"Irreducible complexity" began as a battle cry of opponents to Darwinism, but following the Darwinists' victory in those battles the Darwinists use the phrase to stand for their ability to resist that particular challenge. Now, merely by trumpeting "irreducible complexity" Darwinists beat back any reference to it. The challenge involves one of Darwinism's greatest implausibilities, that throughout the emergence of a complex organ or capability each of the community of genes needed to code for it had to be beneficial enough by itself to survive natural selection, independent of the benefit it would eventually contribute as part of the community of genes.
I usually decline to associate myself with this challenge, but examination of a dinosaur skull at the American Museum of Natural History shattered my resolve. Right at the center of the underside of that colossal structure was a perfect sphere inches across, the ball of the ball and socket joint on which this colossal skull would have balanced. Not only was it as big as that taking the weight of a truck, it was supported by just the ribs of "bone" you'd add to reinforce it. The resemblance to brute Victorian engineering was eerie. It seemed implausible to me that such a structure could be specified by the selection individually of genes more suited to coding for proteins. Bear in mind, that ball, as well as the corresponding socket, had to grow from an infant-appropriate size to the adult size without deforming while bearing the collossal weight of the massive skull.
The issue at stake is not merely whether Darwinism is true, it's also that having to anticipate implausibilities like this in our account of our own evolution makes it hard for us to draw any conclusions from that account about our nature. At some point I think we're entitled to insist Darwinists fashion their theory into an account that illuminates rather than confounding. Implausibility should not be a source of pride, as it seems to have become for Darwinists.