| I can answer some of this, but others may have more complete answers. Several factors come into play: 1)The two potential hybrids would have to bloom at the same time. 2)They would have to be equally attractive to the same pollinators 3)Pollen from a different species will often grow slower down the flower's pistil(s) than the species pollen (i.e. if a red raspberry flower has both red and black pollen on it, the red pollen will 'outcompete' the black pollen. 4)Some crosses only work in 1 direction (for the red X black raspberry cross only one of them can successully act as the female parent. [I can't recall which one]. 5) Cross species hybrids seed germination and seedling vigor is sometimes(often?) less than the non-hybrid seedling 6) They hybrid's are often sterile themselves. Don't confuse terms like "Hybrid" corn with cross species hybrids. Hybrid corn refers to a cross between two 'landraces' (inbred strains) of corn that the cross between them results in a strain of corn w/ hybrid vigor. In this case both species involved in the cross was Zea mays. In cross-species hybrid is another ball of wax. | 
