Postzygotic reproductive barriers act after fertilization. A hybrid zygote forms, but the resulting offspring has reduced survival or reproductive success.

Three common patterns are:

• hybrid inviability: hybrid embryos or offspring fail to develop normally or survive poorly;
• hybrid sterility: hybrids survive but produce few or no functional gametes;
• hybrid breakdown: first-generation hybrids can be viable and fertile, but later-generation descendants have reduced fitness.

These barriers can arise because genes and chromosomes that function well within each parental lineage interact poorly when combined.

Example

A horse and a donkey can produce a mule. The mule is generally healthy, so fertilization and development occurred successfully. However, mismatched chromosome sets interfere with normal meiosis, so most mules are sterile. This is a postzygotic barrier, specifically hybrid sterility.

Postzygotic isolation does not prevent all gene flow automatically. If some hybrids reproduce successfully and backcross—mate with individuals from one of the parental populations—alleles can still cross the species boundary.

The defining contrast is:

$$\boxed{\text{prezygotic: prevents zygote formation}}$$

$$\boxed{\text{postzygotic: reduces fitness after a zygote forms}.}$$