In the RNA world, RNA evolved to perform all sorts of different jobs — and that is exactly what Andy’s lab is trying to do now: evolve RNA molecules that perform useful jobs, such as binding to a tumor cell or stopping viral replication. This technique is called directed evolution.

How do biologists “evolve” RNA in a test tube? The same way that a population of organisms evolves in the real world: natural selection. It works like this:

Natural selection in the wild				Artificial selection in the lab
1. There is variation in a population of organisms.				1. There is variation in a population of RNA molecules.
2. Some variants are more likely to survive than others.				2. RNAs that perform a particular job are more likely to be selected than others. Selected RNAs are removed from the original pool.
3. Survivors reproduce.				3. Selected RNAs are copied.
4. The population has evolved and now contains more individuals with the selected trait.				4. The population of RNA has evolved and now contains more useful molecules.

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5. After many rounds of selection, the entire population has the selected trait.				5. After many rounds of selection, the entire population consists of useful molecules.

So natural selection can operate on any varying set of individuals that reproduces via genetic inheritance — even if an “individual” is simply a molecule like RNA and is not technically alive (whatever “alive” is)!