Answer: hybridisation between related species is unlikely to contribute to adaptive speciation.
Explanation: any population has natural genetic variation. The available resources are insufficient for all plants (and conversely, not all offspring survive). Natural selection favours variations better suited to the conditions.
Although hybridisation is more common in plants than animals, and can lead to speciation, adaptive radiation from an ancestral species is the general response to environmental change, such as from rainforest to savanna. There is low probability of selective advantage from hybridisation of two ancestral species adapted to niches within the original habitat when the conditions in those niches changes significantly.