Many parasites face different selection pressures within individual hosts to the ones they face when migrating between hosts. Within hosts, parasites evolve and compete for resources - with the most virulent strains coming to dominate. However, this may be a poor strategy for spreading between hosts. If a parasite steals host resources to the point where the host becomes bed ridden, the chances for infecting other hosts via social contact may go down. Steven Frank once explained these dynamics as follows:
A long period of within-host evolution, with many rounds of parasite competition and selection, may favor the origin and spread of increasing competitiveness between parasites, leading to greater virulence. That evolution of increasing virulence occurs during the time of an infection within a single host. Such evolutionary increase of virulence can kill the host and, in consequence, kill the parasites themselves. In that regard, the newly evolved virulence is short-sighted, because it provides a local advantage to the parasites in the short run but leads to their extinction in the long run.
Cooperation between parasites within hosts can be explained as a type of kin selection - since the parasites involved are typically all close relatives.
Intriguingly, memes may exhibit similar effects - as part of cultural kin selection. Memes are commonly copied within brains. They undergo selection within brains and compete for space, attention and other resources. However, the selection pressures that act on memes within brains may be different from the selection pressures that apply to memes that move between brains. Because copies of memes that descend from a common ancestor within an human are kin, they are more likely to cooperate with each other while they are together inside the same organism - in order to maximise their transmission rates between hosts.
One point of uncertainty concerning this idea is the extent to which memes are copied within brains. Most of our knowledge of meme dynamics comes from studying them as they move between brains. Less is known about what happens within brains - partly due to the primitive state of the associated neuroscience and brain scanning technologies. It is not necessarily obvious that memes are copied much within brains - since the brain could be doing something like copying pointers to memes - or deriving new memes from existing ones (to borrow some metaphors from computer science).
However, some cases of meme copying within the brain are fairly clear. Long-term memory is one likely candidate. Copying seems likely to help explain long-term memory's fidelity in the face of entropic forces. An occasional cycle through short term memory may be involved in the refresh in some cases. Also, some of the low level mechanisms supporting long-term memory appear to involve copying.
Another case involves short term memory. Many people talk to themselves - in a process which acts like a short circuit in the process of talking out loud and hearing what is said. They also do things like repeat phone numbers to themselves - to help them remember the digits by keeping them active in short-term memory. Here, it is pretty obvious that copies are being made. When you have a song in your head (an earworm) something similar is usually happening.
The dynamic behaviour of meme copying within hosts is illuminated by certain mental illnesses in which things break down. Schizophrenia, paranoia, depression, OCDs and other mental illnesses appear to involve massive internal over growths of memes. These memes constantly occupy short term memory, use it to make copies of themselves, resulting in unhealthy obsessions. If the meme is "I am worthless" the patient becomes depressed, while too many copies of the "they are out to get me" meme tends to result in paranoia. Mental illnesses are "natural experiments", which scientists can make use of to gain understanding in areas where it would be unethical to perform experimental interventions. An excellent book on this topic is Genes, Memes, Culture, and Mental Illness: Toward an Integrative Model by Hoyle Leigh.
In the organic realm, parasites may sacrifice themselves for close relatives in the same host. In particular some mind-control parasites enter the brains of their hosts in a sacrificial move that leads to their own destruction, but the propagation of their kin. This happens with Cordyceps fungus in ants, for example. In extreme cases, memes can do much the same thing. Suicide bombing memes and patriotism memes are not averse to sacrificing their hosts so that their relatives can flourish. Disturbingly, in such cases, meme overgrowths similar to those that occur in mental illnesses are appear to be part of the normal reproductive strategy.
For those concerned about mental health issues, meme overgrowths within minds can be counteracted by a healthy memetic immune system.
This article is based on an excerpt from my forthcoming "Memes" book.
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