The majority of variation segregating in natural populations is quantitative, and its expression depends on genetic background, environment, and interactions with these two factors. Traditionally, the evolution of quantitative traits has been described using statistical genetic techniques. However, one of the greatest advantages of these approaches is also one of their primary limitations: it is possible to estimate genetic variation and covariation in traits without any direct knowledge of the underlying loci or molecular genetic details. In like fashion, it is possible to estimate the pattern of natural selection on quantitative traits in the absence of knowledge of loci that determine fitness.
Advances in collecting and analyzing molecular data promise to reveal the molecular genetic basis of quantitative trait variation. Identifying the molecular elements responsible for variation in quantitative traits may provide insight into basic evolutionary and genetic processes, including the minimum number and effect of genes that contribute to variation in quantitative traits, how the expression of variation at specific loci varies across environments, whether selection at specific loci differs across environments, whether pleiotropy or linkage disequilibrium underlie genetic correlations and potential evolutionary constraints, as well as the genetic basis of similarity (or dissimilarity) of trait expression among congeners.