Analogous to the theory of
genetic evolution, which operates in the realm of
biological configurations of things, but instead operates in the realm of
ideas and
thought.
While biological systems are made up of many individual genes, thought systems are composed of memes. Each person is not only a carrier of a set of genes, but a set of memes as well. This set of memes, however, is easier to change within a person - parts of it are also easier to pass on to other individuals. Just as few individuals are genetically identical, few are also memetically identical.
In general, in can be said that a set of genes that better enables an individual to survive is more likely to be passed on to later generations of offspring. The same can be said of a set of memes. However, just as there are genetic urges to mate that may put at risk a person's survival, there are also memetic urges to propagandize that may also put biological survival at risk.