Until I ran across this utterly fascinating page at the homepage of John Baez, I had no idea that there were little replicating structures even simpler than viruses. It turns out that there are a whole host of such things! I really liked reading that page for the first time because it made me think of organisms and natural selection in a much more general way. When you get right down to the core of the matter, all you can really say is that patterns that tend to exist, tend to exist. ‘Patterns’ can really be things of any degree of abstraction, whether molecules, chemical reactions, types of bacteria, humans, weather patterns, or societies of dogs.

I recommend you read about subcellular entities. You can get the page in PostScript here. Baez's homepage has lots of other interesting things as well.