In a recent article, Martin Fowler is trying to explore the applicability of evolutionary design - a practice commonly used in Extreme Programming (XP) - to SOA implementations. He starts by discussing two common design paradigms, planned and evolutionary:

Planned design works out the design of software in one phase and builds (programs) it afterwards. In this case changing the design is hard once you've begun construction. Evolutionary design assumes regular change of the design even once you've done significant programming. I concluded that the practices of XP provided a disciplined approach to evolutionary design, thus making it much more practical than people had realized. This change did not remove software design (it is not dead) but did significantly change how we think about design.