Name the barrier by where reproduction fails
- The idea
- Predict
- See it
- Why
- Check
The idea
“If two species make almost no hybrids, that alone tells me which barrier is at work.”
Every barrier ends the same way, with few or no fertile hybrids. Only the stage where the drop starts tells them apart.
The short version
The mistake: If two species make almost no hybrids, that alone tells me which barrier is at work.
What's actually true: Each barrier acts at one stage, before or after a zygote forms. The first stage where hybrids fall behind tells you which.
- Reproduction runs in stages: meet, mate, fertilize, develop, breed. A barrier breaks one, and every later stage looks weak because few hybrids get that far.
- Prezygotic barriers keep a zygote, or fertilized egg, from forming. They include habitat, temporal, behavioral, mechanical and gametic isolation.
- Postzygotic barriers act after. Inviable hybrids die as they develop. Sterile ones live but make no working gametes. In hybrid breakdown, the hybrids' offspring fail.
On the exam: Find where hybrids first fall behind. Before a zygote forms, it's prezygotic. After, it's postzygotic.