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see biogenetic lawbiogenetic law,
in biology, a law stating that the earlier stages of embryos of species advanced in the evolutionary process, such as humans, resemble the embryos of ancestral species, such as fish.
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The genealogical history of organisms, both living and extinct. Phylogeny represents the historical pattern of relationships among organisms which has resulted from the actions of many different evolutionary processes. Phylogenetic relationships are depicted by branching diagrams called cladograms, or phylogenetic trees. Cladograms show relative affinities of groups of organisms called taxa. Such groups of organisms have some genealogical unity, and are given a taxonomic rank such as species, genera, families, or orders. For example, two species of cats—say, the lion (Panthera leo) and the tiger (Panthera tigris)—are more closely related to each other than either is to the gray wolf (Canis latrans). The family including all cats, Felidae, is more closely related to the family including all dogs, Canidae, than either is to the family that includes giraffes, Giraffidae. The lion and tiger, and the Felidae and Canidae, are called sister taxa because of their close relationship relative to the gray wolf, or to the Giraffidae, respectively.

Cladograms thus depict a hierarchy of relationships among a group of taxa (illus. a). Branch points, or nodes, of a cladogram represent hypothetical common ancestors (not specific real ancestors), and the branches connect descendant sister taxa. If the taxa being considered are species, nodes are taken to signify speciation events. The goal of the science of cladistics, or phylogenetic analysis, is to discover these sister-group (cladistic) relationships and to identify what are termed monophyletic groups—two or more taxa postulated to have a single, common origin.

Phylogenetic treesenlarge picture
Phylogenetic trees

The acceptance of a cladogram depends on the empirical evidence that supports it relative to alternative hypotheses of relationship for those same taxa. Evidence for or against alternative phylogenetic hypotheses comes from the comparative study of the characteristics of those taxa. Similarities and differences are determined by comparison of the anatomical, behavioral, physiological, or molecular [such as deoxyribonucleic acid (DNA) sequences] attributes among the taxa. A statement that two features in two or more taxa are similar and thus constitute a shared character is, in essence, a preliminary hypothesis that they are homologous; that is, the taxa inherited the specific form of the feature from their common ancestor. However, not all similarities are homologs; some are developed independently through convergent or parallel evolution, and although they may be similar in appearance, they had different histories and thus are not really the same feature. In cladistic theory, shared homologous similarities are either primitive (plesiomorphic condition) or derived (apomorphic condition), whereas nonhomologous similarities are termed homoplasies (or sometimes, parallelisms or convergences). This distinction over concepts and terminology is important because only derived characters constitute evidence that groups are actually related.

As evolutionary lineages diversify, some characters will become modified. Examples include the enlargement of forelimbs or the loss of digits on the hand. Thus, during evolution the foot of a mammal might transform from a primitive condition of having five digits to a derived form with only four digits (illus. b). Following branching at node 1, the foot in one lineage undergoes an evolutionary modification involving the loss of a digit (expressed as character state A). A subsequent branching event then produced taxa 1 and 2, which inherited that derived character. The lineage leading to taxon 3, however, retained the primitive condition of five digits (character state a). The presence of the shared derived character, A, is called a synapomorphy, and identifies taxa 1 and 2 as being more closely related to each other than either is to taxon 3. Distinguishing between the primitive and derived conditions of a character within a group of taxa (the ingroup) is usually accomplished by comparisons to groups postulated to have more distant relationships (outgroups). Character states that are present in ingroups but not outgroups are postulated to be derived. Systematists have developed computer programs that attempt to identify shared derived characters (synapomorphies) and, at the same time, use them to construct the best phylogenetic trees for the available data.

Knowledge of phylogenetic relationships provides the basis for classifying organisms. A major task of the science of systematics is to search for monophyletic groups. Some groups, such as birds and mammals, are monophyletic; that is, phylogenetic analysis suggests they are all more closely related to each other than to other vertebrates. However, other traditional groups, such as reptiles, have been demonstrated to be nonmonophyletic (some so-called reptiles, such as dinosaurs and their relatives, are more closely related to birds than they are to other reptiles such as snakes). Classifications based on monophyletic groups are termed natural classifications. Phylogenies are also essential for understanding the distributional history, or biogeography, of organisms. Knowing how organisms are related to one another helps the biogeographer to decipher relationships among areas and to reconstruct the spatial histories of groups and their biotas. See Animal evolution, Animal systematics, Biogeography, Taxonomic categories



the historical development of organisms. The term was introduced by the German evolutionist E. Haeckel in 1866.

