Read What Stops Mating Before Fertilization
These PYQs ask you to locate the barrier to mating: are potential mates apart in time, separated by courtship signals, or pairing preferentially with their own form even in the same place?
Reproductive Isolation is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
First locate the barrier
A prezygotic barrier acts before a zygote forms. If the organisms share a habitat but reproduce at different times, the barrier is temporal. If distinct mating signals keep them from pairing, it is behavioral. Reduced hybrid viability and hybrid breakdown happen after fertilization, so they cannot explain a question where mating is prevented. Do not call a same-habitat case allopatric just because mating is limited.
Time versus song in the source questions
The 13-year and 17-year Magicicada emerge as adults on different cycles despite occupying the same habitat: temporal isolation is the tested answer. Eastern and Western meadowlarks have different breeding songs that prevent mating: behavioral prezygotic isolation is the tested answer. In the frog oscillograms, compare the actual call patterns: among the options, the divergent ewingi II and verreauxi IV pair best supports co-occurrence with distinct signals. The traces alone do not measure genetic variation or prove free interbreeding.
Read finch pairing without overclaiming
The Santa Cruz finch figure shows small- and large-beaked morphs with few intermediates, and like tends to pair with like. That combination is consistent with disruptive selection in sympatry and assortative mating. It is not evidence of geographic separation, selection for intermediates, or perfectly efficient reproductive isolation. Preferential pairing can reduce gene flow without proving that gene flow is zero.
Read spatial separation on a speciation map
On a speciation map, first identify which axis changes. A lineage differing only in spatial location has geographic separation without the mapped ecological or mating divergence. That pattern is allopatric speciation. Do not choose reproductive isolation as a catch-all label: the figure specifically tests spatial differentiation. Choose C.
OlympIQ exam lens
- ✓Same habitat + different emergence periods: temporal isolation.
- ✓Different mating songs that prevent pairing: behavioral prezygotic isolation.
- ✓A song oscillogram can support a mating-signal inference, not a direct genetic-diversity claim.
- ✓Few intermediates + like-with-like pairing supports disruptive selection and assortative mating.
- ✓Do not infer complete isolation or allopatry from preferential pairing in one location.
- ✓Spatial differentiation alone on the source map indicates allopatric speciation.
Ask exactly what the evidence establishes. A prezygotic barrier is not hybrid inviability; sharing an island is not allopatry; and assortative pairing is not proof that two forms never exchange genes.
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