We already know that different species cannot interbreed – have fertile offspring-, so gene flow is stopped.  Why does that happen?

This is due to the prezygotic and postzygotic reproductive isolation. These are barriers which prevent interbreed happening. They avoid it either before fertilization, in prezygotic ones; or after fertilization has occurred, postzygotic.

The prezygotic reproductive isolation mechanisms are :

  • Habitat: living beings from different areas cannot mate as they cannot possibly meet either.
  • Behavioural: during the selection of a possible mate, individuals from different species may discard each other as they don’t have the same mating rituals.
  • Temporal: individuals from different species may be in season at different times in the year so they won’t be interested in copulating at the same time.
  • Mechanical: different species may have different sex organs which aren’t compatible. It just won’t fit.
  • Gametic: for fertilisation to occur, the sperm must reach the ovum. For this to happen they usually attract each other through chemical means, but the chemicals might vary from one specie to another. In this case gametes won’t recognise each other and fertilization won’t take place.

The postzygotic reproductive isolation mechnisms are the following:

  • Hybrid viability: sometimes the hybrid dies prematurely.
  • Hybrid fertility: even if an offspring is produced from the mating of different species usually they are infertile as they generally have a random mixed number of chromosomes (so it’s not the same even between hybrids).
  • Hybrid breakdown: if the hybrid results to be fertile, the hybrid population might dissappear along time as from one generation to the next they may result weaker, less fertile, etc.

The sources of information are these videos:

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