Natural selection is a process that causes adaptive characteristics
to become more common in any system that meets three basic criteria.
1), there must be variation in some trait or characteristic in the population.
2), traits tend to be inherited, or passed on, when individuals reproduce.
And 3), variation in the trait is related to variation in how much individuals will reproduce,
what we call differential reproduction.
So for selection you need variation, inheritance, and differential reproduction.
This easily applies to plants, animals, and the other organisms that reproduce using genes.
Let's say you have a bunch of single-cell bacteria that normally feed on carbon
and you put them in a new environment that has a lot of sulfur.
When would you expect selection to favour the ability to feed on sulfur?
You would if, 1), there is variation among bacteria in the ability to eat sulfur,
2), the offspring of sulfur-eating bacteria also tend to eat sulfur,
And 3), bacteria that eat sulfur tend to leave more offspring than those that can't.
In this example, offspring are probably similar to their parents because they inherit genes from them.
But it turns out that selection occurs in many types of systems, not just genetic ones.
Take stories. Some stories have features: a good character, an interesting plot;
anything that makes them more memorable than other stories.
So one, there's variance.
When people retell a story, the version they tell
will usually have most of the same features as the version they heard. When people retell a story, the version they tell
will usually have most of the same features as the version they heard.
So 2), there's inheritance.
Finally, stories with features that make them more memorable
tend to get remembered and re-told more often than less memorable stories.
So 3), there's differential replication or reproduction of stories.
So in this system, selection would cause stories with more memorable features
to become relatively more common over time.
Keep in mind that selection isn't the only force affecting the frequency of different traits in a population.
Innovation, mutation, and random processes like drift are also important.
Selection isn't even the only process that drives adaptation.
Individuals can adapt to new circumstances by learning from experience, for example.
And there are, in fact, deep similarities between the mathematics of natural selection
and other adaptive processes like Trial and Error Learning and Bayesian Learning.
Understanding the role of selection and other dynamical processes
in biological, social, and technological systems
is some of the work that we do here at the Santa Fe Institute.