It extends far further than that. There are cases of interspecies breeding in mammals (A + B = C). What species are the offspring (C + C = C?)? What if such offspring continue to breed? Sometimes, the offspring are not sterile! Do they become a new species. What if, over a few generations, you get a healthy mix of (CCBA + ACBC = ????)?
See https://en.wikipedia.org/wiki/Polar_bear#Taxonomy_and_evolut...
The idea of categorizing things is a convenience that our minds and our society employ to keep things simple. It works in a large number of circumstances. However, it is important to realize that these categories are not 100% rigid and that situations arise that are not simple to categorize. The result is sometimes broadening the definition of a particular category, creating a new category, or ignoring the uncomfortable situation. In reality, it is important to acknowledge that reality does not answer to our whims and needs to categorize things and just accept that clear lines can't always be drawn.
Re:Against the naming of fungi (2013)
by grawprog on Wednesday October 31, 2018 @09:15AM
(#18345359)
Then there's also species with distinct morphological populations within the se habitat.
http://www.zoology.ubc.ca/~schluter/stickleback/stickleback_...
Wow, I knew there were similar problems for prokaryotes (esp. w/ horizontal gene transfer), but didn't know it extended to fungi and never quite thought about it that way.