XVII.5 The spreading of memes is governed by the same laws as the spreading of contagious diseases

In general, the same rules apply to the spreading of memes as to the spreading of infectious diseases and the relevant processes can also be described by the same formal models (Anderson 1993).The most important parameter that determines the efficiency with which a meme will spread is its basic reproduction constant R0, a dimensionless constant equal to the average number of individuals “infected” by the relevant meme by one bearer of the particular meme in the “naive” population, i.e. a population whose members had not previously encountered the meme.If this constant is larger than 1, the given gene can spread in the population and be retained in it for a long time, even if it is harmful to its bearers, i.e. if it reduces their fitness.The actual reproduction constant, R, in a population in which a certain fraction of individuals, q, was “infected” by the meme in the past, is understandably lower and decreases linearly with increasing fraction of infected individuals.If R decreases to a value of 1, the fraction of infected individuals remains constant in the population and the particular meme is retained endemically in the population.Each meme is differently “infectious” and each has a different threshold intensity, NT, i.e. number of susceptible individuals, at which it can begin to spread in the population.A simple equation exists between the threshold density and R0


               NT  = N/R0                                                                                   (1)


where N is the size of the population.If “meme infection” has occurred in the population, the value of R0 can be calculated from the fraction of individuals that remained unaffected by the meme (s), as it holds that


    R0 =[1/(1-s)] ln(1/s)                                                                                       (2)


On the basis of R0we can then easily calculate the size of the fraction of persons in the population that it would be required to make immune to the particular meme through an effective campaign so that this meme would not be able to spread by horizontal transmission.


              p   > (1-1/R0 )                                                                                 (3)


If, for example, a certain meme affects an average of 70% of individuals during its natural horizontal spreading in the population, then it would be necessary to “immunize” 41% of the so-far unaffected population in advance to prevent a future meme epidemic.Because the fraction of immunized persons gradually decreases in a natural way after the end of the epidemic, either through the deaths of immune individuals or forgetting, some meme epidemics can have a regular cyclic character, where the periodicity of the cycle will depend on the size of the population.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.