An educational experience 4.6 billion years in the making,” says the clever tagline for WGBH’s big September series Evolution. The way the Boston producers have been preparing for the reaction from creationists, you’d think they expect the controversy surrounding it will last almost that long. “Evolution is two steps away from abortion,” said Anne Zeiser, director of national strategic marketing for WGBH, placing the flash point at which evolution ignites fundamentalist outrage. Recent polls show that 45 percent of Americans say they believe in creationism, and many Christians view Charles Darwin’s theory of natural selection as an assault on their faith. Evolution, a seven-part, eight-hour co-production of WGBH’s Nova science unit and billionaire Paul G. Allen’s Clear Blue Sky Productions, will kick off the PBS fall season Sept.