How many processes are responsible for phenotypic evolution?

In addressing phenotypic evolution, this article reconsiders natural select ion, random drift, developmental constraints, and internal selection in the new extended context of evolutionary developmental biology. The change of perspective from the "evolution of phenotypes" toward an "evolution of onto genies" (evo-devo perspective) affects the reciprocal relationships among t hese different processes. Random drift and natural selection are sibling pr ocesses: two forms of post-productional sorting among alternative developme ntal trajectories, the former random, the latter nonrandom. Developmental c onstraint is a compound concept; it contains even some forms of natural ("e xternal" and "internal") selection. A narrower definition ("reproductive co nstraints") is proposed. Internal selection is not a selection caused by an internal agent. It is a form of environment-independent selection dependin g on the level of the organism's internal developmental or functional coord ination. Selection and constraints are the main deterministic processes in phenotypic evolution but they are not opposing forces. Indeed, they are con tinuously interacting processes of evolutionary change, but with different roles that should not be confused.