The evolution of new species is termed speciation.
Allopatric speciation takes place when a part of the population becomes geographically separated (geographical isolation) from the parental population. For example, a group of birds lives at the base of the mountain, some members fly up and get geographically isolated. Variation and natural selection act differently on the two because the environment in which the two live is different.
Sometimes a genetic barrier (reproductive barrier) prevents reproduction between a section of a population of a species with other members. Such a section of population usually arises in plants because of polyploidy.
Polyploidy is a mutation in which the normal diploid number of chromosomes become doubled or trebled in a section of the population of a species due to certain irregularities during cell division. The polyploid section of the population is then unable to interbreed (mate and reproduce) with their diploid ancestors and becomes a new species.