Quote Originally Posted by Yukimura
Don't Shinigami come from the souls or dead humans or the desendents of souls of dead humans (those noble clans probably aren't reproducing by just adopting williy nilly). I wouldn't call them different species as much as different stages of the same life cycle, like a butterfly and a caterpiller for instance.
Their bodies are composed of an entirely different matter, their life cycles are not the same, their rate of growth is different, heck almost everything about them is different so might as well consider them as different spicies.