Cope's rule

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.

Effects of growth
Directional selection appears to act on organisms' size, whereas it exhibits a far smaller effect on other morphological traits. (It should be noted that this effect may be a result of sample bias.) This selectional pressure can be explained by a number of advantages, both in terms of mating success and survival rate.

For example, larger organisms find it easier to avoid or fight off predators and capture prey, to reproduce, to kill competitors, to survive lean times, and to resist rapid climatic changes. They may also potentially benefit from better thermal efficiency, increased intelligence, and a longer lifespan.

Offsetting these advantages, larger organisms require more food and water, and shift from r to K-selection. Their longer generation time means a longer period of reliance on the mother, and on a macroevolutionary scale restricts the clade's ability to evolve rapidly in response to changing environments.

Capping growth
Left unfettered, the trend of ever-large size would produce organisms of gargantuan proportions. Therefore, some factors must limit this process:
 * 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.


 * There are probably also physically imposed limits to the size of some organisms; for instance, insects must be small enough for oxygen to diffuse to all parts of their bodies, birds must be light enough to fly, and the length of giraffes' necks may be limited by the amount of pressure it is possible for their hearts to generate.


 * Finally, there may be a competitive element, in that changes in size are necessarily accompanied by changes in ecological niche. For example, carnivores over 21 kg must prey on organisms larger, not smaller, than themselves.  If such a niche is already occupied, competitive pressure may oppose the directional selection.  The three Canidae clades show a trend towards larger size before becoming extinct.

Validity
Cope recognised that clades of Cenozoic mammals appeared to originate as small individuals, and that body mass increased through a clade's history. 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. Large size enhances the ability to avoid predators and capture prey, enhances reproductive success, and improves thermal efficiency. Moreover, in large carnivores, interspecific competition for food tends to be relatively intense, and bigger species tend to dominate and kill smaller competitors. 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. Moreover, the evolution of predator size is likely to be influenced by changes in prey size, and a significant trend toward larger size has been documented for large North American mammals, including both herbivores and carnivores, in the Cenozoic."

- Van Valkenburgh et al., Cope's Rule, Hypercarnivory, and Extinction in North American Canids

In some cases, the increase in body size may represent a passive, rather than an active, trend. In other words, the maximum size increases, but the minimum size does not; this is usually a result of size varying pseudo-randomly rather than directed evolution. This does not fall into Cope's rule sensu stricto, but is considered by many workers to be an example of "Cope's rule sensu lato". In other cases, an increase in size may in fact represent a transition to an optimal body size, and not imply that populations always develop to a larger size.

However, many palaeobiologists are sceptical of the validity of Cope's rule, which may merely represent a statistical artifact. 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. 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 example, among genera of Cretaceous molluscs, an increase in size is no more common than stasis or a decrease. In many cases, Cope's rule only operates at certain taxonomic levels: for example, an order may obey Cope's rule, while its constituent families do not.

Despite many counter-examples, Cope's rule does hold in many instances. For example, all marine phyla except the molluscs show a size increase between the Cambrian and Permian. And Cope's rule also appears to hold in clades where a constraint on size is expected. For instance, one may expect the size of birds to be constrained, as larger masses mean more energy must be expended in flight. However, birds do appear to follow Cope's law.

In island biogeography
In the case of island ecosystems such a mechanism of course exists. It is still rare however, as the island needs to be the "right" size - too small an island would enforce strict partitioning of resources, while too large an island would offer just too many ecological niches. Furthermore, the flora and fauna must be suitable to maintain the right trophic web, with few strong or obligatory mutualisms that would release species from a trophic connection to a sufficient number of others. In short, there must be a wide range of predators and prey but not so many that there is much benefit in evolving adaptation towards hunting a particular kind of prey or an escape mechanism from most predators.

Because home range is correlated to body size, large carnivorous mammals do not colonize islands very efficiently; those that do tend towards dwarfism. On prehistoric Gargano Island in the Miocene-Pliocene Mediterranean, on islands in the Caribbean like Cuba, and on Madagascar and New Zealand on the other hand, most or all apex predators were birds like eagles and owls. In such locales, there are occasional - albeit rather brief (a few million years or so) - bouts of evolution in which a large proportion of the local (vertebrate) fauna increased in size (see also island gigantism).