Example 1
Two frog groups breed in different seasons and do not meet to mate. Name the barrier.
Temporal isolation is pre-zygotic: timing prevents fertilisation from occurring.
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Year 12 · SACE Stage 2
Species and reproductive barriers
Classify species criteria and pre- versus post-zygotic barriers.
Method and worked examples
For sexually reproducing organisms, a biological species can be defined through actual or potential interbreeding that produces fertile offspring under appropriate natural conditions. This criterion has limits for fossils, asexual organisms and isolated populations whose breeding compatibility is unknown. Morphology, biochemical features and genetic evidence provide other criteria.
Pre-zygotic barriers prevent formation of a zygote. Examples include breeding at different times, different courtship behaviour, incompatible reproductive structures and gametes that cannot fuse. Post-zygotic barriers act after fertilisation, such as poor hybrid survival or hybrid sterility. A hybrid that survives but cannot reproduce illustrates a different barrier from failure of gamete fusion. Classify the stage at which the barrier acts rather than assuming every cross-species mating has an identical outcome.
Example 1
Example 2
Two animals produce a healthy hybrid that is sterile. Classify the barrier.
Hybrid sterility is post-zygotic because a zygote formed and developed, but the hybrid cannot produce offspring normally.
Watch out: Appearance is useful evidence but may not reflect reproductive isolation or genetic relationships.