Darwin's theory of evolution by natural selection states that evolution occurs through the natural selection of heritable variation. Development plays the key physiological role connecting the heritable genotypes, passed from one generation to the next, to the phenotypes that are made available for selection. While at times the developmental variations underlying a selected trait may be neutral with respect to selection, it is through its effects on heritable variation that developmental tinkering affects evolution. We can gain a deeper understanding of the evolutionary process by considering the role of development in structuring variation and, through its effects on variation, structuring evolution. Both evolutionary theory and empirical studies show that features that interact in development tend to be inherited together and, hence, to evolve together. Gene mapping studies show that this modular inheritance pattern is due to modular pleiotropic gene effects, individual genes affecting a single modular unit, and that there is heritable variation in the range of features encompassed by these modules. We hypothesize that modular pleiotropic patterns are sculpted by natural selection so that functionally-and developmentally-related traits are affected by module-specific genes.