Some like to define living organisms as that which
2) has inheritance, and
3) has variation.
In other words, living things would be those which evolve by natural selection. Rosie Redfield (blog) espoused this view in a recent talk at the Evolution 2012 conference in Ottawa (#evol2012 Twitter feed). Jerry Joyce (lab page) did the same at the 74th symposium of Quantitative Biology at Cold Spring Harbor Labs in 2009.
But this is folly.
First of all, I can easily give an hypothetical example of something that must clearly be alive, but which does not evolve. I’ll defer that to the end of this post.
But I can also give an example of something that most people will not agree is alive, namely languages. Metaphorically, I can accept that languages are alive. “Danish is such a beautiful language, alive with raunchy adjectives and verbs that sing.” Or something. But not actually alive in a literal sense. It is spoken by beings that are alive, but is no more alive than thoughts or books, even if it does evolve (note that languages evolution really isn’t of the Darwinian kind, either, just like memes aren’t).