In organic evolution, many microbes spend most of their time evolving within their hosts, only occasionally bursting forth in an attempt to spread between them. Sometimes they lie dormant for extended periods - in an attempt to evade the host's immune system.
Similarly, much memetic evolution actually takes place within the minds of human hosts. Ideas sometimes grow, develop and fester within their hosts for months or years before entering into a transmission phase.
Like many parasites, memes often face alternating selective environments. They need one set of adaptations to survive within their hosts and another set of adaptations to spread between them. The results are often a compromise.
Also, like many parasites inside their hosts, memes within minds are often surrounded by their kin - often their close kin. For example, each time you hum a catch song, more very-similar memory traces are laid down, reinforcing the memory of the tune. As with organic parasites, the kin-related dynamics associated with within-brain evolution can be strong.