Speciation refers to the creation a new species. Through this process, the earliest groups of similar organisms were able 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.
Types of speciation are defined by 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. Allopatric speciation is the most common form of speciation seen today. Parapatric speciation, which is extremely rare, occurs when populations are separated by an extreme change in habitat rather than a physical barrier.
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.