Evolution of Quantitative Traits

Evo Bio Exam 3

Quantitative Genetics

• the study of continuous traits

• Ex:

  • height

  • color

  • speed

  • growth rate

  • fitness

• the assumption is that these traits are polygenic ( controlled by many loci)

Selection differential (S)

• mean trait value of the total population - mean trait value of the reproducers

• describes the strength of selection in phenotype units

• gives the difference in phenotypic mean of breeders and non-breeders

• standardize fitness data (divide all fitnesses by the highest fitness)

• steeper the slope = stronger selection or fitness gradient

Response to selection (R)

• trait mean before selection - trait mean in the next generation

For a given S, what determines R?

Darwins postulates

• trait exhibits variation

• trait variation affects fitness

• trait variation is heritable

• for evolution by natural selection one needs traits that exhibit heritable variation for fitness

Breeder’s equation

R = h²S

• the evolutionary response to selection (R) is a product of the heritability of a trait (h²) ( in that population at that time) and the selection differential (S)

Heritability

• fraction of the total phenotypic variance in a population due to the genetic variance that causes resemblance between parents and offspring

• range = 0-1.0

High heritability

• high resemblance between parents and offspring

• can predict offspring phenotype from parental phenotype

For a given selection differential, the higher the heritability in a population…

the larger the response to selection

2 Sources of phenotypic variation

• Genetic (G)

• Environmental (E)

• Vp = Vg + Ve

  • phenotypic variance = genetic variation + environmental variances

  • always sums up to 1.0

Broad sense heritability (H)

Vg/Vp = Vg/ Vg + Ve

Deviations from the regression line may be due to:

• Ve (environmental variance)

• Non-additive genetic variance

  • dominance

  • epistasis

    • these obscure the relationship between allelic and phenotypic variation, decrease the resemblance between parents and offspring, and thereby impede the response to selection

Three components of genetic variation

V= Va + Vd + Vl

• Va = additive variance

• Vd = dominance variance

• Vl = epistasis variance

Va : Additive variance

• alleles are codominant - have additive effects

• effects are independent of genetic background

• this produces resemblance between parents and offspring and thus contributes to the response to selection

• allelic variation consistently associated w/ phenotypic variation

Vd : Dominance variance

• allele are dominant/ recessive - nonadditive at the same locus

• ex: Aa has same trait value (same phenotype) as AA

V: Epistatic variance (interactive variance)

• genetic variance resulting from epistatic (non-additive) interactions between alleles at different loci

• effects are dependent on genetic background

• allelic variation not CONSISTENTLY associated w/ phenotypic variation

What does epistasis do to the resemblance between parent & offspring?

• reduces resemblance by masking or modifying phenotypic expression of genes

How does epistasis affect heritability?

• can hide genetic variance?

Heritability complications

• environmental effects

• non-additive effects

• Vp= Va + Vd + Vi + Ve

Narrow-sense heritability

• h² = Va/Vp

• produces the resemblance between parents and offspring and thus contributes to the response to selection

Additivity

• because alleles have independent effects, offspring resemble parents

• consistent relationship between alleles and phenotype means there will be a good response to selection

• strong environmental effects can cloud the resemblance between parents and offspring (lower Va/Vp), meaning that the relationship between allelic and phenotypic variation is less consistent- lowering heritability and reducing the response to selection

Deviations from the regression line may be due to

• Ve (environmental variance)

• Non-additive genetic variance

Epistasis

• the effect of an allele at one locus depends on the alleles at one or more other loci

• add effects of alleles at each locus

• multiply effects between loci

Epistasis: Little resemblance between parents and most offspring

Poor correlation caused by:

• recombination

• independent assortment

• non-additivity

Recombination

• eliminates LD, decreasing resemblance between parents and offspring by changing the allelic combinations at interacting loci

• b/c loci 1 and 2 interact non-additively and the allele combinations at these loci change each generation, offspring do not resemble parents and these cannot contribute to the response to selection