The data given on p. 643 only shows a correlation for sure that bilateral flower shape is connected with increased speciation— some other feature connected to bilateral shape may be the actual driver of evolution. However, since this was found over and over in different groups, it is likely that bilateral flower shape does indeed drive speciation.
Work Step by Step
Keep in mind that without a proven mechanism (the text on p. 642 only suggests one), only a correlation can be proven. Data would be needed which directly showed many cases of bilateral flower evolution causing speciation.