Final answer:
The sterile hybrids resulting from the cross-pollination between two orchid species demonstrate a post-zygotic barrier that maintains species separation by ensuring that these hybrids cannot reproduce.
Step-by-step explanation:
When two closely related orchid species cross-pollinate, it is possible for them to produce hybrid offspring. However, since these hybrids are unable to produce seeds of their own, there is a post-zygotic barrier that ensures reproductive isolation. This barrier maintains the distinctness of each species despite the occasional cross-pollination event. The mechanisms for these barriers could be genetic, where hybrid karyotypes are unbalanced, or vital functions are disrupted.
The absence of endosperm in orchid seeds further complicates hybrid viability, as it requires a symbiotic relationship with a fungus to provide necessary nutrients, a connection that might fail in hybrids. Additional pollen-packing adaptations, like pollinia, indicate specialization for specific pollinator interactions, all of which reinforce speciation and prevent the easy blending of species despite occasional hybridization. Such post-zygotic reproductive barriers result in the inability of hybrids to contribute to the gene pool, effectively preventing the two parent species from merging into one.