The Evolutionary Explanation of Rationality

I take the following argument (word for word) from Steven Stich’s fantastic book The Fragmentation of Reason:

(1)Evolution is caused by natural selection.
(2)Natural selection will choose the best-designed (i.e., that most fitness-enhancing) system available in the gene pool.
(3)Over evolutionary time, a huge and varied set of options will be available for natural selection to choose among, and this set is very likely to include one or more that closely approximate a theoretical optimum.
(4)Systems produced by evolution can be expected to be about as well designed as it is possible to be.
(5)Our inferential system was produced by evolution.

Stephen Stich, The Fragmentation of Reason (Cambridge: MIT Press, 1990), 63.

For context, Stich is attempting to construct an evolutionary explanation of our belief that the human mind is rational (to whatever degree). He critiques every point, but what is most damaging to the case is that it is no way explains why the mind must be rational. There needs to be some ancillary proposition that states something like “Rationality has a monopoly on survival enhancing behavior.” I’m not positive that is a very helpful way to word it (I’m open to suggestions), but the missing premise needs to include something to the effect of survival-enhancing behavior necessitating rational beliefs. Since any missing premise built around that concept is probably wrong (and open to clear counterexample) there can be no evolutionary explanation of rationality (even if our minds evolved by the mechanisms of contemporary evolutionary theory).