There is no empirical evidence of supernatural design in biological systems. There is no adequate way of testing the hypothesis that such design exists.
The thing people have trouble with in evaluating this situation is that one does see "design" of some sort in biology. My hands are flexible and dextrous enough to type, for instance, and that seems like design.
This argument is essentially what William Paley advanced in his "Natural Theology" an argument which Darwin was familiar with (Darwin cites Paley exactly once, on an unrelated point). Darwin's insight was that small changes added together and recombined in a population could lead to structures which perform certain functions very well, well enough that they appear "designed" for the purpose.
Nonetheless, Billy Dembski, Phil Johnson, Mikey Behe and their gang of miscreants revived Paley's half-assed argument and are trying to force their "natural theology" into science classes.
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