Something similar happens with mitochondria in males. All the ancestral mitochondria were in female germ-line cells. The mitochondria of males find themselves in somatic cells for the first time, and they experience maleness for the first time too.
This difference in selection pressures is a result of "germline sequestration" - the separation from before birth of somatic cells from germ line ones. In many plants, there is no such separation. Many somatic cells remain totipotent. For example, in strawberry plants, most somatic tissues can go on to form runners or seeds.
Another closely related case arises with parasites. The ancestors of a parasite all spread from one body to another. However, inside a body they face different selection pressures from the primary ones that molded their ancestors. There, what pays is doing well inside the body. Something like generating itchy scabs may be counter-productive in this context - since it just activates the host immune system. Within bodies there's often selection against between-host transmission, and for better exploitation of the host resources. It might be a bit of a stretch calling this "somatic selection" - since the soma involved is the body of the host - but the process involved is essentially the same.
This brings us to cultural evolution. Memes face similarly dual sets of selection pressures. The ancestors of memes all spread between minds - but once inside a individual mind they face selection to survive and reproduce within that mind. In that competition, the selection pressures are different. Once inside a mind, there's a local competition for attention - and in that context, efforts expended on spreading between minds turn into a waste of resources.
In the organic case, a common consequence of this type of selection is the rapid evolution of avirulence within individuals. Persistent viral infections gradually become asymptomatic - not because they have gone away, or because the immune system gets better at combating them - but because they evolve away from the ancestral virulent type under the selection pressures within the body - which favour other traits.
I expect that that this happens with memes too. There are some confounding factors with memes - since things like news-related memes are time-sensitive and naturally lose their virulence over time anyway. However, some memes are more 'timeless'. One candidate for this effect is religious evangelism. This theory would be consistent with the idea that levels of evangelism will gradually decline - after an initial indoctrination period. In the secular world, evangelism associated with causes and charities may well behave in the same way. In general, this theory predicts that people are more likely to teach others things shortly after learning them - and that this should be especially true of material with 'evangelical' content.
The idea that 'old' infections lose their ability to cause pathological symptoms associated with their transmission raises some theraputic possibilities. For example, maybe you could relieve the symptoms of warts by putting some virus from an old person into a young person using a syringe. In fact, we already do something similar to this with vaccinations. A closely-equivalent cultural practice would appear to be organised education - which systematically transfers memes from the old to the young. However, education isn't specifically about combating the effects of bad memes.
Of course, the idea of fighting bad memes with good ones is not new. We use the same principle whenever we consume probiotic products containing live bacterial cultures. However, cultural somatic selection proposes a specific place to look for the good memes: old people. Particularly old people who once showed symptoms, but who are now asymptomatic.