German wikipedia defines a biological species as a group where individuals can reproduce offspring with other members of the group, but not with individuals outside of the group.
First of all, to the best of my knowledge, proper sexual reproduction only happens with Eukaryotes. Then this means that no bacteria ever reproduce offspring with other individuals, and therefore each bacterium is its own species.
But that is a meaningless definition. If each bacterium is its own species, then the categorization into species becomes meaningless.
On top of that, bacteria have "pseudosexual" horizontal gene transfer (HGT) which allows them to exchange genetic material with any other bacterium (if the circumstances are right; if i understand this correctly). So all bacteria are in a single species if you look at it that way.
I understand that bacteria normally don't undergo HGT with all other bacteria because some might only open up at hot temperatures while others only open up in cold temperatures - thus creating a natural barrier. But it is also my understanding that while such barriers exist, they're not permanent and can be overcome in nature (without human intervention) for example due to certain virus infections and similar circumstances.
Long story short:
Wouldn't it make more sense to just consider that the concept of "species" only apples to eukaryotes and not to bacteria at all? Wouldn't that save all of us a headache? Maybe we should consider bacterial species to be less strict that eukaryotic species. Maybe we should describe bacteria by their individual features and give that group a name, instead of expecting that diverging lines of evolution cannot ever come together again.
We can’t even give a solid definition of what life is, let alone how to divide that into species in a reliable and consistent way. Biology is a really messy science and strict definitions just don’t cut it. There are always exceptions and edge cases that violate the rules we come up with. So yes, this applies to all life, not just eukaryotes. Bonus points for those who can name situations where we’ve tried to classify complex things and failed to come up with anything that works perfectly.
Biology is just far too complex for simple rules like that. Whatever appealing definitions you come up with are always rough and inaccurate. If you expect a biology definition to always work in every situation, you’re going to be disappointed. This isn’t the kind of simple matter where simple definitions are possible. Spoiler: there are many.
Indeed. In intro biology they teach species concepts and then the minute you advance to the next level you are slammed with all the flaws and fuzziness inherent in the definitions.
Biology is, frankly, the most difficult science for this very reason. Fuzzy categories are necessary for us to start understanding complexity, but the categories can become a hindrance too when you are really trying to push into new territory.
Treat all classification schemes as provisionally useful, but never mistake them for reality.
Biology is actually heading in a very good direction now that DNA sequencing is cheap, proteomix is being established and so on. Psychologists aren't so lucky with their fuzzy fMRI images and extra fuzzy terms and largely opinion based models. At the moment, the complexity of the human mind is just overwhelming and psychologists are struggling to produce even the simplest qualitative results.
While biology can't really categorize species in a consistent manner, at least we know what the basic building blocks are. In psychology, all you have is a collection of conflicting opinions about what the basic blocs could be. Check back in a 100 years and pretty much everything has been redefined several times. I can imagine it's a bit like what happened with taxonomy once we started sequencing everything.