Until you start to use evolution. What a species is, begins to blur as soon as you try to establish evolutionary lines. When is a whale not a whale but just a water enthusiast mammal? somewhere between 50 and 35 million years ago. Exactly when, it’s anyone’s guess. Taxonomy is indeed part of biology, though.
What a species is, begins to blur as soon as you try to establish evolutionary lines.
It doesn’t because “species” is definied as an animal that can have fertile offspring with other members of it’s species. Looking at evolution doesn’t change that definition, it just shows that it’s not a very good definition on an evolutionary timescale. Our concept of species in taxonomy only makes sense within small timeframes.
When is a whale not a whale but just a water enthusiast mammal?
First we have to establish what you mean by “whale” and translate that to the proper order/clade. Then you look at what was the first described fossile in the group is. And that’s your answer. And yes, that answer will change with new fossil discoveries or reclassifications based on other information happen. But as long as you keep up to date with them, the current way we use taxonomy gives quite binary definitions of the majority of lifeforms.
Taxonomy is indeed part of biology, though.
It sure is. But it’s just an arbitrary classification system within the greater field. It is like an “index”, so you can look up what information belongs to the thing you’re looking at. But it doesn’t actually hold much information about biology of the thing itself.
Species actually don’t have a rigid definition that works across all organisms. The most common definition is the one you gave but sometimes it simply doesn’t work, for example any organism the doesn’t use sexual reproduction doesn’t fit this definition. Clarification of extinct populations would also be an issue. Even considering organisms this is usually used with, there are exceptions. For example; domesticated cattle and American bison, coyotes and wolves, and most cat breeds with various wild species.
But it doesn’t actually hold much information about biology of the thing itself.
What do you mean “biology of the thing itself”? Are you talking about morphology which is a different part of biology. And taxonomic trees are often made based on morphological features so there is a connection.
Taxonomically speaking, the first whale was the last common ancestor of all (modern) whales, whether this was a land dweller or already aquatic isn’t important from a taxonomic point of view
Except you’re still at odds with what a “species” even is because you’ll have a bunch of fossils that exist over several million years as one “species” that definitely looks different at the beginning than it did at the end because evolution is such a gradual process that there never really is a clean break between species.
You are aware that whale isn’t a single species, are you? I’m not commenting on how blurry the species definition is, I’m aware of that. I’m commenting on the question about the first whale