What's surprising in the new Pew evolution numbers is the trend—a more than 10-point drop in belief among Republicans. What explains it?

Pew doesn't speculate but remarks on the confusing result:

Differences in the racial and ethnic composition of Democrats and Republicans or differences in their levels of religious commitment do not wholly explain partisan differences in beliefs about evolution. Indeed, the partisan differences remain even when taking these other characteristics into account.

One possibility is that respondents who identified as Republican and believed in evolution in 2009 are no longer identifying as Republicans. Fewer scientists, for example, are reportedly identifying with the GOP, and the overall trend is for fewer Americans to call themselves Republicans. But both Gallup and separate polling from Pew found approximately the same party ID in 2009 and 2013.

Another is that the rise of "intelligent design" education has helped to swing younger Americans against evolution. Yet the age breakdown remains similar in 2009 and 2013, with respondents ages 18 to 29 most likely to believe in evolution.

What does that leave? Maybe the gap represents an emotional response by Republicans to being out of power. Among others, Chris Mooney has argued that beliefs on politically contentious topics are often more rooted in opposition to perceived attacks than anything else—an instance of "motivated reasoning." Given that Democrats have controlled the White House and Senate since 2009, this could be backlash to the political climate, though it will be hard to tell until Republicans control Washington again.

Of course, motivated reasoning might help explain why many Democrats also believe in evolution.