Evolution Lecture 20, Chapter 13

Topics for today:

Evolutionary response to selection in quantitative traits

1. How many genes?
2. How do polygenic traits evolve?
3. How strong is natural selection?

Fig. 13.9

What is a quantitative trait?

What traits are not polygenic?

Examples of polygenic traits in humans

How many loci contribute to polygenic are traits?

QTL Mapping (Quantitative trait loci)

Mimulus lewisii

Mimulus cardinalis

Basic requirements for this technique

• Two different phenotypes (in this case separate species)
• Genetic map available for both species
• Many molecular markers for each species with known locations on the genetic map

Basic steps to identify QTLs

1. Cross to the F1 – all intermediate
2. Cross to the F2 - recombination causes variation to segregation
3. Find associations between the phenotypic attributes and the molecular markers
4. If an individual expresses the phenotype and also possesses the marker, then you know that a gene that influences the trait resides in that region of chromosome.

Fig 13.5

What regions of chromosome 3 affect bristle number in Drosophila?

• Artificial selection for high and low bristle number
• Many additive QTL
• Some strong epistatic effects

Number of QTLs found

· 33 sternopleural (thorax)

· 31 abdominal

· 11 pleiotropic

Fig. 13.6

(QTL review paper linked on web)

QTLs are well-described for agricultural species

• Example of QTLs that are associated with salt tolerance in rice
• Can find an interaction QTL viewer for cows at http://bovineqtlv2.tamu.edu/index.html

How do polygenic traits evolve?

• R = h²_NS
• R = Response to selection = change in a trait between generations
• h²_N = Narrow-sense heritability
• S = Strength of selection = change within a generation

What is heritability?

• Broad-sense heritability: h²_B = V_G = V_A + V_D + V_I

V_P V_A + V_D + V_I + V_E

• Narrow sense heritability: h²_N = V_A = V_A

V_P V_A + V_D + V_I + V_E

• Only the variance attributable to additive genes is transmitted across generations
• Gene interactions, including dominance (subscript D) and epistasis (subscript I), are created anew depending upon the specific combination of alleles that offspring acquire from their parents

Heritability can be estimated by artificial selection

• R = h²_NS
• S is determined experimentally
• R is observed
• h²_realized = R

S

Fig. 13.8

How do you measure selection?

Two measurements

1. i = intensity of selection (before and after selection)

= z_a –z_b

ÖV_P

2. b = partial regression coefficient between relative fitness and trait (effect holding all other traits constant)

Strength of selection measured on beak size

• No reproduction in drought year
• Strength of selection = change within generations

Fig 13.11

Strength of selection

• Estimates from many studies
• Generally weak
• Fecundity & mating > survival
• Even weak selection can have strong effects over time

Fig 13.14