Heritability h2 = VA/Vp Proportion of total phenotypic variance attributed to variation in breeding values. Expresses the extent to which genes are transmitted from parents to offspring

Detecting the genetic component of phenotypic variation Resemblance between parents and offspring indicates degree to which a trait is inherited Drosophila wing length offspring (O) bOP parental average (P) heritability (narrow sense): h2 = VA VP h2 = bOP Fraction of variation in parents that is explained by variation in their genes (VA). • heritability can also be estimated with offspring–midparent regression

Bill depth in the Ground Finch 1976 h2 = 0.9 1978

Heritability is often measured in the lab; estimates are higher than would be expected in nature. Almost all characteristics in almost all species are genetically variable to some extent.

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