This is an oldish argument, inspired by a student comment, but I kind of like my present formulation of it. Start with this fine inductive argument:
- All the known explanations of present species are evolutionary.
- So, all the explanations of present species are evolutionary.
- All the present species have evolutionary explanations.
We could derive (3) from (2) if we had the further thesis:
- All the present species have explanations.
We could get scientific support for (4) as follows. Randomly samples species, and make sure that we can find an explanation for each randomly sampled species. But we haven't done this.
Yet, I say, we know (3) to be true, and hence we know (4) to be true as well, since (3) obviously entails (4) and this doesn't look like one of the exceptional cases that are counterexamples to closure.
But how do we know (4) to be true? I think a priori. And the best non-ad hoc story about that is that we derive it from the Principle of Sufficient Reason.