Not necessarily.... I routinely hybridize moths from around the world (different species) and many times create viable offspring from two totally isolated, obviously separate species - related sure - but defintely separate.
"If you can successfully breed, you aren't a different species."
Not entirely true. Take the Horse, Ass, Zebra species as an example. They are the same genus, different species, and they most definatelu can breed and produce offspring.
quote:
Now differences in chromosome number do not serve as reproductive barriers between all species. For example, lets look at some of the equine species ( horses and donkeys). Domesticated horses have 32 pairs of chromosomes and Donkeys have 31. Yet, they can produce offspring, mules, which have 31.5 pairs of chromosomes. One of the horse chromosomes goes unpaired. Wild mountain zebras have 16 pairs of chromosomes, while the last species of wild horse (Przewalski's Horse) has 33 pairs. However, all of these equine species can produce hybrid offspring. In all of these crosses but one, the offspring are sterile. It has long been argued that this sterility is due to the difference in chromosome number, but hybrids of the wild (33 pairs) and domesticated horse (32 pairs) are fertile, and have 32.5 pairs of chromosomes. So clearly, something more than just differences in chromosome number is contributing to the species interbreeding barrier.
from:http://madsci.wustl.edu/posts/archives/may2001/989331026.Ev.r.html