Quantitative Genetics

Question 0

Stabilizing selection

Answer 0

Individuals with intermediate value of a trait have highest fitness
Fitness function - similar to bell curve
Middle values high fitness and extreme values low fitness
Mean does not change
Variation of trait is reduced

Question 1

Directional selection

Answer 1

Individuals at one extreme of a trait distribution have highest fitness
Fitness function - slanted line like y=x line
Lower trait values have lower fitness and higher trait values have higher
Mean changes
Variation of a trait decreases

Question 2

Disruptive selection

Answer 2

Individuals at both extremes of trait have highest fitness
Fitness function - upside down bell curve
Ends have highest fitness and center have lowest fitness
Mean does not change
Variation of trait increases

Question 3

Calculate response to selection (R) from mean trait values across generations

Answer 3

R = h2S where h2 is heritability and S is the selection differential 
S = offspring generation mean - previous generation mean

Question 4

How to determine the size of R?

Answer 4

1. The heritability of the trait

2. The strength of selection

Question 5

Response to selection (R)

Answer 5

The change in the trait value from one generation to the next, a measure of the evolution of the trait

Question 6

Heritability (h2)

Answer 6

The fraction of phenotypic variation (genetic variation + environmental variation) in a trait that is due to genetic variation
Genetic variation/phenotypic variation

Question 7

Selection differential (S)

Answer 7

A measure of the strength of selection, the difference between he mean trait value of selected (successfully reproducing) individuals and the mean trait value of the entire population

Question 8

Heritability = 0

Answer 8

No genetic variation but genes are still involved and expressed

Question 9

Heritability = 1

Answer 9

Lots of genetic variation

Question 10

Estimating heritability of continuous traits from measurements within families

Answer 10

Slope of offspring height vs parent height show heritability

Question 11

Measuring S

Answer 11

Mean trait of successful parents - mean trait of entire population

Question 12

Predict response to selection with S and h2

Answer 12

R = h2S

Question 13

When R is less than S

Answer 13

Heritability is closer to 0 since heritability is equal to R/S

Question 14

When R is greater than S

Answer 14

Heritability = 1 since R = S means that R/S is 1

Question 15

Heritability and affect affect on response to selection

Answer 15

Low - R is smaller

High - R is larger

Question 16

S and affect on R

Answer 16

Low - smaller

High - larger

Question 17

Traits associated with fitness and the level of heritability

Answer 17

Traits that are important for fitness, under strong selection, should have little genetic variation and low levels of heritability (continually under selection)

Question 18

Fisher’s fundamental theorem of natural selection

Answer 18

Rate of change in the trait (and I’m the average fitness) is proportional to the amount of genetic variation, must be enough genetic variation for the population to respond to selection, traits closely related to fitness and continually under selection may not have much heritability remaining

Question 19

Explanations why genetic variation in traits is related to fitness in natural populations

Answer 19

1. Disruptive selection, heterozygote superiority and frequency-dependent selection can maintain genetic variation
2. Beneficial mutations have not gone to fixation yet (mutations or environment may be new)
3. Selection has not eliminate mildly deleterious mutations (mutation-selection balance, migration)