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Speciation refers to the formation of a new species and is the process that has resulted in a diversity of life on Earth. Through this process, the earliest groups of similar organisms were able to branch out and populate the world with millions of different varieties of life. Though vital to the concept of evolution, the term "species" has been defined several different ways throughout history. The oldest definitions were based on the physical similarity of individuals. Today, however, the main defining characteristic of a species is reproductive isolation. A species is defined by the biological species concept as a group of organisms that are able to interbreed and exchange genetic information to produce viable, fertile offspring.
Types of speciation are defined by the way in which populations become isolated. Sympatric speciation occurs when populations of a species that share the same habitat become reproductively isolated from each other. Allopatric speciation occurs when populations of a species become geographically isolated, and it is the most common form of speciation seen today.
However, such processes of speciation are insufficient to explain the diversity of life we see on Earth today. Many modern species instead owe their existence to the phenomenon of adaptive radiation, the process by which several new species evolve rapidly through repeated colonization of a new habitat.