Phylogeny and its principles are studied by phylogenetics, whose main objective is to reconstruct the evolutionary transformations of animals, plants, and microorganisms and then to determine the origin of specific organisms and the relationships among the taxonomic groups to which these organisms belong. Toward this end, Haeckel developed a method of triple parallelism, which makes it possible to reconstruct the historical development of a given taxonomic group by comparing the data of morphology, embryology, and paleontology. When embryological data were used to reconstruct the evolutionary transformations of organisms, it became necessary to study the relations between individual and historical development and to define more precisely what was meant by phylogeny.

In 1922 the British evolutionist W. Garstang defined phylogeny as a series of ontogenies in successive generations that were connected by the parent-child-grandson relationship. This concept was developed by I. I. Shmal’gauzen, who regarded phylogeny as a “historical series of known (selected) ontogenies.” The interpretation of phylogeny as a historical series of ontogenies controlled by natural selection makes it possible to trace the development of any taxonomic group. The thoroughness to which such research can be pursued depends on the selection of characteristics that establish the phylogenetic continuity of forms, on the availability of paleontological data, and on the objectives of the research.

An investigation of the historical changes that took place in the characteristics of a given taxonomic group makes it possible to reconstruct the phylogeny of the group. However, the reconstructed phylogeny often contains errors, owing to the uneven rates at which different characteristics have evolved and to the necessity of extending known data about a limited number of characteristics to the phylogeny of an entire organism and then to the phylogeny of a taxonomic group. Consequently, the data of many biological sciences, such as molecular biology, biochemistry, genetics, biogeography, and ecology, are increasingly used to reconstruct a phylogeny. These data compensate for the incompleteness of the paleontological information and permit a more accurate reconstruction of a phylogeny than that achieved by the classical method of triple parallelism.

Analysis of the adaptational aspect of phylogenetic transformations is of particular importance and can markedly increase the reliability of phylogenetic reconstructions. The study of phylogeny is the basis for constructing a natural system, for developing an evolutionary theory, and for investigating individual taxonomic groups in more detail; it is also important in the study of historical geology and stratigraphy.

The phylogeny of various taxonomic groups of animals, plants, and microorganisms has been studied unevenly owing both to the unevenness of the paleontological and embryological data available and to the manner in which certain groups have traditionally been studied. Paleontologists are familiar with less than 3 percent of the conjectured number of existing species (about 4.5 million). The incompleteness of the paleontological data makes it difficult to reconstruct a phylogeny. The phylogeny of vertebrate animals and higher plants has been studied most fully because these groups are comparatively recent and because of the availability of paleontological data. Phylogenetic relationships up to the level of genera and species have been accurately established for some groups of vertebrates, including hominids and proboscideans. Among invertebrates, only some groups of mollusks, brachio-pods, and arthropods have been studied in detail. However, the relationships among the phyla of the animal kingdom, like the phylogenetic relationships among such lower taxonomic groups as the mammalian orders, are still disputed. The phylogeny of the lower plants has been studied least of all owing to their ancient origin and the incompleteness of the paleontological and embryological data. An important objective of phylogenetics is to study the origin of intraspecific communities of existing species. The results of such research will provide a fuller understanding of the theory of speciation.


Severtsov, A. N. Morfologicheskie zakonomernosti evoliutsii. Moscow-Leningrad, 1939.
Severtsov, A. N. Glavnye napravleniia evoliutsionnogo protsessa, 3rd ed. Moscow, 1967.
Fedotov, D. M. Evoliutsiia i filogeniia bespozvonochnykh zhivotnykh. Moscow, 1966.
Ivanov, A. V. Proiskhozhdenie mnogokletochnykh zhivotnykh. Leningrad, 1968.
Shmal’gauzen, 1.1. Problemy darvinizma, 2nd ed. Leningrad, 1969.
Takhtadzhian, A. L. Proiskhozhdenie i rasselenie tsvetkovykh rastenii. Leningrad, 1970.
Iudin, K. A. “O poniatii ‘priznak’ i urovniakh razvitiia sistematiki zhivotnykh.” Tr. Zoologicheskogo in-ta, 1974, vol. 53.
Raup, D., and S. Stanley. Osnovy paleontologii. Moscow, 1974. (Translated from English.)



The evolutionary or ancestral history of organisms.
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