Define Modelling the evolution of correlated characters?

Current discoveries in genomics and proteomics have illustrated that the link between genotype and phenotype is much more complex than many had considered. Though, complexity is nothing new to physiological geneticists and evolutionary biologists. Case in point: evolutionary biologists and before them, animal and plant breeders have long predictable that most traits are parts of correlated suites of characters. For example, birth weight and gestational length are often correlated in many species. Selection, whether natural or artificial, that works upon one trait can as well change other traits that are not directly under selection.