Glossary for Biological Systematics

Adaptive radiation - rapid speciation and diversification of a group into unoccupied ecological niches.

Agamospermy - production by plants of seeds in which the embryo is derived (without fertilization) from diploid cells of the parent.

Allelic substitution - the replacement at a genetic locus of one allele by another as a result of mutation or recombination; the basic process of genetic change in a population.

Allometry - change in a measurable aspect of an organism (such as shape) with increase or decrease in size.

Allopatry - the condition of two organisms or groups of organisms inhabiting different places.

Alloploid - see Allopolyploid.

Allopolyploid - a polyploid originating through the addition of unlike chromosome sets, usually in conjunction with hybridization between two species.

Anagenesis - evolutionary change in a single lineage not associated with a speciation event.

Analogy - Non-homologous similarity of structure resulting from similarity of function.

Ancestral character - see Plesiomorphy.

Aneuploidy - an evolutionary change in chromosome number not in multiples of the haploid genome.

Apomixis - asexual reproduction. See also Parthenogenesis, Agamospermy.

Apomorphy - A homology which has originated as an evolutionary novelty within a group under study.

Artificial classification - a classification that has no existence independent of human perception, being based on attributes chosen by the classifier for a specific purpose. See Natural classification.

Association coefficient - a type of similarity coefficient used with binary (presence/absence) data only. Based on the number of matching character states. Maximum similarity = 1.0, minimum similarity = 0.0.

Autapomorphy - an apomorphy possessed by a species or clade that is shared with no other species or clade.

Autoploid - see Autopolyploid.

Autopolyploid - a polyploid originating through multiplication of the same chromosome set.

Backcross - the offspring of a hybrid crossed with one or the other of its parents; in the case of interspecific hybrids, with a member of one or the other parent species.

Binomial nomenclature - the system for naming organisms developed by Carl Linnaeus, in which every organism has a generic name and a specific epithet.

Bivalent - two synapsed chromosomes in prophase I of meiosis.

C-index - the minimum number of character changes divided by the patristic distance.

Carbonization - process of fossilization in which heat and pressure of the rock layers drive off all volatile elements from remains of organisms, leaving only carbon.

Category - a level of the taxonomic hierarchy.

Character - an attribute of an organism (contrast with Character state).

Character clique - a group of compatible characters.

Character evolution - see Character transformation.

Character matrix - matrix consisting of an array of numbers representing for each character the character state possessed by each OTU.

Character state - condition of a character in a particular organism (contrast with Character).

Character transformation - change of a character from one state to another though the evolutionary process.

Chromosome repatterning - evolutionary change in chromosome number or structure. See also Polyploidy, Aneuploidy, Disploidy.

Circular reasoning - see Tautology.

Clade - see Monophyletic group.

Cladistic classification - classification of organisms based on evolutionary kinship.

Cladogenesis - the origin of a branch on a phylogenetic tree; the pattern of cladogenesis results from the process of speciation.

Cladogram - A graphic hypothesis of the phylogeny of a group of organisms, based on the distribution of synapomorphies in the group. Unless other information is added, the sequence of branch points, which represent cladogenesis, is the only information portrayed.

Class - a group of objects that share defining properties.

Classification - arrangement of entities into orderly groups.

Clone - a group of physiologically independent but genetically identical individuals, or a member of such a group.

Clustering - In phenetic classification, the conversion of a similarity matrix to a phenogram, by mathematically grouping organisms according to their phenetic similarities.

Coal ball - A carbonate rock surrounding carbonized plant (or rarely animal) material, generally found as an inclusion in coal strata.

Coefficient of similarity - see Similarity coefficient.

Compatibility analysis - a method for forming cladograms from the largest group of compatible characters.

Compatible characters - characters that are consistent with the same cladogram. See also Congruence.

Compression fossil - The organic remains of an organism which have been pressed flat between rock strata.

Congruence - the property of characters that support the same cladogram. See also Compatible characters.

Convergence - The independent origin of apparently similar apomorphies from different plesiomorphies.

Correlation coefficient - a type of similarity coefficient usable with all types of data. A measure of the correlation of the character states of one OTU to those of the other. Maximum similarity = 1.0; minimum similarity = -1.0.

Dendrochronology - The science of determining the absolute age of tree rings by comparing sequences of wide and narrow rings with those of trees of known ages.

Derived character - see Apomorphy.

Determinate growth - cessation of ontogeny when an organism reaches its final adult form; characteristic of animals. See Indeterminate growth.

Developmental canalization - limitation of the possible adaptive evolutionary changes a species can undergo by innate constraints of its ontogenetic (developmental) pathway.

