A recent article in the New Yorker includes a good overview of why identifying species can be problematic:
"You Name It: Carl Linnaeus and the effort to label all of life" by Kathryn Schulz, August 21, 2023
Extended quote from article:
"What Linnaeus sought to do was organize nature according to its fundamental, intrinsic divisions--to carve it at the joints, in Plato's famous formulation. But what he actually did, for the most part, was impose artificial categories on the natural world for the convenience of scientists.
This is not a retroactive assessment; Linnaeus himself knew full well the limitations of his classification method. To achieve a system completely in accordance with nature was, he wrote, "the first and last wish of botanists." But the more closely you looked at her bounty the more difficult that prospect became--so, in the meantime, "artificial systems are absolutely necessary."
In philosophy, this tension between intrinsic and imposed categories takes the form of a debate between nominalism and realism. Realists believe that nature is full of real and discrete categories, from 'amphibian" to "zinc," and that the job of the scientist is to discern them accurately. Nominalists believe that nature lacks clearly defined categories, and that we simply impose those distinctions upon it--creating, as it were, the illusion of joints where none really exist.
This is not just the position of post-truth relativists. "I look at the term 'species' as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other": that is Charles Darwin, in the second chapter of "On the Origin of Species." That book, of course, trumpeted to the world a very large problem with the entire notion of a species. According to evolutionary theory, species are constantly changing--emerging, diverging, going extinct.
The very concept of a species is in radical flux, too, with more than twenty competing definitions in circulation. Choosing a definition is not just a matter of what goes in the dictionary under "species"; which one you use will determine how you divide up nature, such that a group of creatures that would be regarded as a species by one standard might not merit the label by another.
All this confusion comes, as Darwin wrote, "from trying to define the undefinable." Yet committed realists continue to promulgate more and more definitions, in the belief that one of them will map perfectly onto some intrinsic and stable feature of nature. Darwin called that idea "laughable," a word that captures the impossibility but not the gravity of arbitrarily imposing categories on living beings.
