Cope's rule, named after American paleontologist Edward Drinker Cope, [1] [2] postulates that population lineages tend to increase in body size over evolutionary time. [3] It was never actually stated by Cope, although he favoured the occurrence of linear evolutionary trends. [4] It is sometimes also known as the Cope–Depéret rule, [5] because Charles Depéret explicitly advocated the idea. [6] Theodor Eimer had also done so earlier. [4] The term "Cope's rule" was apparently coined by Bernhard Rensch, [1] based on the fact that Depéret had "lionized Cope" in his book. [4] [lower-alpha 1] 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 and on a clade level: clades comprising larger individuals are more prone to extinction, which may act to limit the maximum size of organisms.
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. [3] This selectional pressure can be explained by a number of advantages, both in terms of mating success and survival rate. [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. [3] They may also potentially benefit from better thermal efficiency, increased intelligence, and a longer lifespan. [3]
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. [3]
Left unfettered, the trend of ever-larger 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. [3] 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, flying birds must be light enough to fly, and the length of giraffes' necks may be limited by the blood pressure it is possible for their hearts to generate. [3] Finally, there may be a competitive element, in that changes in size are necessarily accompanied by changes in ecological niche. For example, terrestrial carnivores over 21 kg almost always prey on organisms larger, not smaller, than themselves. [11] If such a niche is already occupied, competitive pressure may oppose the directional selection. [3] The three Canidae clades (Hesperocyoninae, Borophaginae, and Caninae) all show a trend towards larger size, although the first two are now extinct. [12]
Cope recognised that clades of Cenozoic mammals appeared to originate as small individuals, and that body mass increased through a clade's history. [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. 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. [11]
In some cases, the increase in body size may represent a passive, rather than an active, trend. [14] 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 ". [15] 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. [13]
However, many palaeobiologists are skeptical of the validity of Cope's rule, which may merely represent a statistical artefact. [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 example, among genera of Cretaceous molluscs, an increase in size is no more common than stasis or a decrease. [15] 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), or more generally, it may apply to only some clades of a taxon. [18] Giant dinosaurs appear to have evolved dozens of times, in response to local environmental conditions. [19] [20]
Despite many counter-examples, Cope's rule is supported in many instances. For example, all marine invertebrate phyla except the molluscs show a size increase between the Cambrian and Permian. [21] Collectively, dinosaurs exhibit an increase in body length over their evolution. [22] 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. Birds have been suggested to follow Cope's law, [23] although a subsequent reanalysis of the same data suggested otherwise. [24]
An extensive study published in 2015 supports the presence of a trend toward larger body size in marine animals during the Phanerozoic. However, this trend was present mainly in the Paleozoic and Cenozoic; the Mesozoic was a period of relative stasis. The trend is not attributable simply to neutral drift in body size from small ancestors, and was mainly driven by a greater rate of diversification in classes of larger mean size. A smaller component of the overall trend is due to trends of increasing size within individual families. [25]
In biological phylogenetics, a clade, also known as a monophyletic group or natural group, is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. In the taxonomical literature, sometimes the Latin form cladus is used rather than the English form. Clades are the fundamental unit of cladistics, a modern approach to taxonomy adopted by most biological fields.
Evolution is the change in the heritable characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certain characteristics becoming more or less common within a population over successive generations. The process of evolution has given rise to biodiversity at every level of biological organisation.
A mammal is a vertebrate animal of the class Mammalia. Mammals are characterized by the presence of milk-producing mammary glands for feeding their young, a neocortex region of the brain, fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which their ancestors diverged in the Carboniferous Period over 300 million years ago. Around 6,400 extant species of mammals have been described and divided into 29 orders.
Macroevolution usually means the evolution of large-scale structures and traits that go significantly beyond the intraspecific variation found in microevolution. In other words, macroevolution is the evolution of taxa above the species level.
Xenarthra is a major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and armadillos. Extinct xenarthrans include the glyptodonts, pampatheres and ground sloths. Xenarthrans originated in South America during the late Paleocene about 60 million years ago. They evolved and diversified extensively in South America during the continent's long period of isolation in the early to mid Cenozoic Era. They spread to the Antilles by the early Miocene and, starting about 3 million years ago, spread to Central and North America as part of the Great American Interchange. Nearly all of the formerly abundant megafaunal xenarthrans became extinct at the end of the Pleistocene.
The three-domain system is a taxonomic classification system that groups all cellular life into three domains, namely Archaea, Bacteria and Eukarya, introduced by Carl Woese, Otto Kandler and Mark Wheelis in 1990. The key difference from earlier classifications such as the two-empire system and the five-kingdom classification is the splitting of Archaea from Bacteria as completely different organisms. It has been challenged by the two-domain system that divides organisms into Bacteria and Archaea only, as Eukaryotes are considered as a clade of Archaea.
Eumetazoa, also known as diploblasts, Epitheliozoa or Histozoa, are a proposed basal animal clade as a sister group of Porifera (sponges). The basal eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as a eumetazoan in the ParaHoxozoa. The competing hypothesis is the Myriazoa clade.
Bergmann's rule is an ecogeographical rule that states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer regions. The rule derives from the relationship between size in linear dimensions meaning that both height and volume will increase in colder environments. Bergmann's rule only describes the overall size of the animals, but does not include body proportions like Allen's rule does.
