Cope's rule states that population lineages tend to increase in body size over evolutionary time. While the rule has been demonstrated in many instances, it does not hold true at all taxonomic levels, or in all clades. Larger body size is associated with increased fitness for a number of reasons, although there are also some disadvantages - both on an individual level, and on a clade level: clades comprising larger individuals are more prone to extinction, which may act to limit the maximum size of organisms that have been observed.