Are species natural kinds?

I have always considered that species are natural kinds and organisms merely instances of a general class. However, I recently learned about the Ghiselin-Hull thesis and now I am not so certain. First I had to overcome some initial skepticism about their argument due to what I consider a poor choice of terminology (I am not a fan of using the term individuals as they do). After some reflection I think they are mostly correct and it is inaccurate to consider species as natural kinds. The thought experiment that convinced me is this: if we found a tiger on Mars, with the same stripes, claws, jaws and the same DNA as tigers on Earth, but which had evolved independently by perfect convergent evolution, would we say that the tiger belongs to the same species as an Earth tiger? Most likely not. If tigers were instances of a class the answer would be yes.

Right now my epistemological position can be summarized in the following points.

• Realism is correct. There are structures and processes in the real world (in the territory).
• Those processes might create objects that share similarities.
• Prototype theory is essentially correct; we classify objects into fuzzy categories based on similarities and differences.
• Gold as a category refers to something in the real world. Species also refer to something in the real world. Both are valid categories.
• Gold and species are different types of structures: the first one reflects a static property of the universe; the second one is the result of many processes over time.
• It makes sense to move the discussion beyond natural kinds vs artificial kinds and start talking about Dennett’s Real Patterns. Species might not be natural kinds in the traditional sense but they are Dennett Patterns (same for gold).

Following some recommendations by Gemini, I put a couple of books in my Amazon basket about this topic. One is The Species Problem: A Philosophical Analysis by Richard A. Richards. The second one is Natural Categories and Human Kinds by Muhammad Ali Khalidi. It would be interesting to see if reading them substantially changes my vision of the world.