Tetanurae is a clade that includes most theropod dinosaurs, including megalosauroids, allosauroids, tyrannosauroids, ornithomimosaurs, compsognathids and maniraptorans. Tetanurans are defined as all theropods more closely related to modern birds than to Ceratosaurus and contain the majority of predatory dinosaur diversity. Tetanurae likely diverged from its sister group, Ceratosauria, during the late Triassic. Tetanurae first appeared in the fossil record by the Early Jurassic about 190 mya and by the Middle Jurassic had become globally distributed.
Pareiasaurs are an extinct clade of large, herbivorous parareptiles. Members of the group were armoured with osteoderms which covered large areas of the body. They first appeared in southern Pangea during the Middle Permian, before becoming globally distributed during the Late Permian. Pareiasaurs were the largest reptiles of the Permian, reaching sizes equivalent to those of contemporary therapsids. Pareiasaurs became extinct in the Permian–Triassic extinction event.
Phenotypic plasticity refers to some of the changes in an organism's behavior, morphology and physiology in response to a unique environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompasses all types of environmentally induced changes that may or may not be permanent throughout an individual's lifespan.
Phylogenetic comparative methods (PCMs) use information on the historical relationships of lineages (phylogenies) to test evolutionary hypotheses. The comparative method has a long history in evolutionary biology; indeed, Charles Darwin used differences and similarities between species as a major source of evidence in The Origin of Species. However, the fact that closely related lineages share many traits and trait combinations as a result of the process of descent with modification means that lineages are not independent. This realization inspired the development of explicitly phylogenetic comparative methods. Initially, these methods were primarily developed to control for phylogenetic history when testing for adaptation; however, in recent years the use of the term has broadened to include any use of phylogenies in statistical tests. Although most studies that employ PCMs focus on extant organisms, many methods can also be applied to extinct taxa and can incorporate information from the fossil record.
In phylogenetics, basal is the direction of the base of a rooted phylogenetic tree or cladogram. The term may be more strictly applied only to nodes adjacent to the root, or more loosely applied to nodes regarded as being close to the root. Note that extant taxa that lie on branches connecting directly to the root are not more closely related to the root than any other extant taxa.
The evolution of biological complexity is one important outcome of the process of evolution. Evolution has produced some remarkably complex organisms – although the actual level of complexity is very hard to define or measure accurately in biology, with properties such as gene content, the number of cell types or morphology all proposed as possible metrics.
There is much to be discovered about the evolution of the brain and the principles that govern it. While much has been discovered, not everything currently known is well understood. The evolution of the brain has appeared to exhibit diverging adaptations within taxonomic classes such as Mammalia and more vastly diverse adaptations across other taxonomic classes. Brain to body size scales allometrically. This means as body size changes, so do other physiological, anatomical, and biochemical constructs connecting the brain to the body. Small bodied mammals have relatively large brains compared to their bodies whereas large mammals have a smaller brain to body ratios. If brain weight is plotted against body weight for primates, the regression line of the sample points can indicate the brain power of a primate species. Lemurs for example fall below this line which means that for a primate of equivalent size, we would expect a larger brain size. Humans lie well above the line indicating that humans are more encephalized than lemurs. In fact, humans are more encephalized compared to all other primates. This means that human brains have exhibited a larger evolutionary increase in its complexity relative to its size. Some of these evolutionary changes have been found to be linked to multiple genetic factors, such as proteins and other organelles.
Hydroidolina is a subclass of Hydrozoa and makes up 90% of the class. Controversy surrounds who the sister groups of Hydroidolina are, but research has shown that three orders remain consistent as direct relatives: Siphonophorae, Anthoathecata, and Leptothecata.
The Avalon explosion, named from the Precambrian faunal trace fossils discovered on the Avalon Peninsula in Newfoundland, eastern Canada, is a proposed evolutionary radiation of prehistoric animals about 575 million years ago in the Ediacaran period, with the Avalon explosion being one of three eras grouped in this time period. This evolutionary event is believed to have occurred some 33 million years earlier than the Cambrian explosion, which had been long thought to be when complex life started on Earth.
The Extended Evolutionary Synthesis (EES) consists of a set of theoretical concepts argued to be more comprehensive than the earlier modern synthesis of evolutionary biology that took place between 1918 and 1942. The extended evolutionary synthesis was called for in the 1950s by C. H. Waddington, argued for on the basis of punctuated equilibrium by Stephen Jay Gould and Niles Eldredge in the 1980s, and was reconceptualized in 2007 by Massimo Pigliucci and Gerd B. Müller.
A biological rule or biological law is a generalized law, principle, or rule of thumb formulated to describe patterns observed in living organisms. Biological rules and laws are often developed as succinct, broadly applicable ways to explain complex phenomena or salient observations about the ecology and biogeographical distributions of plant and animal species around the world, though they have been proposed for or extended to all types of organisms. Many of these regularities of ecology and biogeography are named after the biologists who first described them.
The Lilliput effect is a decrease in body size in animal species that have survived a major extinction. There are several hypotheses as to why these patterns appear in the fossil record, some of which are: the survival of small taxa, dwarfing of larger lineages, and the evolutionary miniaturization from larger ancestral stocks. The term was coined in 1993 by Adam Urbanek in his paper concerning the extinction of graptoloids and is derived from the island of Lilliput inhabited by a miniature race of people in Gulliver’s Travels. This size decrease may just be a temporary phenomenon restricted to the survival period of the extinction event. In 2019 Atkinson et al. coined the term the Brobdingnag effect to describe a related phenomenon operating in the opposite direction, whereby new species evolving after the Triassic-Jurassic mass extinction originated at small body sizes before undergoing a size increase. The term is also from Gulliver's Travels where Brobnignag is a land inhabited by a race of giants.
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