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Natural selection is a process 
that causes adaptive characteristics

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to become more common in any system 
that meets three basic criteria.

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1), there must be variation in some trait 
or characteristic in the population.

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2), traits tend to be inherited, 
or passed on, when individuals reproduce.

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And 3), variation in the trait is related to variation 
in how much individuals will reproduce,

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what we call differential reproduction.

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So for selection you need variation, 
inheritance, and differential reproduction.

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This easily applies to plants, animals,
and the other organisms that reproduce using genes.

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Let's say you have a bunch of single-cell bacteria
that normally feed on carbon

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and you put them in a new environment
that has a lot of sulfur.

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When would you expect selection 
to favour the ability to feed on sulfur?

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You would if, 1), there is variation among bacteria 
in the ability to eat sulfur,

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2), the offspring of sulfur-eating bacteria 
also tend to eat sulfur,

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And 3), bacteria that eat sulfur
tend to leave more offspring than those that can't.

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In this example, offspring are probably similar to their parents
because they inherit genes from them.

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But it turns out that selection occurs 
in many types of systems, not just genetic ones.

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Take stories. Some stories have features:
a good character, an interesting plot;

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anything that makes them 
more memorable than other stories.

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So one, there's variance.

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When people retell a story, 
the version they tell

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will usually have most of the same features
as the version they heard. 
When people retell a story, 
the version they tell

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will usually have most of the same features
as the version they heard.

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So 2), there's inheritance.

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Finally, stories with features 
that make them more memorable

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tend to get remembered and re-told 
more often than less memorable stories.

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So 3), there's differential replication
or reproduction of stories.

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So in this system, selection would cause 
stories with more memorable features

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to become relatively more common over time.

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Keep in mind that selection isn't the only force 
affecting the frequency of different traits in a population.

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Innovation, mutation, and random processes 
like drift are also important.

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Selection isn't even the only process 
that drives adaptation.

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Individuals can adapt to new circumstances 
by learning from experience, for example.

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And there are, in fact, deep similarities 
between the mathematics of natural selection

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and other adaptive processes 
like Trial and Error Learning and Bayesian Learning.

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Understanding the role of selection 
and other dynamical processes

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in biological, social,
and technological systems

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is some of the work that we do
here at the Santa Fe Institute.