Here's a breakdown:
* Reproduce naturally: They can mate and produce offspring without human intervention.
* Create fertile offspring: The offspring must be able to reproduce as well.
For example, dogs are a species because they can mate and produce puppies that can also reproduce. However, a horse and a donkey cannot be considered the same species because they can mate and produce a mule, but mules are sterile.
It's important to note that this definition is not perfect and has limitations:
* Asexual reproduction: Species that reproduce asexually, like bacteria, don't fit this definition.
* Extinct species: We can't observe their reproductive capabilities.
* Hybridization: Some species can interbreed and produce fertile offspring, blurring the lines between them.
Other species concepts exist, like the morphological species concept (based on physical appearance) and the phylogenetic species concept (based on evolutionary history). However, the biological species concept remains the most widely used and accepted.
