Evolution depends on selection as much as on replication and innovation: if all novelties had an equal chance of success then there would be no gradual development, in culture any more than in nature. The information that is preserved and copied in cultural traits must, in order for evolution to occur, be subject to some sort of struggle for survival. Putting this another way, meme theory needs some sort of criterion of memetic “fitness” – something that ideas and skills have in common, in virtue of which their relative success rates can be subject to systematic study – for without this it breaks down into the trivial statement that out of many new ideas and skills some survive whilst others do not.

Glancing back to the natural world, we know that the general fitness criterion for genes is the influence they have on an organism's longevity and fertility – its ability to find food and sex – and that this will be affected not only by the genes that it possesses but also by the rest of the gene pool and the external environment.

So what is the general fitness criterion for the cultural population of traditions, ideas, tunes and designs? It makes sense to say that, as for genes, memetic success will depend on three separate factors: the content of the meme itself; the way in which it fits with other memes; and the external environment – the minds and surroundings of the people whose attention it is trying to attain.