macroevolution

n.

Large-scale evolution occurring over geologic time that results in the formation of new taxonomic groups.

macroevolutionary mac'ro·ev'o·lu'tion·ar'y (-shə-nĕr'ē) adj.

Large-scale patterns and processes in the history of life, including the origins of novel organismal designs, evolutionary trends, adaptive radiations, and extinctions. Macroevolutionary research is based on phylogeny, the history of common descent among species. The formation of species and branching of evolutionary lineages mark the interface between macroevolution and microevolution, which addresses the dynamics of genetic variation within populations. Phylogenetic reconstruction, the developmental basis of evolutionary change, and long-term trends in patterns of speciation and extinction among lineages constitute major foci of macroevolutionary studies.

Phylogenetic reconstruction

Phylogenetic relationships are revealed by the sharing of evolutionarily derived characteristics among species, which provides evidence for common ancestry. Shared derived characteristics are termed synapomorphies, and are equated by many systematists with the older concept of homology. Characteristics of different organisms are homologous if they descend, with some modification, from an equivalent characteristic of their most recent common ancestor. Closely related species share more homologous characteristics than do species whose common ancestry is more distant. Species are grouped into clades according to patterns of shared homologies. The clades form a nested hierarchy in which large clades are subdivided into smaller, less inclusive ones, and are depicted by a branching diagram called a cladogram. A phylogenetic tree is a branching diagram, congruent with thecladogram, that represents real lineages of past evolutionary history.

A cladogram or phylogenetic tree is necessary for constructing a taxonomy, but theprinciples by which higher taxa are recognized remain controversial. The traditionalevolutionary taxonomy of G. G. Simpson recognizes higher taxa as units of adaptiveevolution called adaptive zones. Species of an adaptive zone share common ancestry, anddistinctive morphological or behavioral characteristics associated with use ofenvironmental resources. Higher taxa receive Linnean categorical ranks (genus, family,order, and so forth) reflecting the breadth and distinctness of their adaptive zones. All taxa must have a single evolutionary origin, which means that the taxon must include the most recent common ancestor of all included species. A taxon is monophyletic if it contains all descendants of the group's most recent common ancestor, or paraphyletic if some descendants of the group's most recent common ancestor are excluded because they have evolved a new adaptive zone. For example, evolutionary taxonomy of the anthropoid primates groups the orangutan, gorilla, and chimpanzee in the paraphyletic family Pongidae and the humans in the monophyletic family Hominidae. Although the humans and chimpanzees share more recent common ancestry than either does with the gorilla or orangutan, the chimpanzees are grouped with the latter species at the family level and the humans are placed in a different family because they are considered to have evolved a new adaptive zone. The Hominidae and Pongidae together form a monophyletic group at a higher level.See also Animal systematics.

Cladistic taxonomy or phylogenetic systematics accepts only monophyletic taxa because these alone are considered natural units of common descent. Linnean rankings are considered unimportant. Taxa recognized using both the Simpsonian and cladistic taxonomies are standardly used in macroevolutionary analyses of extinction and patterns of diversity through time. The Simpsonian versus cladistic taxonomies often lead to fundamentally different interpretations, however. For example, extinction of a paraphyletic group, such as dinosaurs, would be considered pseudoextinction by cladists because some descendants of the group's most recent common ancestor survive. Birds are living descendants of the most recent common ancestor of all dinosaurs. The dinosaurs as traditionally recognized, therefore, do not form a valid cladistic taxon. See also Aves; Dinosaur; Phylogeny.

Developmental processes

Comparative studies of organismal ontogeny are used to find where in development the key features of higher taxa appear and how developmental processes differ between taxa. Evolutionary developmental biologists denote the characteristic body plans of taxa by the term Bauplan. The major characteristics of animal phyla and their developmental and molecular attributes appear to have arisen and stabilized early in the history of life, during the Cambrian Period. Subsequent evolutionary diversification builds upon the Bauplan established early in animal evolution. See also Cambrian.

Particularly important to the evolutionary diversification of life are historical processes that generate change by altering the timing of organismal development, a phenomenon called heterochrony. Heterochronic changes can produce either paedomorphic or paeramorphic results. Paedomorphosis denotes the retention of preadult characteristics of ancestors in the adult stages of descendants; peramorphosis is the opposite outcome, in which the descendant ontogeny transcends that of the ancestor, adding new features at the final stages. Heterochronic changes can be produced by changing the rates of developmental processes or the times of their onset or termination.

Developmental dissociation occurs when different kinds of heterochronic change alter the development of different parts of the organism independently. Extensive dissociation can fundamentally restructure organismal ontogeny, producing ontogenetic repatterning. However, it is rare that a single heterochronic transformation affects all parts of the organism simultaneously. For most taxa, novel morphologies are produced by a mosaic of different heterochronic processes and by changes in the physical location of developmental events within the organism.

Long-term trends

Traditional Darwinian theory emphasizes natural selection acting on varying organisms within populations as the main causal factor of evolutionary change. Over many generations, the accumulation of favorable variants by natural selection produces new adaptations andnew species. Macroevolutionary theory postulates two additional processes analogous tonatural selection that act above the species level and on much longer time scales. Anevolving lineage ultimately experiences one of two fates, branching speciation orextinction. Lineages that have a high propensity to produce new species and an ability towithstand extinction will dominate evolutionary history.

The higher-level process of differential speciation and extinction caused by the varyingcharacteristics of species or lineages has been called species selection. Because theprecise meaning of the term species is controversial, the more neutral terms lineageselection and clade selection are sometimes substituted for species selection. Most speciesshow an evolutionary duration from a few million to approximately 10 million years in thefossil record between geologically instantaneous events of branching speciation. Speciesselection therefore generally occurs on a time scale of millions of years, rather than thegenerational time scale of natural selection. Species selection may be the primary factorunderlying morphological evolutionary trends at this scale if lineages evolve by punctuatedequilibrium, in which most morphological evolutionary change accompanies branchingspeciation, and species remain morphologically stable between speciational events. See also Speciation.

The fossil record reveals mass extinctions in which enormous numbers of species frommany different taxa are lost within a relatively short interval of geological time. Somelineages may be better able to survive mass extinction events than others, and thecharacteristics that make a lineage prone to survive mass extinction may be very differentfrom those that influence species selection between events of mass extinction. Catastrophicspecies selection denotes differential survival and extinction of lineages during events ofmass extinction as determined by character variation among lineages. Prior to theCretaceous mass extinction, dinosaur taxa dominated mammalian taxa, whereas mammalssurvived the mass extinction and then diversified extensively. The characteristics of theancestral mammals may have permitted them to survive environmental challenges to whichdinosaurs were susceptible. See also Extinction (biology); Fossil; Mammalia; Paleontology; Permian.

Because natural selection, species selection, and catastrophic species selection candiffer in the biological characteristics they promote, higher-level processes may undo orreverse evolutionary trends arising from lower-level processes. See also Organic evolution.

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Macroevolution is large-scale change that can generate entire new groups of related species, also known as a clade. One example would be the movement of plants onto land; all terrestrial plants are descended from that event, which occurred during the Devonian period 400 million years ago.

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The noun has one meaning:

Meaning #1: evolution on a large scale extending over geologic era and resulting in the formation of new taxonomic groups

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