Here's why:
* Species: A species is defined as a group of organisms that can interbreed and produce fertile offspring. This biological definition makes it the most natural and distinct grouping. While there are some exceptions and debates, it generally holds true.
* Other categories: Higher categories like genus, family, order, class, phylum, and kingdom are human-created classifications based on shared characteristics. These categories are useful for understanding relationships and organizing the diversity of life, but they don't necessarily reflect discrete units in nature.
For example, the distinction between a dog and a wolf is clear because they cannot interbreed and produce fertile offspring. However, the distinction between a mammal and a reptile is more arbitrary, based on shared characteristics that are not necessarily absolute.
