Allopatric vs sympatric speciation

• Allopatric speciation


  Geographical isolation - a population of a species is split into two by a geographical barrier preventing exchange of genetic material
• Sympatric speciation


  Formation of a new species within the same location due to isolating mechanisms like behavioural isolation or temporal isolation
• Kaibab squirrels


  • Example of allopatric speciation - ancestors had wide distribution, then Grand Canyon formed and cut off a small population which evolved distinct characteristics
• Apple maggot flies


  • Example of sympatric speciation in process - ancestral flies fed on hawthorns, then started using apples, leading to reproductive isolation and genetic divergence
• Mechanisms of reproductive isolation

  • Geographical isolation
  • Temporal isolation
  • Behavioural isolation
• For speciation to occur, the gene flow between the populations must be minimal or non-existent
• Geographical isolation


  Separation of species by natural or human-made physical barriers, as seen in Galapagos finches
• Temporal isolation


  Differences in timing of reproductive cycle like mating seasons or gamete production, preventing interbreeding even in same location (e.g. cicada species)
• Behavioural isolation


  Differences in mating rituals like courtship dances or mating calls, preventing recognition and interbreeding (e.g. fireflies)
• Prezygotic barriers


  Barriers that prevent the formation of the zygote
• Postzygotic barriers


  Barriers that occur after the zygote is formed, including zygote inviability or sterility