Proponents of Intelligent Design have exploited a vexing question at the heart of Darwin’s theory. Now, say two leading biologists, scientists can — and must — answer back.

In a new book, ”The Plausibility of Life,” published this week by Yale University Press, Marc Kirschner, founding chair of the Department of Systems Biology at Harvard Medical School, and John Gerhart, a biologist at the University of California, Berkeley, discuss the origins of complicated biological features and present their theory of “facilitated variation,” which they believe addresses a major question in evolution: How can small genetic changes develop into complex, useful body parts?

In a sign of the times, they also rebut claims of irreducible complexity made by Intelligent Design advocates.