L 6

S, the selection differential, is

the difference caused by selection in mean phenotype

within a generation

R, the selection response, is

the change in the trait mean across

successive generations

If none of the variation in a trait is

genetic (show equation)

No change in mean i.e. R = 0 * S

if all variation in a trait was additive genetic

variation

Then the transmission in the change

In mean would be (equation)

perfect, i.e. R = 1 * S

The breeder’s equation.

R = h2S

S – change in phenotypic mean in

parental generation due to selection

R- Response: change in mean in offspring

generation

h2- Narrow sense heritability,

Mean flowering time before selection: 60 days

Selection due to drought within one generation

moved the mean flowering time to 53 days

h2 = 1/2

R = ?

Predicted change in next generation

R = h2S

= ½ * (53-60)

= -3.5

breeders equation where we can apply the breeder’s equation over many

generations

n = generations

• Rn = n h2 S

Dwarfing of Red Deer on Jersey:

• Evolved from a mean weight of 200kg to 36kg.

• Within in 6,000 years.

• Generation time of red deer = 5 years

Assume h2 = 0.5 and that this is constant.

R (change in the trait mean)= 36kg - 200kg = -164 kg

n= 6,000/5 =1,200 generations

Rn/1,200 = -0.136 kg change across generations

Rn/n = -0.136 = 0.5 x S

-.136/.5

S = -0.27 kg per generation!

S is difference caused by selection in mean phenotype

within a generation