Prezygotic reproductive barriers reduce gene flow by preventing mating or fertilization before a zygote forms.

Several mechanisms can produce this isolation:

• habitat isolation: populations use different habitats and rarely encounter one another;
• temporal isolation: breeding occurs at different times of day, season or year;
• behavioral isolation: courtship signals or mate-recognition behaviors differ;
• mechanical isolation: reproductive structures do not fit or transfer gametes effectively;
• gametic isolation: sperm and egg, or pollen and ovule, fail to recognize or fuse successfully.

These mechanisms need not be absolute. Even a partial reduction in mating or fertilization can lower gene flow enough for populations to diverge.

Example

Suppose two insect populations occupy the same region but one breeds in early spring and the other in late summer. They may be physically capable of producing viable offspring in a laboratory, yet in nature their breeding schedules rarely overlap. The barrier is therefore prezygotic temporal isolation.

The defining feature is simple:

$$\boxed{\text{prezygotic barrier} \Rightarrow \text{gene flow is reduced before fertilization}.}$$