In spite of this, these species can still hybridize and produce fertile offspring.

A "successful" hybrid is one demonstrated to produce fertile offspring.

And which of the hybrids have produced fertile offspring?

The two types do not readily interbreed, but fertile offspring can occur.

A species is a group that can produce fertile offspring by crosses within but not outside itself.

This suggests that the two populations interbred and produced fertile offspring.

And of course there would be little point, if it did not, for he could sire no offspring, fertile or not.

That is, the two species may have been enough alike to mate and produce fertile offspring.

If individuals can interbreed and produce fertile offspring, they belong to the same species.

The four species can interbreed and produce fertile offspring.