The distinction between r- and K- selection has been on the MCAT in the past. The terminology derives from the Verhulst equation of population dynamics in which 'r' is the growth rate of the population and 'K' is its carrying capacity. As a general rule, r-selected species produce many offspring, each of which is comparatively less likely to survive to adulthood, while K-selected species invest more heavily in fewer offspring, each of which has a better chance of surviving to adulthood. Primates invest considerable time and energy into each singly born offspring. This reproductive strategy is a quintessential example of K-selection. Social structure built around parent-offspring interaction is much more common in K-selected species than r-selected species.