I wanted to share photos of my pink- and yellow-flowered S. rubra wherryi clones, along with a red-flowered clone from the same location (Washington County, Alabama) for comparison. This is the first time I've had all three flowering together. Also including a photo of the variant plants.
Although the plants look like pure S. rubra wherryi, the flower scapes of the variants are on average several inches taller than on the red-flowered clones, and S. alata does grow within pollination range. My hypothesis is that many generations ago, a hybrid was created that subsequently crossed only with the rubra parent, to the point where the only expression of the hybrid parentage is in the flower color.
I think that this also explains the red-flowered S. flava from Bay County, Florida that some of us have in cultivation (originally from Meadowview BRS) as well as the pink-flowered S. alata I have seen in DeSoto National Forest in Stone County, Mississippi. I have seen late-flowering alatas overlap with very early-flowering S. psittacina. The psittacina are comparatively rare in these immense fields of S. alata, so it would make sense that any hybrid would tend to breed with the alatas in subsequent generations.