I liked all the answers but I am compelled to add my two cents: Hybrid is usually defined as a cross between two different breeding populations and species is usually defined as a breeding population (in essence). But this might seem to imply that a hybrid is impossible by definition. Also, if you have two different geographically separated clades of the same species that don't normally breed with each other, you might be tempted to call them different species (by definition). Clearly, the definitions are not very clear and are an attempt to cram a portion of the natural world into simple concepts. As such, you might consider the concepts of species and hybrid to be non-disjoint - that is, something that is a cross between two 'subspecies' of the same species would be both a species and a hybrid. Also, one way to visualize the quantitative differences between the F1 and F2 hybrids is to consider the bell curves of any given trait (e.g. petal length) in the two species and in the hybrids. Each species has a mean and standard deviation for the petal length and the mean petal length for the F1 and F2 hybrids will be each the average of the two species mean petal lengths. But the total variation of the F1 hybrid is expected to be close to that of the species, while the total variation of the F2 hybrid is expected to span that of both species. Both are bell curves, but the F1 is a steep, narrow curve while the F2 has a very broad curve, extending all the way to the extremes of both species. $.02 Tim