Macroevolution

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Macroevolution

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 the cladogram, that represents real lineages of past evolutionary history.

A cladogram or phylogenetic tree is necessary for constructing a taxonomy, but the principles by which higher taxa are recognized remain controversial. The traditional evolutionary taxonomy of G. G. Simpson recognizes higher taxa as units of adaptive evolution called adaptive zones. Species of an adaptive zone share common ancestry, and distinctive morphological or behavioral characteristics associated with use of environmental 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 illustration). See Animal systematics

Phylogenetic relationships of anthropoid primates showing traditional family-level taxaenlarge picture
Phylogenetic relationships of anthropoid primates showing traditional family-level taxa

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 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 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 and new species. Macroevolutionary theory postulates two additional processes analogous to natural selection that act above the species level and on much longer time scales. An evolving lineage ultimately experiences one of two fates, branching speciation or extinction. Lineages that have a high propensity to produce new species and an ability to withstand extinction will dominate evolutionary history.

The higher-level process of differential speciation and extinction caused by the varying characteristics of species or lineages has been called species selection. Because the precise meaning of the term species is controversial, the more neutral terms lineage selection and clade selection are sometimes substituted for species selection. Most species show an evolutionary duration from a few million to approximately 10 million years in the fossil record between geologically instantaneous events of branching speciation. Species selection therefore generally occurs on a time scale of millions of years, rather than the generational time scale of natural selection. Species selection may be the primary factor underlying morphological evolutionary trends at this scale if lineages evolve by punctuated equilibrium, in which most morphological evolutionary change accompanies branching speciation, and species remain morphologically stable between speciational events. See Speciation

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

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

Macroevolution

 

in the evolution of forms of life, the totality of processes occurring above the species level—that is, after practically total interspecies isolation has been established and the leveling of attained differences by crossbreeding has ceased.

The German scientist R. Wohlthereck (1920), the Soviet scientist Iu. A. Filipchenko (1927), and the German R. Goldschmidt (1940) first used the term “macroevolution” in connection with the two types of genetic variation accepted by them (intraspecies variation, or mutations subject to Mendel’s laws; and special, or non-Mendelian variation). According to them, these types of genetic variation were the factors determining the origin of taxons above the species level. Most biologists studying the mechanism of evolution believe that identical microevolutionary processes are the basis for the formation of species, genera, families, and so on, and that therefore, there is no reason to draw a distinction between macroevolution and micro-evolution.

REFERENCES

Timofeev-Resovskii, N. V., N. N. Vorontsov, and A. V. lablokov. Kratkii ocherk teorii evoliutsii. Moscow, 1969.
Philiptschenko, Y. Variabilitdt und Variation. Berlin, 1927.
Goldschmidt, R. The Material Basis of Evolution. New Haven, Conn. 1940.

N. V. TIMOFEEV-RESOVSKII

macroevolution

[¦mak·rō‚ev·ə′lü·shən]
(evolution)
The larger course of evolution by which the categories of animal and plant classification above the species level have been evolved from each other and have differentiated into the forms within each.