A meme is a self replicating piece of information. Examples include jokes, ad campaigns, religions, political ideology, mathematics, and poetry. Any piece of information which alters it's host in such a way that the host is more likely to propagate the information to new hosts
The definition of memes encompasses viruses, both biological and electronic. In a looser sense it also encompasses all life. The information in our genes was imperfectly copied down through our ancestors.
All memes evolve due to the pressures of Darwinism. Those memes more effective at self replication tend to squeeze out less successful memes from their niche. This is a well known fact about biological memes(live organisms), but is seldom thought about on the more abstract level (ie religion, politics, scientific ideas). Much of the interaction between organisms can be observed between informational memes, such as competition, cooperation, parasitism, and symbiotic relationships