How are living things classified according to species?

Answer 1

AnswersThere are multiple answers to the question of what is a species. One of the lessons of evolution is that the idea of species is transient. No population of organisms can be said to truly be the same species as it's ancestors. It's important to note that the idea of a species is a human concept. Nature doesn't play that nice.

One of the more common definitions and easiest to explain is the biological definition. This basically states that a species is "a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature." In other words, a group that can breed with each other. (This definition has obvious failings with asexual organisms.)

Some of the challenges to species definitions come from organisms that form "ring species" which, even if forming an incomplete ring, still challenge how we think of a species.

Other definitions define a species based on various similarities, either in form, behaviour, or biochemistry.