Postzygotic isolation is a barrier that acts after fertilization, so a zygote forms but the hybrid offspring cannot survive, stay fertile, or produce healthy descendants. In General Biology I, it shows how species stay separate.
Postzygotic isolation is a reproductive barrier in General Biology I that acts after fertilization. The egg and sperm may fuse, but the resulting hybrid does not successfully become a reproducing member of either population.
That matters because species do not stay separate only by avoiding mating. Sometimes two populations can mate, make a zygote, and still fail to pass genes into the next generation. Postzygotic isolation is one of the reasons gene flow stops even when mating occurs.
The most common forms are hybrid inviability, hybrid sterility, and hybrid breakdown. In hybrid inviability, the embryo or young hybrid does not survive well enough to reach adulthood. In hybrid sterility, the hybrid grows up but cannot produce functional gametes. A classic example is a mule, which is healthy enough to live but cannot reproduce.
Hybrid breakdown is a little different. The first-generation hybrid may look fine and may even be fertile, but its offspring are weak, sterile, or less viable. That means the problem can show up one generation later, which is why some crosses seem successful at first but still fail to maintain a stable lineage.
Mechanistically, postzygotic isolation usually comes from genetic incompatibilities. Two species may have genes that work well within their own genomes, but when mixed, those gene combinations can disrupt development, meiosis, or fertility. The hybrid is not "bad" in a general sense, it is mismatched for the combined genetic background.
In the bigger speciation story, postzygotic isolation helps keep populations on separate evolutionary paths. Even if a rare hybrid forms, the barrier limits how much the two gene pools blend, which supports the formation and maintenance of distinct species.
Postzygotic isolation is one of the cleanest ways to see how speciation works at the gene level. In General Biology I, you are not just memorizing that species differ. You are tracing how barriers keep gene flow low enough for populations to diverge.
This term also helps you sort out what happens before and after fertilization. Prezygotic barriers stop mating or fertilization from happening at all. Postzygotic barriers come later, so they explain cases where reproduction looks successful at first but still fails to produce a lasting lineage.
It shows up a lot in evolution questions, especially when you are comparing populations, hybrid animals, or examples of reproductive isolation. If a hybrid cannot survive, cannot reproduce, or produces weak descendants, that is evidence that the two parent populations are becoming genetically incompatible.
It also connects to class discussions about why species boundaries are not just based on appearance. Two organisms can look similar, live nearby, and even mate, but if their hybrids do poorly, they still function as separate species under the biological species concept.
Keep studying General Biology I Unit 18
Visual cheatsheet
view galleryprezygotic isolation
Prezygotic isolation happens before fertilization, so it blocks mating, gamete transfer, or successful egg-sperm fusion. Postzygotic isolation comes after the zygote forms. A good way to separate them is timing: prezygotic barriers stop a hybrid from forming, while postzygotic barriers let a hybrid form but reduce its survival or fertility.
hybrid inviability
Hybrid inviability is one type of postzygotic isolation. The hybrid embryo or offspring develops poorly and does not survive to reproductive age. When you see this term, think about developmental failure caused by incompatible genes rather than a problem with mating or fertilization.
hybrid sterility
Hybrid sterility is another postzygotic barrier, but the hybrid survives to adulthood. The key issue is that it cannot make functional gametes, so it has no direct offspring. Mules are the standard example, and they show how a living hybrid can still be an evolutionary dead end.
biological species concept
The biological species concept defines species by their ability to interbreed and produce viable, fertile offspring. Postzygotic isolation is one of the main reasons this definition works in practice. If hybrids are inviable or sterile, the groups fail the biological species test even if mating sometimes occurs.
A quiz question might give you a mating scenario and ask whether the barrier is prezygotic or postzygotic. If the prompt says fertilization happened but the offspring died, were sterile, or produced weak descendants, you should identify postzygotic isolation and name the specific type. In a short answer or lab-style question, you may also need to explain how that barrier reduces gene flow between populations.
For image-based or case-based questions, look for clues like a mule, a hybrid plant with poor fertility, or a cross that works once but fails in the next generation. The move is not just to label the term, but to trace the sequence from zygote formation to failed reproduction. That shows you understand the mechanism, not just the vocabulary.
These are easy to mix up because both are reproductive barriers, but the timing is different. Prezygotic isolation prevents fertilization, while postzygotic isolation happens after fertilization and affects the hybrid's survival or fertility.
Postzygotic isolation is a reproductive barrier that acts after fertilization, so a hybrid can form but still fail to survive or reproduce.
The main forms are hybrid inviability, hybrid sterility, and hybrid breakdown.
This term matters because it explains how gene flow stops even when two populations are able to mate.
A classic example is a mule, which is a viable hybrid but usually sterile.
In evolution, postzygotic isolation supports speciation by keeping populations genetically separate over time.
Postzygotic isolation is a reproductive barrier that acts after fertilization. The hybrid may form, but it does not survive well, cannot reproduce, or produces weak offspring. In General Biology I, it is a core idea for explaining how new species form.
The three main types are hybrid inviability, hybrid sterility, and hybrid breakdown. Hybrid inviability means the hybrid does not survive, hybrid sterility means it survives but cannot make gametes, and hybrid breakdown means later generations become weak or sterile.
Yes. A mule is a classic example of hybrid sterility, which is a type of postzygotic isolation. The horse-donkey hybrid can live and grow, but it usually cannot produce viable offspring because its chromosomes do not pair properly in meiosis.
Prezygotic isolation blocks fertilization from happening, while postzygotic isolation acts after fertilization. If you are given a scenario where a zygote forms but the offspring fails later, that points to postzygotic isolation, not prezygotic isolation.