intelligent design, theory that some complex biological structures and other aspects of nature show evidence of having been designed by an intelligence. Such biological structures are said to have intricate components that are so highly interdependent and so essential to a particular function or process that the structures could not have developed through Darwinian evolution, and therefore must have been created or somehow guided in their development. Although intelligent design is distinguished from creationism by not relying on the biblical account of creation, it is compatible with a belief in God and is often explicitly linked with such a belief. Also, unlike creationists, its proponents do not challenge the idea that the earth is billions of years old and that life on earth has evolved to some degree. The theory does, however, necessarily reject standard science's reliance on explaining the natural world only through undirected natural causes, believing that any theory that relies on such causes alone is incapable of explaining how all biological structures and processes arose. Thus, despite claims by members of the intelligent-design movement that it is a scientific research program, the work of its adherents has been criticized as unscientific and speculative for inferring a pre-existing intelligence to explain the development of biological structures instead of attempting to develop adequate falsifiable mechanistic explanations. In addition, the theory has been attacked on the grounds that many aspects of nature fail to show any evidence of intelligent design, such as “junk” DNA (see nucleic acid) and the vestigial webbed feet of the frigate bird (which never lands on water).