Directional selection appears to act on organisms' size, whereas it exhibits a far smaller effect on other morphological traits,[10] though it is possible that this perception may be a result of sample bias.
[10] For example, larger organisms find it easier to avoid or fight off predators and capture prey, to reproduce, to kill competitors, to survive temporary lean times, and to resist rapid climatic changes.
At one level, it is possible that the clade's increased vulnerability to extinction, as its members become larger, means that no taxon survives long enough for individuals to reach huge sizes.
[13] Discussing the case of canid evolution in North America, Blaire Van Valkenburgh of UCLA and coworkers state: Cope's rule, or the evolutionary trend toward larger body size, is common among mammals.
Progenitors of hypercarnivorous lineages may have started as relatively small-bodied scavengers of large carcasses, similar to foxes and coyotes, with selection favoring both larger size and enhanced craniodental adaptations for meat eating.
[3][16] Purported examples of Cope's rule often assume that the stratigraphic age of fossils is proportional to their "clade rank", a measure of how derived they are from an ancestral state; this relationship is in fact quite weak.
[17] Counterexamples to Cope's rule are common throughout geological time; although size increase does occur more often than not, it is by no means universal.
For instance, one may expect the size of birds to be constrained, as larger masses mean more energy must be expended in flight.
[24] An extensive study published in 2015 supports the presence of a trend toward larger body size in marine animals during the Phanerozoic.