Speciation occurs when organisms diverge after experiencing different sets of environmental pressures. Organisms belong to different species when they can no longer interbreed to produce viable offspring.
Biologists normally define types of speciation by the degree to which groups are separated. In allopatric speciation, a barrier separates groups and they begin to evolve independently as they face different environments. Geological processes, such as the destruction of land bridges or mountain formations, often result in speciation when groups of individuals become separated. Allopatric speciation can also occur when habitats become fragmented due to human activities. When organisms leave a population and colonize an island, they may face different conditions than on the mainland, which results in the formation of two species.
Sympatric speciation occurs when individuals inhabit the same area. This often happens with plants since they are prone to developing extra sets of homologous chromosomes, which makes them unable to produce viable offspring with one another. Through this mechanism, speciation can occur even though individuals are not physically separated. In animals, sympatric speciation takes place when organisms begin to exploit different prey items and never come into contact with other members of the species.
In addition to these natural mechanisms, artificial speciation can be induced by humans who breed organisms together for certain traits. Dachshunds, for example, were bred for their short stature, which enabled them to hunt small mammals that lived in burrows.