Detecting the genetic component of 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 Breed only these 16% 68% 14% 14% Standard deviation units S = selection differential

Response to selection when bOP = 1 selected nonselected 16% > 1 Note standard deviation ( = 2 cm

Response to selection for a less variable population Note standard deviation ( = 1 cm

Response to selection when when bOP < 1

Response to selection under a more intense selection program 2% > 2

Summary of Graphs Response of a quantitative trait to selection depends on: the relationship between fitness and phenotype the phenotypic variance the degree to which the trait is heritable R = h2 S

Selection on polygenic characters How do selection response and heritability change over time? R = h2S generation h2 (low line) 1–9 0.50 10–25 0.23 26–52 0.10 53–76 0.15 • 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

The slope of the best fit line is the selection gradient. N = 30 mice (0 or 1) Absolute fitness Mean fitness (10/30 = 0.33) [(20 x 0) + (10 x 1)] / 30 = 0.33 The slope of the best fit line is the selection gradient.

Ground finches and seeds during the drought of 1977

Heritability of beak depth in Geospiza fortis.

Finches before and after the drought

Selection on multiple traits and correlated characters 3-Dimensional Selection Gradient Drought of 76-77 Darwin’s finches narrow wide shallow deep

Two-Dimensional Graphs

Selection can only move in direction shown by this arrow. most fit 2-character phenotype, but can’t be achieved because characters are correlated