The crucial event, for the origin of a new species, is reproductive isolation.

Biologists need to understand how a barrier to interbreeding can evolve between the new species and its ancestors.

There are three main theories as to how this can happen:

• Allopatric speciation in which the new species evolves in geographic isolation from its ancestor.

• Parapatric speciation: the new species evolves in a geographically contiguous pair of populations.

• Sympatric speciation: the new species evolves within the geographic range of its ancestor.

Figure: in (a) allopatric speciation the new species forms geographically apart from its ancestor; in (b) parapatric speciation, the new species forms in a contiguous population; and (c) in sympatric speciation a new species emerges from within the geographic range of its ancestor.