Evolution and the Principle of Sufficient Reason

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:

1. All the known explanations of present species are evolutionary.
2. So, all the explanations of present species are evolutionary.

But (2) isn't all that the evolutionary biologist claims about present species. She claims more strongly:

3. All the present species have evolutionary explanations.

Claim (3) is stronger than claim (2). Let's suppose that half of the present species have evolutionary explanations and the other half have no explanations at all. Then (2) is true, but (3) is not.

We could derive (3) from (2) if we had the further thesis:

4. All the present species have explanations.

Indeed, (3) is equivalent to the conjunction of (2) with (4). But the evidence in (1) does not do anything to support (4). It only supports (2).

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.