Three wild Penstemon hybrids are documented herein: Penstemon clevelandii × spectabilis, P. centranthifolius × spectabilis, and P. centranthifolius × eatonii. Each putative hybrid was compared to its respective parental species using manifold floral and vegetative characters for which the parents differed. For each hybrid hypothesis, characters were counted as being intermediate or not intermediate. There were significantly more intermediate characters in all 3 cases (one-sided sign tests, P < 0.05). These Penstemon species evidently do hybridize naturally, and for nearly all characters the hybrids are consistent with the view that the parents differ largely by additive genetic effects.
Wilson, Paul and Valenzuela, Michael
"Three naturally occurring Penstemon hybrids,"
Western North American Naturalist: Vol. 62
, Article 3.
Available at: https://scholarsarchive.byu.edu/wnan/vol62/iss1/3