This is a staggering and incredibly frustrating statistic for me. It comes from a survey carried out by LifeWay research, sampling 1,000 ministers "from randomly selected Protestant churches." A few other stats coming out of this phone survey can be found on CNN's Belief Blog here.
If you don't understand why such results would upset me, you probably have not read much on the wardrobe before. I just can't seem to get away from this topic. If you're interested in reading a bit on evolution, I'd point you towards two older posts on this blog (one of which will point, in turn, to the blog of Rachel Held Evans): happy birthday Charles Darwin! and another post on Evolution.