The biological species concept is the most commonly used and acceptable definition of species, which explains that a group of organisms can be referred to a species if members of the group can interbreed successfully to produce viable offspring that are also fertile. It explains that there tend to be reproduction barriers that would prevent different species of organism from successfully interbreeding. Also, that two organisms have the same appearance, do not necessarily mean they belong to the same species. For example, a horse and a donkey were interbreed to produce an offspring called the mule, but, the mule was found to be infertile. A horse and a donkey are different species, even though they have close appearance.
