
- the form of variation envisaged by Darwin to be of fundamental importance for evolutionary
change was “continuous”, or what we know call “polygenic” or “quantitative”.
- quantitative characters exhibit continuous variation among individuals.
- unlike discrete characters, it is not possible to assign phenotypes to distinct groups.
- the statistical approach to studying such traits is referred to as quantitative genetics.
- the vast majority of morphological, physiological, and behavioral characters are polygenic and
thus understanding this class of variation is fundamental for evolutionary biologists.
- there are two characteristics of quantitative traits:
1. they are influenced by many genetic loci.
2. they exhibit variation due to both genetic and environmental effects.
- we are just now beginning to understand what polygenes are, and just how many such loci are
involved in affecting the expression of a quantitative character.
- our ignorance is profound.
- new insights are being provided by the ability to map QTLs, or "quantitative trait loci".
- this is done by melding traditional breeding experiments of quantitative genetics with fineresolution linkage maps. [A linkage map is a detailed "map" of the location of genetic markers
dispersed throughout the genome of the species.]
- identifying and characterizing QTL loci is going to dominate the field of quantitative genetics
for many years to come.
- there appears to be nothing magical about QTLs
- they possess possess multiple alleles, exhibit varying degrees of dominance, and experience
selection and drift.
- some QTLs exhibit stronger effects than others – these are called major effect and minor
effect genes, respectively.
- the total number and relative contributions of major effect and minor effect genes underlies the
genetic architecture of the trait.
- mapping QTLs is expensive, labor intensive and fraught with statistical problems!
What is heritability?
- the key to understanding selection on quantitative characters involves the concept of
heritability.
- heritability estimates the proportion of the total phenotypic variation that is due to
genetic rather than environmental effects.
- more formally, it is possible to separate the total phenotypic variation of a quantitative trait into
two components: environmental and genetic.
- let
VP = total phenotypic variance
VG = genetic variance
VE = environmental variance
VP = VG + VE
- then
heritability = VG / VP (broad-sense)
- this is called “broad sense” heritability.
- we will be considering another type of heritability called “narrow sense” heritability.
- this is different from broad sense heritability in incorporating only the additive component of
genetic variance.
- VG can be partitioned into three components due to additive (VA), dominance (VD), and
epistatic (VE) effects.
VG = VA + VD + VE
heritability = h2 = VA/VP (narrow-sense)
- of these three components, it is only the additive component that responds to selection.
Example of additive gene action
- suppose two genes (A and B) contribute to a quantitative character (say, abdominal bristle
number in Drosophila melanogaster)
- for alleles that act in an additive fashion, the heterozygote exhibits a phenotype that is exactly
intermediate between the two alternative homozygotes:
B1B1
A1A1 0
A1A2 2
A2A2 4
B1B2
B2B2
1
3
5
2
4
6
- suppose we selected for increased bristle number in a population with a low mean number of
bristles.
- this would result in the substitution of the A2 and B2 alleles.
Estimating heritability
- how do we estimate heritability?
- there are many different ways, but the most common is to compare the resemblance between
parents and their offspring.
- suppose we are interested in obtaining an estimate of heritability for growth rate for a species.
- one way to do this would be to perform pair matings between different individuals and then rear
the progeny of these crosses under similar conditions (to hold the environmental effect constant
among families).
- we allow the progeny of these matings to reach the same age as the parents.
- we then take the mean of each pair of parents - this is called the midparent value.
- we then take the mean of each set of progeny - we can call this the offspring value.
Cross
F
1
100
2
160
3
150
etc…
M
“Midparent
value”
“Offspring
value”
140
110
140
120
135
145
132
126
153
- data from many families is preferred.
- we can then estimate heritability by regressing the offspring values against the midparent
values.
- the slope of this regression line is an estimate of heritability.
- here are some other comparisons:
Comparison
Slope
Midparent-offspring
Parent-offspring
Half-sibs
First cousins
h2
1/2h2
1/4h2
1/8h2
- as the groups become less related, the precision of the h2 estimate is reduced.
- once we have an estimate of heritability, we can predict the response to selection by the
following prediction equation:
R = h2 S
- here, R is called the response to selection and S is the selection differential.
- the selection differential is equivalent to the strength of selection acting on the trait.
- it is quantified by the difference between the mean of the selected group and the original
population.
- the heritability usually remains constant over a sizable number of generations giving us a
constant and predictable response to selection.
Example: Beak depth in the large ground finch, Geospiza magnirostris
- in this species, the heritability of beak depth is about 0.72.
- the initial population mean was 8.82 mm.
- in one episode of intense selection, the mean beak depth of survivors was 10.11 mm.
Selection differential = S = 10.11 – 8.82 = 1.29 mm
Response to selection, = h2S = (0.72)(1.29) = 0.93 mm
- thus, beak depth in the next generation is predicted to be 8.82 + 0.93 = 9.75 mm
- this assumes that the finch population will not experience drift, mutation, or migration.
Heritability estimates in nature
- what are heritability estimates in natural populations?
- they are commonly very high.
- here are some heritability estimates for a population of medium ground finches in the
Galapagos studied by Peter Boag (1983) in comparison to a population of N. American mainland
song sparrows studied by Smith and Zach (1970):
Character
Body weight
Wing length
Tarsus length
Bill length
Bill depth
Bill width
Ground Finch
0.91
0.84
0.71
0.65
0.79
0.90
Song Sparrow
0.04
0.13
0.32
0.33
0.51
0.50
- why would the heritabilities of these traits be so much higher in the Galapagos species?
- in a review paper, Mousseau and Roff (1987) compared the heritabilities of different kinds of
quantitative traits:
Trait
Life history
Physiological
Sample size
341
104
Mean heritability
0.262
0.330
Behavioral
Morphological
105
570
0.302
0.461
- these studies illustrate the abundance of additive genetic variation in natural populations for
most quantitative characters.
Selection on quantitative characters
- natural selection acting on quantitative traits can take three basic forms: stabilizing, directional,
or disruptive.
1. Stabilizing. e.g., gall size in goldenrods caused by the fly, Eurosta altissima.
- this is probably the most common form of selection.
- here, individuals with intermediate phenotypes have the highest fitness and those with extreme
phenotypes the lowest fitness.
- selection thus does not change the mean of the character but reduces its variance.
- small galls made by E. altissima are preferentially attacked by parasitoid wasps.
- large galls are attacked at a higher rate by birds.
- the result is a substantial reduction in the variance of gall size with intermediate sized galls
surviving at much higher rates.
2. Directional. e.g., selection on corn oil content.
- this form of selection increases or decreases the mean of a quantitative character in a consistent
manner over a number of generations.
- it also reduces the amount of variation in the population, but this depends on the intensity of
selection and the heritability of the trait.
3. Disruptive. e.g., seed-cracker finches in Cameroon.
- disruptive selection occurs with extreme values of a phenotypic trait have the highest fitness.
- unlike stabilizing and directional, this form of selection acts to increase the amount of
phenotypic variation of the selected trait but may not change the mean.
- it is thought to occur less frequently than the other two.