Prezygotic isolation is any reproductive barrier that stops two species from mating or completing fertilization, acting BEFORE a zygote forms. It keeps populations reproductively isolated and is one of the mechanisms behind speciation (CED 7.10).
Prezygotic isolation is a barrier that keeps two species from making a fertilized egg in the first place. The key word is pre-zygotic: it all happens before the sperm and egg combine into a zygote. Either the two species never attempt to mate, or they try but fertilization fails.
There are several flavors, and the AP CED expects you to recognize them. Temporal isolation means they breed at different times. Habitat isolation means they live in different places and never meet. Behavioral isolation means their mating rituals don't match (think mismatched courtship signals). Mechanical isolation means their body parts physically don't fit. And gametic isolation means the sperm and egg are chemically incompatible even if mating occurs. All of these stop gene flow early, which is exactly what's needed for two populations to become separate species under the biological species concept.
This lives in Unit 7: Natural Selection, specifically topic 7.10 Speciation. It directly supports learning objective AP Bio 7.10.C, which asks you to explain the mechanisms that drive speciation, and the essential knowledge EK 7.10.C.2 that lists pre-zygotic and post-zygotic mechanisms as the things that maintain reproductive isolation and prevent gene flow. It also ties back to EK 7.10.A.1, since speciation literally requires two populations to become reproductively isolated. The big theme: evolution doesn't just change a population, it can split one into two, and isolation barriers are how that split sticks.
Keep studying AP Biology Unit 7
Reproductive Isolation (Unit 7)
Prezygotic isolation is one half of reproductive isolation. The other half, postzygotic, kicks in after a zygote forms (hybrids that are weak, sterile, or short-lived). Together they're the wall that keeps gene pools separate so speciation can finish.
Allopatric Speciation (Unit 7)
When a mountain or river splits a population (allopatric), habitat isolation is a prezygotic barrier doing the work. Geographic separation prevents mating before any zygote could even form, so the two groups drift apart genetically.
Behavioral, Temporal, and Mechanical Isolation (Unit 7)
These aren't competitors to prezygotic isolation, they ARE prezygotic isolation. Each is just a specific way mating or fertilization fails: wrong courtship dance, wrong season, or parts that don't fit physically.
Genetic Drift and Mutation (Unit 7)
Once a prezygotic barrier shuts off gene flow, mutation and genetic drift can push the two isolated populations in different directions. Isolation sets the stage; these processes do the divergence.
Multiple-choice questions love to test whether you can sort a scenario into the right bucket. Expect stems like "Which of the following is NOT a form of prezygotic isolation?" or a scenario about two orchid species that grow meters apart but use different pollinators, where the answer is a prezygotic mechanism. The classic trap: a stem describes hybrids that form but then die from mismatched jaws or are sterile. That's POSTzygotic, because a zygote already formed. Your job is to ask one question first, "Did a fertilized egg get made?" If no, it's prezygotic. If yes but the hybrid fails, it's postzygotic. No released FRQ uses this term verbatim, but it's exactly the kind of mechanism you'd cite when explaining how reproductive isolation drives speciation in a written response.
Prezygotic = before the zygote (mating or fertilization fails). Postzygotic = after the zygote (the hybrid forms but is inviable or sterile). The deciding question is whether a fertilized egg ever formed. Two orchids that never get pollinated by the same insect? Prezygotic. A mule that's born but can't reproduce? Postzygotic.
Prezygotic isolation blocks mating or fertilization BEFORE a zygote forms, so no hybrid is ever created.
Its main types are temporal, habitat, behavioral, mechanical, and gametic isolation, and you should be able to match each to a scenario.
It's one of the two reproductive isolation categories (EK 7.10.C.2); postzygotic is the other and acts after fertilization.
Always ask 'Did a fertilized egg form?' to tell prezygotic from postzygotic on a test question.
By stopping gene flow early, prezygotic barriers help two populations stay reproductively isolated, which is the requirement for speciation (EK 7.10.A.1).
It's any reproductive barrier that prevents two species from mating or completing fertilization, so a zygote never forms. Examples include temporal, habitat, behavioral, mechanical, and gametic isolation, and it's covered under topic 7.10.
Postzygotic. A sterile hybrid like a mule means a fertilized egg already formed and developed, so the barrier acts after the zygote, not before. Prezygotic barriers stop reproduction before fertilization happens.
Prezygotic acts before a zygote forms (mating doesn't happen or sperm and egg don't combine), while postzygotic acts after, when a hybrid forms but is inviable or sterile. The deciding factor is whether a fertilized egg was ever made.
They're all prezygotic barriers. Behavioral isolation is mismatched courtship signals, temporal isolation is breeding at different times or seasons, and mechanical isolation is body parts that physically don't fit during mating.
Speciation requires two populations to stop exchanging genes (EK 7.10.A.1). Prezygotic barriers cut off gene flow early, letting mutation and genetic drift push the two groups apart until they become separate species.
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