Neutralists agree that selection is the only adaptive mechanism we know. But they argue that adaptive change is rare, at the molecular level, relative to neutral drift. In short, neutralists view molecular variation as follows Most mutations are deleterious. Advantageous mutations occur, and are responsible for adaptation, but they are so rare compared to neutral ones that they have little effect on the overall patterns. The cost of selection argument and selectionist response The attraction of the neutral theory is that neutral drift does not require extra genetic deaths For substitutions, which we see as differences between species.
Substitutions occur by drift at rate u neutral .
Again that could accommodate the unexpectedly high levels of variation observed, though our estimates of N and u neutral are very crude, so it's hard to say. There are possible answers to this dilemma. See Ridley 7.7.1. Look for evidence of selection, or for evidence of design that would imply selection. Already had a couple examples.