Differentia - phrase used by Carl Linnaeus and other ‘Aristotelian’; taxonomists which describes how a particular organism differs from other organisms in a genus.

Disploidy - evolutionary change in chromosome structure without a corresponding change in chromosome number.

Distance coefficient - a type of similarity coefficient usable with all types of data. Distance between OTUs is calculated in n-dimensional space, where n = number of characters. Maximum similarity = 0.0; minimum similarity = infinity.

E.U. (Evolutionary unit) - basic unit of phylogenetic reconstruction.

Ecocline - gradual transition between ecotypes; a continuous change in form as a response to a continuous change in an environment.

Ecological isolation - the separation of geographically co-occurring organisms by different ecological preferences.

Ecological time - the span of time over which ecological processes take place, generally tens, hundreds, or thousands of years. See also Geological time.

Ecotype - a variant of a species, the outstanding differences of which are genetic and which have evolved in response to environmental factors.

Electrophoresis - a method for separating proteins or nucleic acids on the basis of their electric charge and size.

Ethnobiology - study of the uses of living organisms by indigenous peoples.

Evolutionary classification - classification of organisms based on levels of evolutionary advancement.

Evolutionary stasis - lack of anagenesis in a species or clade over time, as a result of lowered selective pressure, stabilizing selection, or developmental canalization.

Exaptation - an attribute of a species that comes to serve an adaptive role other than that which mediated its establishment.

Exemplar - a part representing a whole, e.g., a single organism which represents an OTU.

Fossil - A remnant or trace of an organism of past geological ages that has been preserved in the Earth’s crust.

Founder effect - phenomenon through which newly established populations are more representative of their founders than of the population from which the founders came. See also Genetic drift.

Genetic drift - change in gene frequencies in a small populations a result of each generation being unrepresentative of its parents.

Genetic swamping - loss of rare alleles in a population as a result of repeated mating of individuals possessing these alleles with wild-type individuals.

Genome - A complete set of chromosomes inherited as a unit from one parent.

Genomic alloploid - an allopolyploid derived from two species with substantial differences in their genomes, i.e., there is little or no chromosome homology between them.

Genotype - The genetic constitution of an individual, the genetic information possessed by the genome.

Geographic speciation - see Vicariance speciation.

Geological time - the span of time over which geological processes take place, generally thousands, millions, or billions of years. See also Ecological time.

Geological uniformitarianism - The hypothesis that geologic processes in the past are no different from processes occurring today unless there is specific evidence to the contrary.

Grade - a level of evolutionary advancement; the members of a grade share an adaptive zone.

Gradualism - Theory that postulates that evolution is primarily anagenetic (microevolutionary), with new variation arising in individuals by mutation and recombination, and natural selection acting primarily on individuals and populations.

Hardy-Weinberg equilibrium - In an effectively infinite and panmictic population and in the absence of mutation and natural selection, the frequency of any given allele will change until it reaches a stable predictable value in the population, and then remain there.

Hierarchy - an ordering of groups in which larger groups encompass sets of smaller groups.

Homology - similarity of features based on common descent.

Homoplasy - false homology. See Convergence, Parallelism.

Hybrid vigor - increased size or fitness of a hybrid, generally a results of its heterozygosity.

Hypothesis - Statement based in inductive generalizations which is subject to deductive falsification.

Impression fossil - The cast or mold of the surface of an organism in usually fine-grained sedimentary rocks.

Indeterminate growth - continuation of ontogeny when an organism reaches its final adult form; characteristic of plants. See Determinate growth.

Introgression - movement of alleles from one species to another through hybrid intermediates.

Inversion loop - Loop formed during synapsis of a normal chromosome with a homologous chromosome having an inversion. If chiasmata form within the loop, abnormal chromosomes with two centromeres or none can result.

Karyotype - the form and appearance of an organism’s genome at metaphase.

Linkage group - a group of genes on the same chromosome among which there will be no recombination except by crossing over.

Macroevolution - Evolution at or above the species level, i.e., of species or clades. See Punctuated Equilibrium.

Microevolution - Evolution below the species level, i.e., of individuals and populations. See Gradualism.

Mimicry - resemblance of one species to another in order to mislead a third.

Modern synthesis - the synthesis of genetics and evolution, accomplished in the 1930s.

Monophyletic group - A group of organisms consisting of a common ancestor and all its descendants.

Monotypic - a taxon containing only one taxon of a lower category.

Morphological gap - The absence of morphological intermediates between two taxa.

Mosaic evolution - evolutionary origin of the features diagnostic of a higher taxon sequentially, so that intermediate organisms may be a mosaic of advanced and primitive features.

