The term species literally means “kind” or “appearance”; species are different kinds or types of living organisms. In many cases, species differences are easy to recognize: a horse is clearly a different species than a chicken. Sometimes, however, species differences are not so clear. Some species of Plethodon salamanders are so similar in appearance that they can be distinguished only by looking at their proteins or genes.

The concept of a species has two primary uses in biology. First, a species is a particular type of organism to which a unique name has been given. For effective communication, biologists must use a standard set of names for the organisms that they study, and species names serve that purpose. When a geneticist talks about conducting crosses with Drosophila melanogaster, other biologists immediately understand which organism was used. The second use of the term species is in an evolutionary context: a species is considered an evolutionarily independent group of organisms.