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 and otherwise really goo 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).
We can of course define for our own purposes life (or living things) as anything we want. Doing that sensibly, however, is key, since science is all about communication. I could define life as anything that grows, anything that moves, anything that catalyzes chemical reactions, etc. Those are all things that most things we would call life do in some way or other. But it would not be sensible, because there are things that are not alive that do those things, too. Fires grow, the wind moves, earth catalyzes. Defining something sensibly means that it should conform to daily use of the term, or in the cases where it does not, it should make sense to refine the vernacular.
So in defining life as something that evolves by natural selection, we would both include things that clearly are not alive in the sense that most people understand it (language), and we would also exclude some things that are clearly alive, but does not evolve.
This latter thing that is alive but does not evolve - what is it, then? It's true that no living organisms that we know of do not evolve, right?
Well, both true and false. First of all, individual organisms do not evolve at all. Populations evolve. Lineages evolve. Individuals develop - from a single cell to an adult human, for example. Organisms are collections of cells, and does not evolve. Does that mean I am not alive? Clearly I am. This definition does not work. It is true that all living things descend* with modification from ancestors that were different from themselves. But what if we one day discovered an organism, looking quite like any other, but which does not die and does not reproduce? Would it not be alive?
Suppose we go to another planet and find one being there, looking exactly like a human being. Everything we can measure about this being confirms that it is just as much alive as you and me. It eats, moves, heals, replenishes, communicates, feels, defecates. Learning more about this being, though, we find that it has no ancestors, and that it does not age. It does not reproduce, and it is the only such being on the planet. Thus, there is no lineage of descent and no population that can evolve. So this being is then not alive? Of course it is. This definition does not work.
For those of you who would object that this example is irrelevant, because no such being that is alive but does not evolve has ever been found: definitions must encompass such thought experiments, or they are useless. If our definitions can not guide us when we are in doubt - in situations where something new is encountered, them they are useless. In that case we might as well just define life as the things that we already know are alive, which just amounts to bookkeeping.
Lastly, see what I did there? I demolished something without providing a solution. Tough luck! I am not required to put forth another definition that I think is better than this one. Just as well as I am not required to come up with some alternative to religion just because I am an atheist.
Is R2-D2 alive? Is it evolving?
* Actually, I prefer to say 'ascend', like the twigs on a tree grows up, and not down. Idiosyncrasies.