The ideas presented in this paper resulted from research supported by NSF grant BIR-9400642 and the Yale Institute for Biospheric Studies. The finical support is gratefully acknowledged. The author thanks the following persons for stimulating discussions on the subject of this paper: Leo Buss, Maria Blanco, Jim Cheverud, Manfred Laubichler, Mary McKitrick, Bernhard Misof, Christian Pazmandi, Louise Roth, Charaf Seddredine, Andreas Wagner. The author also wants to thank Rudi Raff for making available an unpublished book chapter on modularity, and Jim Cheverud, Mary McKitrick, and Miriam Zelditch for valuable suggestions to improve the manuscript. This is contribution #23 of the Center for Computational Ecology.