Species

The species is the smallest taxonomic group and the basic unit of classification. Members of a species share many similar features, including similar DNA, and they are more closely related to each other than to members of any other species.The most widely used definition is that a species is a group of organisms that can interbreed to produce fertile offspring. For example, a horse and a donkey can mate, but their offspring, the mule, is infertile (sterile). Horses and donkeys are therefore separate species.All the members of one species living in the same area at the same time form a population. A species may be made up of many populations in different places.Deciding what counts as a species matters because:
• counting species is how biologists measure species richness and biodiversity
• conservation laws and protection are based on species
• clear species identification is needed in medicine, agriculture and pest control.However, defining a species is surprisingly difficult. The interbreeding definition cannot be tested for fossils or for organisms that reproduce asexually, and some organisms of the same species look very different. Because no single definition works for all organisms, biologists use several species concepts, each based on different evidence: physical features, the ability to interbreed, or evolutionary history and DNA. These concepts are covered in the following lessons.