---
title: "Postzygotic Barriers — AP Bio Definition & Exam Guide"
description: "Postzygotic barriers block successful hybrids AFTER a zygote forms, keeping species separate. Learn how they drive speciation and show up on the AP Bio exam."
canonical: "https://fiveable.me/ap-bio/key-terms/postzygotic-barriers"
type: "key-term"
subject: "AP Biology"
---

# Postzygotic Barriers — AP Bio Definition & Exam Guide

## Definition

Postzygotic barriers are reproductive isolating mechanisms that act after a hybrid zygote forms, lowering the survival or fertility of hybrid offspring so two species stay genetically distinct.

## What It Is

Postzygotic barriers are reproductive isolating mechanisms that kick in *after* [fertilization](/ap-bio/key-terms/fertilization "fv-autolink") happens between two different species. The key word is "zygote." Pre means before, post means after, so a postzygotic barrier means the egg and sperm actually combined into a zygote, but something goes wrong from there.

Think of it this way: a zygote forms, but the hybrid never gets to pass its genes on. Maybe the embryo doesn't develop properly (reduced hybrid viability), maybe the hybrid grows up fine but is sterile like a mule (reduced hybrid fertility), or maybe the first hybrid generation is okay but its [offspring](/ap-bio/unit-7/intro-natural-selection/study-guide/v9Lf9qQpmpSXvd2ZUOqH "fv-autolink") fall apart (hybrid breakdown). All three keep the two species' gene pools from blending, which is the whole point of reproductive isolation in [speciation](/ap-bio/unit-7/speciation/study-guide/EvkCBpDW4LggHrVIepHo "fv-autolink").

## Why It Matters

This term lives in [Unit 7](/ap-bio/unit-7 "fv-autolink"): Natural Selection, where speciation is a major idea. [Reproductive isolation](/ap-bio/key-terms/reproductive-isolation "fv-autolink") is the engine that splits one species into two, and postzygotic barriers are one of two flavors of that isolation (the other being prezygotic). Understanding them helps you explain *how* new species form and stay separate, which is exactly the kind of mechanism-based reasoning Unit 7 rewards. While the closest CED anchor here is the origins-of-life and evolutionary framing of 7.12.A, the broader payoff is being able to argue why gene flow stops between diverging populations.

## Connections

### Reduced Hybrid Viability and Reduced Hybrid Fertility (Unit 7)

These are the two most common postzygotic barriers. Viability means the hybrid embryo dies or is weak, and fertility means the hybrid lives but can't reproduce (the mule is the classic example). They're not separate ideas from postzygotic barriers, they ARE postzygotic barriers.

### Hybrid Breakdown (Unit 7)

This is the sneaky third type. The first-generation hybrid looks fine and can even reproduce, but the next generation is weak or sterile. It shows that a barrier can hide one generation deep before it shows up.

### [Mechanical Isolation (Unit 7)](/ap-bio/key-terms/mechanical-isolation)

This is a [PREzygotic barrier](/ap-bio/key-terms/prezygotic-barrier "fv-autolink"), the opposite timing. Mechanical isolation blocks mating before a zygote ever forms (physical incompatibility), while postzygotic barriers only matter once fertilization already happened. Knowing which side of fertilization a barrier acts on is the whole distinction.

### [Genetic Diversity (Unit 7)](/ap-bio/key-terms/genetic-diversity)

Postzygotic barriers protect the separate gene pools of two species by stopping their DNA from mixing successfully. By keeping species distinct, they help maintain the [genetic diversity](/ap-bio/key-terms/genetic-diversity "fv-autolink") across an ecosystem rather than collapsing everything into one blended population.

## On the AP Exam

Expect this in multiple-choice questions that hand you a scenario and ask you to classify the barrier. The trick is always timing: did the barrier act before or after a zygote formed? A sterile mule, a hybrid embryo that dies, or a healthy hybrid with weak grandkids all point to postzygotic. On free response, you might be asked to explain how reproductive isolation leads to speciation, and naming a postzygotic barrier as a concrete mechanism strengthens that answer. No released FRQ uses this exact term verbatim, but the speciation reasoning it supports shows up across Unit 7.

## Postzygotic barriers vs Prezygotic barriers

Both stop two species from successfully reproducing, but the dividing line is fertilization. Prezygotic barriers (like mechanical or temporal isolation) prevent a zygote from ever forming. Postzygotic barriers let a zygote form, then sabotage the hybrid afterward. If a fertilized egg exists, you're in postzygotic territory.

## Key Takeaways

- Postzygotic barriers act AFTER a zygote forms, while prezygotic barriers act before fertilization.
- The three main types are reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.
- A sterile mule is the textbook example of reduced hybrid fertility, a postzygotic barrier.
- These barriers keep two species' gene pools separate, which is what reproductive isolation and speciation are all about.
- On the exam, identify the type of barrier by asking whether a zygote already formed.

## FAQs

### What are postzygotic barriers in AP Bio?

They're reproductive isolating mechanisms that act after a hybrid [zygote](/ap-bio/key-terms/zygote "fv-autolink") forms, lowering the survival or fertility of the offspring. The three types are reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.

### Is a mule a postzygotic barrier?

Yes. A mule is the offspring of a horse and a donkey. It's healthy but sterile, which is reduced hybrid fertility, a classic postzygotic barrier.

### How are postzygotic barriers different from prezygotic barriers?

The dividing line is fertilization. Prezygotic barriers (like mechanical or temporal isolation) stop a zygote from ever forming, while postzygotic barriers let the zygote form and then make the hybrid weak or sterile.

### Do postzygotic barriers prevent mating?

No. Mating and fertilization actually happen with postzygotic barriers, since a zygote forms. The barrier only kicks in afterward by reducing the hybrid's fitness. Preventing mating is the job of prezygotic barriers.

### Why do postzygotic barriers matter for speciation?

They block gene flow between two diverging populations even when individuals can still cross-fertilize. By keeping hybrid offspring from passing on genes, they help maintain two separate species rather than one blended gene pool.

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