adaptation 4 - quantitative traits Flashcards
Quantitative traits
Are influenced by the combined activity of several genes (polygenic)
Often influenced by the environment, to varying degrees
Have patterns of continuous trait variation, rather than having discrete categories
Often have a mean and variance
possible barriers for response to changing conditions
pikas: low elevation is a barrier to moving to other more northern mountain ranges, bc cant stand heat
meaning of adaptation in 336
undergoing genetic change
in other contexts, used instead of acclimating
distribution of traits affected by different factors
general normally distributed
Why are quantitative traits normally distributed?
Bc affected by multiple loci and environmental factors
Evolution of traits with normal distributions by natural selection
- trait must vary btwn individuals
- trait must affect fitness
- trait must be heritable
3 forms of selection on quantitative traits
- directional selection
- stabilizing selection
- disruptive selection
related to how selection impacts one generation
Fitness function
shows fitness of individuals with certain “values” of a quantitative trait
Directional selection
individuals with more severe “values” of a quantitative trait have greater fitness
variation shifts in one direction or another
stabilizing selection
variation decreased
disruptive selection
variation increases
average effect
determines how much the mean of a trait Z changes when we know an individual carries this particular allele, assuming PAIRED AT RANDOM with another allele from population
Breeding Value
(A)
Breeding value of an individual (or genotype) measures the average contribution of that individual to the trait mean of its offspring, if crossed with an average range of mating partners
Breeding value equation
(mean of pop) + 2*([mean of offspring] - [mean of population])
what does breeding value measure
the degree to which an individual’s phenotype is expected to be transmitted to their offspring
what is phenotype a function of
P = genes + environment
BREAKS DOWN Into
(A + D + I) + E
What could genetic component of phenotype be broken down into?
A - additive genetic effects (breeding value)
D - dominance effects
I - Interaction effects (epistasis btwn loci)
def. additive effects of alleles
average effect of all alleles that contribute to a phenotype
def dominance effects
effects of dominance interactions among alleles at each locus
def epistasis
The effect of interactions among alleles at different loci
why do we focus on additative effects in the evolution of quantitative traits
non-additive effects are disrupted every generation therefore no predictable effect on offspring
env. not genetically based
therefore, natural selection acts on additive effects
Artificial selection vs natural
breeders focus on picking individuals for crosses to move the mean trait values/change traits of group
natural selection acts on differential success of individuals with different trait values, affects how a trait mean will change in response
phenotypic variance
(VP)
describes the variability among a set of individuals for a particular trait
NOT always do to genetic differences (VG), not all genetic differences passed to next generation (VA)
componenets of phenotypic variance
VP total phenotypic variance
VG total genetic variance
VA genetic var due to additive effects of alleles
VD gen var due to dominance among alleles
VI - gen var due to epistatic interactions of alleles at diff loci
VE - phenotypic var due to environmental effect
VP = VA + VD + VI + VE
VA =
= genetic var due to additive effects
= variance in breeding values
= measure of ability of population to evolve/shift population mean from one generation to the next
heritability of a quantatative trait calculation
“narrow sence heritability” - proportion of pheno var caused by additive
H^2 = VG/VP
can estimates of heritability be applied to whole species?
NO!!!! only apply to population and environments in which they were estimated
can heritability change within pops?
YES!!!!!
DO HERITABILITY ESTIMATES APPLY To individuals?
NO!!! only populations!!!
measure of avg relationship of traits values in parents and offspring across the population as a whole
goes heritability indicate generic basis
NO does not indicate the degree to which a trait is genetically based
determine herotability by gra[hing
plotting trait value (pheno) of offspring against trait score (pheno) of parents, slope of line is heritability
range of heritability
0 to 1 (proportion of pheno var accounted for by genetic var)
The breeder’s equation describes…
how much we expect a trait mean to change over gens, given the heritability of trait, and knowledge of the strength of selection acting on that trait
the breeders equation
R = h^2 S
where
S = “selection differential”, measures strength of selection within a generation
R = “Response to selection”, measures change in trait mean from one generation to the next
how could breeders equation be used
using heritability estimate and strength of selection to predict response to selection (R)
using changes in mean trait value (R0 in response to selection) of known strength to est heritability (h^2)
Selection Differential
Measures the strength of selection
calculated by S = Xb - Xp
where XP = trait value of population before selection
XB = mean trait value of reproducing individuals
Strong v weak selection
strong - very few individuals can reproduce
weak - many many individuals can reproduce
H^2 = 0
No response to selection
H^2 not 0 or 1
scaled response to selection
h^2 = 1
response = S
3 approaches to estimating heritability
- experimental/breeding analysis of phenotypic variance components: est heritability using variance in breeding value (h^2 = VA/VP
- phenotypic correlations among relatives, e.g., parent-offspring regression in which slope = h^2
- measuring response to selection across generations, knowing R and S, can est. h^2 using h^2 = R/S
