Organisms that belong to the same species are able to interbreed and produce fertile, viable offspring

• Reproductive isolation occurs when barriers prevent two populations from interbreeding and producing offspring
  

• Over time, genetic differences will accumulate between the reproductively isolated populations, leading to eventual speciation
  

There are two main categories of reproductive isolation barriers:

• Pre-zygotic barriers occur before fertilisation (no interbreeding means no offspring are produced)

• Post-zygotic barriers occur after fertilisation (offspring are produced but are either not viable or infertile)
  

Pre-Zygotic Barriers

Geographic isolation occurs when two populations occupy different habitats or separate niches within a common region

• Lions and tigers typically occupy different habitats and do not interbreed (usually)
  

Behavioural isolation occurs when two populations exhibit different specific courtship patterns or mating rituals

• Certain groups of birds will only respond to a species-specific mating call (e.g. eastern and western meadowlarks have distinct birdsongs)
  

Temporal isolation occurs when two populations differ in their periods of activity or reproductive cycles

• Leopard frogs and wood frogs reach sexual maturity at different times in the spring and hence cannot interbreed
  

Post-Zygotic Barriers

Hybrid inviability occurs when offspring are produced but fail to develop to reproductive maturity

• Certain types of frogs form hybrid tadpoles that die before they can metamorphose into a frog
  

Hybrid infertility occurs when hybrid offspring are viable but unable to produce functional gametes (sterility)

• Mules are sterile hybrids resulting from a female horse and a male donkey
  

Isolation Barriers