Response to Selection

Bio 508: EvolutionRobert Page

Slides Courtesy of Dr. Voss

h2 = VA/Vp

Describes the extent to which offspring resemble theirparents

Proportion of total phenotypic variance attributable toadditive genetic variance

Heritability

Resemblance between parents and offspring indicates degree to which a trait is inherited

heritability (narrow sense):

h2 = VA

VP

h2 = bOP

Drosophila wing length

Offs

prin

g av

erag

e (O

)

bOP parental average (P)

Detecting the genetic componentof phenotypic variation

Fraction of variation in parents that is explained by variation in their genes (VA).

19761978 h2 = 0.9

Bill depthin the

Ground Finch

Almost all characteristicsin almost all species aregenetically variable tosome extent.

Heritability is often measured in the lab;estimates are higherthan would be expected in nature.

Detecting the genetic componentof phenotypic variation

Genetically variable characters can be altered by selection.

The response to selection is proportional to the amount of genetic variation in the character.

Truncation Selection

S = selection differential

Breed only these 16%

Standard deviation units

68%

14% 14%

nonselected

selected

Response to selection when bOP = 1

Note standard deviation ( = 2 cm

16% > 1

Note standard deviation ( = 1 cm

Response to selection for a less variable population

Response to selection when when bOP < 1

2% > 2

Response to selection under a more intense selection program

Summary of Graphs

Response of a quantitative trait to selection depends on:

1. the relationship between fitness and phenotype2. the phenotypic variance3. the degree to which the trait is heritable

R = h2 S

Selection on polygenic characters

R = h2S

generation h2 (low line)1–9 0.5010–25 0.2326–52 0.1053–76 0.15

How do selection response and heritability change over time?

• Long-term selection may eventually exhaust standing additive genetic variation

• Continued response depends on mutational input

Response to Selection for Increased Bristle #

316% increase in phenotype !

Relationship among heritability, R, and S