A species is the basic unit in classification and in the study of evolution.
In practice a species is recognized and defined by its phenetic appearance. However, the theoretical problem of what actually constitutes a species is a controversial issue in evolutionary biology.
The phenetic, biological, recognition, ecological and cladistic species concepts are all different answers to this problem. Of these the biological species concept, according to which a species is a set of interbreeding organisms, is the most widely used definition.
There is some disagreement as to whether species really exist in nature or are categories imposed by humans. However, almost all biologists think that the species is a fundamental natural unit.
How new species are formed, speciation, is a major issue in biology.
Biologists always identify species by their phenetic appearance: different types of oak tree are recognized by leaf patterns, for example.
The biologist Michael Donoghue firmly believes that the species is a real catergory in nature.
Are species real?