> To continue the series: what would it be to have *three* kinds of
> Genes, memes and... ?
I'm not sure that memes are qualitatively different from genes in the way we often think. Both can be thought of as patterns in the physical world.
Genes are patterns of nucleotides in DNA molecules. The molecules engage in chemical reactions such that they replicate and preserve their patterns.
Memes are patterns of neural configurations in people's brains. The physical substance of the brains engages in activity such that patterns in one person's brain come to be copied analogously in another person's brain, such as when an idea is verbally passed from one person to the other.
The difference, then, is in where the pattern lies, as well as in the physical mechanism by which the pattern comes to be replicated. It ought to be possible to identify other instances in which patterns of matter come to be copied in other matter, and these would then be other candidates for the genes/memes continuum.
One of the old hypotheses for the formation of life was by Cairns-Smith, who proposed that patterns in clay crystals would replicate. Clays sometimes exist in moving water which continually dissolves them away in some places while adding new material in other places. The new material tends to copy the patterns of the existing clay substrate, in terms of defects and layers with slightly different properties. Such patterns would then be candidates as replicators beyond genes.
In our own bodies, there are speculative theories of non-genetic replicators, called prions. These are supposed to be protein molecules which exist naturally in the body but which have a specific harmful configuration which is different from their benign configuration. Somehow molecules which get into the harmful configuration are able to trigger other molecules also to adopt that harmful shape, leading to the progression of the disease. "Mad cow" disease has been proposed to be caused by this mechanism. However I don't believe this theory has ever been validated.
Some forms of inheritance might be neither genetic nor memetic. For example, if the father is a coal miner, his sons may become coal miners simply because that is the most economically attractive option available.
Some drug addicts give birth to drug-addicted babies. If such babies are more likely to grow up to be drug addicts themselves, then this could be a replicated pattern which would not be memetic, since it would be based on physiological changes in the body, but not genetic either, since the genome is not involved.
At sports events, it used to be popular to do "the wave", where spectators would stand and sit in a rhythmic pattern, making a wave of standing people move around the stadium. This was not a memetic replicator in the way that, say, a popular song would be, but rather it was simply a form of behavior which produced this continually replicating pattern.
I think if you look around you can identify many other instances in which patterns replicate without the direct intervention of genes or memes.