Defining a species of virus is problematic because many biologists do not consider them to be alive. It's the "organism" part of the species definition (any species definiton) that's a problem.
Defining a species of bacteria under the biological species concept is problematic because the BSC defines a species as a collection of organisms that can reproduce only with other members of the same species. Bacteria reproduce asexually, so they're not reproducing WITH anyone. However, there are other problems with the BSC. For example, lions and tigers are clearly separate species, but they can interbreed. So biologists tend to favor other species definitions (like the morphological species concept), which can effectively classify lions and tigers (as well as several other species that are able to interbreed). These species definitions have no trouble classifying bacteria.
Why bacteria and virus don't fall neatly into typical definition of species.?
it's because they are not complete organisms, so as to be classified in the species 'tree'.
Reply:Bacteria are single-celled organisms. They are classified like any other organism (genus, species, etc.)
Viruses are a different matter. Viruses are not classified like living organisms. They do not have a genus, species, etc.
There are many who believe that viruses are living organisms. I do not agree with those people, since a virus cannot reproduce on its own. It uses the host cell's "machinery" to manufacture more viruses. A virus is basically a nasty package of genetic material.
ivy
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