Here’s the problem with the European Blackberry Complex, which includes R. fruticosa, R. armeniaca, R. bifrons, R. ulmifolia, and others. (Some simplification in my explanation here.)
Most species reproduce sexually. A very common, widely used definition of the species is a group of organisms that breed with one another, producing fertile offspring. Even if they looks a bit different, they’re the same species if they breed together. Consider variation within the species humans, or mallard ducks, or dogs, etc. We can tell where the species boundaries are by who breeds with whom (producing fertile offspring). Within sexually reproducing species, genes get mixed up, so you can have blonds with blue eyes, blonds with brown eyes, brunettes with brown eyes, brunettes with blue eyes, etc.
Some species don’t have sex; they are asexual. Many of these reproduce by bulbs, rhizomes, or other obviously asexual means, but some plants are apomictic, producing seeds without sex. In asexual species, the genes don’t get mixed up, so you might get only blue-eyed blondes and brown-eyed brunettes, no other combinations, for example. Mutations introduce some variation to asexual species. To accommodate that variation, we tend to have broader species concepts for completely asexual species, though not all biologists agree.
Members of the European Blackberry Complex reproduce in a way that really messes with our species concepts. They mostly set seed asexually. What seem to be distinct individuals are actually clones, and a single clone might cover a whole county or more. However, blackberries also have sex. They may have sex even with distantly related members of the complex. The offspring of such crosses can look a bit different from the blackberry clones that were already spreading around the area. Are these clonal populations best thought of as different species or just different genetic individuals? How different do they have to be before we name them? Should every slightly different type be called a species? (Some people would argue yes, which is why you have 2000 named dandelion species in Europe.) If not, how much difference is necessary to name a species? This really messes with biologists minds. We don’t agree. We probably will never agree.
Meanwhile, the blackberries go their merry way, spreading asexually through their stems (canes), asexually through seeds, and occasionally through sexually produced seeds. They do not care about our human need for neat, mutually exclusive names for talking about the kinds of blackberries.
This is why when people disagree about the species concepts of Rubus bifrons, R. armeniacus, and R. ulmifolius, I Don’t Care. Call it what you want. I’ll try to follow the herd when/if there is evidence of agreement about what species names we will arbitrarily apply to these plants that don’t have species concepts the way we wish they would.
