Wikipedia lists the world records and extremes of the vocal range,
can extrema be reached by a lot of practice or were they born with that?
Overall vocal type (e.g. soprano vs contralto or tenor vs bass) seems to be what you were born with and http://en.wikipedia.org/wiki/Voice_type#Classifying_singers warns of the dangers of misclassification of vocal type, and by implication, attempting to sing at the extremes of one's natural range.
Practice is certainly required to reach the expected range of one's natural vocal type and even then, I think it's easier to extend upward than downward. My own comfortable range without practice used to be G2 to C4 or D4 but when I was regularly singing in a choir was more like F2 to F4 or G4. It would likely have been dangerous to go higher and physically impossible to go lower.
I would think practice should be more about increasing tessitura than absolute range. However, my experience is entirely as a chorist and not as a solo singer.