The mechanisms of reproductive isolation that work before zygote formation are categorized as pre-zygotic isolation mechanisms. One such mechanism is the temporal isolation.
Here, two diverging populations exhibit differences in the timing of reproduction so as to prevent the possible interbreeding between them.
In the given example, two species of clams present in the same habitat prevent interbreeding among themselves as they release their gametes at a different time of year. One species release gametes in water in early spring while the other does the same during late summer.
Since gametes are not released at the same time, they cannot fuse together and interbreeding is not possible between the given two species.
