An individual’s phenotype—the set of its visible traits—is determined by a combination of its genotype and environmental factors.
Variation in quantitative traits can be caused by just a few or by a very large number of loci. When variation results from many genes, the trait can evolve far past its original range of variation by changes in allele frequencies, without contributions from new mutations.
A fitness function shows the relation between the value of a trait and the average fitness that individuals with that value have. A fitness function can result in selection that is directional (favoring an increase or decrease of a trait’s mean), stabilizing (selection against extreme individuals, which decreases variation in the population), or disruptive (selection against intermediate individuals, which increases variation).
The force of directional selection on a trait is measured by the selection gradient, which is slope of the regression line that relates relative fitness to the trait value. The selection gradient can be used to predict the rate at which a trait will evolve and to test hypoth-eses about adaptation.
The rate at which the mean value of a trait will evolve is given by the breeder’s equation, and it depends on the amount of genetic variation (measured either by the additive genetic variance or the heritability) and the strength of directional selection.
Almost all quantitative traits have standing genetic variation and will evolve when selection acts on them. When selection acts on traits that do not have heritable variation, new mutations must arise before the trait will evolve.
Artificial selection has been essential to civilization. Selective breeding has caused many species of domesticated animals and plants to evolve dramatically new forms, very different from those of their wild ancestors. The results of artificial selection demon-strate that selection can produce very large changes in relatively short periods of time. Natural selection can do the same in natural populations.
Genetic covariance (or correlation) between traits causes evolutionary side effects: selection on one trait will cause others to evolve. This can result in trade-offs and constraints, in which adaptation in one trait has negative fitness effects on other traits. Genetic cor-relations result from pleiotropy and linkage disequilibrium.
Some traits show phenotypic plasticity, the situation in which the phenotype produced by a genotype is altered by the environment that an individual experiences. Plasticity of some traits has evolved adaptively, but in other cases the response to the environment is not adaptive.
Genetic variation in quantitative traits can be caused by a small or a large number of quantitative trait loci (QTL). These chromo-some regions can be localized by QTL mapping, in which variation at genetic markers is correlated with a trait’s phenotypic value.
The number and types of loci that contribute to additive genetic variation within populations may often be quite different than those involved in adaptive differences among species.