Two sympatric species of wildflowers grow the same habitat but hybrids are extremely rare in nature despite the facts that viable hybrids can easily be produced in the lab. The rarity of hybrids is...

Two sympatric species of wildflowers grow the same habitat but hybrids are extremely rare in nature despite the facts that viable hybrids can easily be produced in the lab. The rarity of hybrids is hypothesized to be the consequence of the difference in the blooming season because one species blooms in April while the other blooms in June. This hypothesis is referring to what type of reproductive isolation?