Unlike zoologists, botanists aren't quite as hung up on whether "good" species (i.e. "real" species) need to be completely reproductively isolated from each other. In other words, two groups of organisms can be valid species based on genetics, or ecological factors, and yet (oops, I had this wrong in the previous draft) hybridize to produce fertile offspring. Often the hybridization comes as a secondary thing, and more often these days, from human disturbance that makes some sort of bridge that brings formerly isolated species together.
When I bought my house in 2005, the sellers had put in a nice little front garden, and landscaped it with xeriscaping species. The plants have largely performed as advertised, I don't water it much, and it looks nice. They planted three kinds of Penstemon, and those have really done well. So much so that I have my own hybrid zone, where parental species, as well as intermediates can be seen.
My camera, sadly, is at my brother's house in MN, so I can't take a picture, but these pictures, from the fabulous Wayne's Word website give you the idea. Notice how F1s (that's "first filial generation, or the first hybrid offspring of pure parents) are a nice 50:50 split. It's not always that clean, but gives bumbling scientists a clue that hybridization is occurring. Then, as hybrids cross with hybrids, and hybrids backcross to parents ("BCs"), you can get a wide variety of forms, and presumably different abilities to adapt to environmental conditions.
How do these biological mix ups occur? Well, you can blame it on the pollinators, who work a patch of flowers and end up transferring pollen between species. That's exactly how it looks in my garden, where there are plants with blue flowers, and pink flowers (who, of course aren't going to change, as they are perennials), and then the intermediate flowers have combinations of blue and pink. Maybe I'll get a cool new Penstemon variety I can propagate.
Happy Father's Day to all the dads out there!!