Species & Reproductive Isolation
For sexually reproducing organisms, a species is a group whose members can interbreed to produce fertile offspring. (Other criteria — morphological similarity, biochemical similarity, and a shared gene pool — help where interbreeding can't be tested.)
Species stay distinct because of reproductive isolating mechanisms, which fall into two groups.
Pre-zygotic — prevent a zygote forming
- Temporal — breed at different times.
- Behavioural — different courtship signals.
- Mechanical — incompatible body/flower structures.
- Gamete — sperm and egg can't fuse.
Post-zygotic — prevent fertile hybrids
- Hybrid inviability — the hybrid dies early.
- Hybrid sterility — the hybrid survives but can't reproduce (e.g. a mule).
Pre-zygotic barriers stop a zygote forming; post-zygotic barriers stop fertile hybrids.