It was one of the most-discussed stories for a while and spurred quite a range of commentary, much of it centering on why this was, as the topic didn’t inherently seem like the kind of opinion prone to much fluctuation. (Though, of course, it just may be.)
One might have wondered if it was partly that party identification was really moving – that is, that people who did not accept evolution were becoming more likely to join the GOP while those who believed in evolution were leaving the party, hence the composition of the party was what was moving. Some reactions were that this stance was becoming more and more publicly identified with membership in the party, and increasingly the “right” answer for a loyal Republican to give, at least to a pollster.
Helpfully, Pew has dug a little deeper into its numbers to explore these ideas – posted on their FactTank blog. As Cary Funk of Pew writes, there may be any number of factors at work both in attitude across the public, and in measurement.