Libertarian science writer Ron Bailey points out that Ron Paul has denied the theory of evolution:

In a South Carolina forum, Paul was asked about his views on evolution, to which he replied, "I think it's a theory, the theory of evolution and I don't accept it as a theory." He also said that he thought it was an inappropriate question to be asking presidential candidates.

I don't believe that scientific illiteracy on this issue is an absolute bar for supporting a presidential candidate. However, for reasons that Bailey explains in his article, it's definitely a negative. Worse, Paul's position on evolution is of a piece with other indicators that he's less a libertarian than a far-right populist. It is consistent with his penchant for right-wing conspiracy theories, such as the supposed plan to form a "North American Union," his opposition to free trade agreements on the ground that they undermine "sovereignty," and his nativist (and highly unlibertarian) position on immigration. Unfortunately, it's also consistent with his having published far-right racist, anti-Semitic, and conspiracy-mongering articles in his political newsletters in the late 1980s and early 90s. Even if Paul didn't endorse their content, he clearly was willing to associate with the sorts of people who believe these things and didn't mind letting them take control of the content of his publications.

Ron Paul isn't all bad. However, it is increasingly clear that association with his presidential candidacy does more harm than good to the cause of libertarianism, a point that I tried to make in my very first post about him. Not only is his candidacy turning out to be a flop politically, as I predicted. It also creates the risk of tarring libertarianism by associating it in the public mind with bigotry, conspiracy-mongering, and xenophobic hostility to free trade and immigration (though the latter, unfortunately, is actually quite popular even outside far-right circles).