Here's Josh's latest from his blog Traveling from Kansas [The Panglossian Paradigm, or as science moves forward, creationists move back]. Note that the opinions on his blog do not necessarily reflect those of NCSE.
For really confused students, I draw on a point Stephen Jay Gould made in Eight Little Piggies (in the essay by the same name), that the number of fingers we have is entirely contingent on history. While one can try to construct an explanation for the superiority of 5 fingers, paleontological history shows that there were potential ancestors of the tetrapod clade (which we are part of) which had as many as eight rays per fin. If they had succeeded, 8 fingers would be the norm, and the Simpsons would look very odd with only 4. As Gould says of historical contingency: "Other configurations would have worked and might have evolved, but they didn't--and five works well enough."There's lot more where that came from so get on over to Travelling from Kansas for more information on the "correct" worldview.
In the essay, Gould is building on a point he made most forcefully in an essay he wrote with Richard Lewontin, "The Spandrels of San Marcos and the Panglossian Paradigm: A Critique of the Adaptationist Program." The point was that biologists were too quick to insist that every feature was adaptive and a result of natural selection. Spandrels are triangular structures produced when two round arches meet. They are necessary byproducts of joining rounded and flat surfaces. Nonetheless, in many churches they are richly decorated and the entire artistic vision for a space can be shaped by the spandrels. One might, Gould points out, be lead to think that the spandrels are there in order to be used for paintings, and not that they are necessary by-products nicely dressed up. The worldview he criticizes treats anything, whether spandrels or five fingers, as the product of intense selection, a perfect solution to the problems it faces.
By coincidence, today's Scientific American question is Why do most species have five digits on their hands and feet?. While there's a bit of catering to an adaptationist perspective the answer to the question is ...
Is there really any good evidence that five, rather than, say, four or six, digits was biomechanically preferable for the common ancestor of modern tetrapods? The answer has to be "No," in part because a whole range of tetrapods have reduced their numbers of digits further still. In addition, we lack any six-digit examples to investigate. This leads to the second part of the answer, which is to note that although digit numbers can be reduced, they very rarely increase. In a general sense this trait reflects the developmental-evolutionary rule that it is easier to lose something than it is to regain it. Even so, given the immensity of evolutionary time and the extraordinary variety of vertebrate bodies, the striking absence of truly six-digit limbs in today's fauna highlights some sort of constraint.Remember the take-home lesson (mostly from Josh's article). Living organisms are not well designed in spite of what the creationists and the adaptationists would have you believe.