Three observations are readily apparent:
First off, Republicans are simply systematically less likely to believe in evolution than Democrats or Independents. And according to Pew, neither the religious nor ethnic makeup of the Republican electorate can fully explain away this difference.
Second, there is almost no difference between Democrats and Independents: in both categories about two-thirds of respondents express a belief in evolution.
Finally, while the beliefs of Democrats and Independents have been relatively stable over the past four years, Republican belief in evolution has fallen by almost one-fifth: while 54 percent of Republicans expressed a belief in evolution in 2009, only 43 percent did so in 2013.
Now there is more than one possible explanations for this final observation. It may be the case that the underlying belief in evolution among Republicans has not really changed sine 2009, but Republican’s willingness to admit to believing (or not believing) in evolution to an interviewer may have changed in the intervening period. Conversely, it is possible that large numbers of Republicans have actually stopped believing in evolution in the past four years. Either trend would be interesting, but would have different implications for how we see the evolution of the Republican electorate.