British biologist Richard Dawkins has sold a follow-up to his incendiary 2006 book The God Delusion to Hilary Redmon at Free Press. According to one source with knowledge of the deal, Mr. Dawkins' literary agent John Brockman told some underbidders at the end of the auction that Free Press, an imprint of Simon & Schuster, is paying $3.5 million for the book.

According to this source, the book—to be called Only a Theory?—will offer empirical evidence for evolution, and will be published next year, on the 150th anniversary of the publication of Charles Darwin's On the Origin of Species.

The God Delusion, which has sold over a million copies and inspired several book-length counterarguments, was published by Houghton Mifflin.

Free Press publisher Martha Levin said she and her staff were "thrilled" about the acquisition.

Ms. Redmon and Mr. Brockman did not return calls seeking comment.