• The evolution of a new species happens when one population of interbreeding organisms splits into two separately breeding populations.

• It has been a matter of controversy whether new species evolve only from sub-populations that are geographically isolated (allopatric) from the ancestral population, or whether they can also evolve from sub-populations that are contiguous with (parapatric), or overlap (sympatric), the ancestral population.

• Allopatric speciation may be by subdivision of the species range or by a peripheral isolate - a small population which becomes cut off at the edge of the species range.

• Parapatric speciation could happen if a steep cline evolved into a hybrid zone and barriers to interbreeding then evolved.

• Sympatric speciation is most likely if selection first establishes a stable polymorphism and then favors assortative mating within each polymorphic type.

• Many new plant species have originated by hybridization of two existing species, followed by polyploidy of the hybrids.

• Reinforcement is the enhancement of reproductive isolation by natural selection: forms are selected to mate with their own, and not with the other, type. Sympatric speciation requires reinforcement to happen; parapatric speciation usually requires it; allopatric speciation can take place with or without reinforcement.