Multivalent - more than two chromosomes synapsed in a unit during prophase I of meiosis; characteristic of many autopolyploids.

Mutation - change of a given allele to another allele; modification of DNA.

Natural classification - a classification that exists in nature independent of human perception. See Artificial classification.

Neontology - the study of extant organisms, as contrasted with paleontology.

Neoteny - paedomorphosis produced by retardation of somatic development.

Nomenclature - a system of names, or the rules by which a system of names is formed.

OTU (Operational Taxonomic Unit) - A group of organisms used in a taxonomic study without designation of taxonomic rank.

Ontogeny - the developmental history of an individual, from zygote or meiospore to death.

Outgroup - see Outgroup comparison.

Outgroup comparison - The principle of hypothesizing plesiomorphy for a character state found both in the monophyletic group under study and the monophyletic group which is its closest relative (the outgroup).

Paedomorphosis - the retention of juvenile characteristics by adult organisms.

Panmixis - the state of a population in which each organism has an equal chance to interbreed with every other organism.

Parallelism - the independent origin of apparently similar apomorphies from the same plesiomorphy.

Paraphyletic group - a group consisting of a common ancestor and some, but not all, of its descendants.

Parsimony - the principle that the simplest explanation, the one that requires the fewest hypotheses, is the one most likely to be correct.

Parsimony analysis - a method for forming cladograms such that there are the fewest number of character transformations.

Parthenogenesis - Production of offspring by female animals from unfertilized ova having the same chromosome number (haploid or diploid) as the parent.

Patristic distance - the difference between two E.U.s expressed as the number of character changes that separate them.

Peripatric speciation - Survival of a group of aberrant individuals on the edge of the range of ancestral species, in conjunction with reproductive or geographic isolation, these individuals eventually achieving lineage independence.

Permineralization - slow replacement of most or all of the organic material in a fossil with mineral crystals (usually silicate).

Petrifaction - see Permineralization.

Phenetic classification - classification of organisms based on their overall resemblance to each other.

Phenogram - a representation of a phenetic character matrix that shows linkage between species based on similarity. Shows no reference to evolution.

Phenon line - A line drawn across a phenogram at a chosen level of similarity, so that groups of OTUs with greater similarity are each placed in taxa of the same rank.

Phenotype - The totality of characteristics of an individual; its appearance as a result of interaction between genotype and environment.

Phenotypic plasticity - Non-genetic variation in organisms in response to environmental factors.

Phyletic evolution - see Anagenesis.

Phyletic extinction - the transformation of an ancestral species into one or more daughter species, none of which resembles the ancestor.

Phyletic gradualism - see Gradualism.

Phylogenetic classification - see Cladistic classification.

Phylogeny - genealogy of species; the pattern of ancestry and descent of species in evolution.

Plesiomorphy - a character state already present in the ancestral species of a group under study.

Polarization - see Character Polarization.

Polyphyletic group - a group of organisms not including the common ancestor of all members of the group.

Polyploid - an organism with more than two genomes.

Population - a group of actually or potentially breeding organisms coexisting spatially and temporally.

Positive assortative mating - preferential mating of individuals with other individuals phenotypically similar to themselves.

Preadaptation - see Exaptation.

Priority - principle of nomenclatural practice that dictates that the valid name of an organism is the first legitimate name applied to it. In plants, priority begins with Species Plantarum (1753), and in animals, Systema Naturae (1758).

Progenesis - paedomorphosis produced by precocious sexual maturation of an organism still in a morphologically juvenile stage.

Punctuated equilibrium - theory that postulates that evolution is primarily cladogenetic, with new variation arising by speciation.

Reciprocal translocation - interchange of chromosome segments between two non-homologous chromosomes.

Recombination - formation of new combinations of genes as a result of the sexual process.

Recombinational speciation - formation of new diploid species from hybrids between existing species.

Regulatory genes - genes that regulate expression of other genes.

Reticulate evolution - the pattern of evolution resulting from recombinational speciation, other diploid hybrid speciation, or allopolyploid speciation.

Reversion - change of an apomorphy to a new character state which resembles the plesiomorphic state.

Segmental alloploid - an allopolyploid derived from two species differing only in segments of their genomes, i.e., there is substantial chromosome homology between them.

Sibling species - species that are not morphologically distinguishable.

Similarity coefficient - A mathematical measure of the similarity between two OTUs. See also Association coefficient, Distance coefficient, Correlation coefficient.

Sister taxa - monophyletic taxa that are each other’s closest relatives, i.e., they share an immediate common ancestral species.

Speciation - formation of new species.

Stabilizing selection - Selection in which heterozygotes are favored over homozygotes, maintaining genetic stability.

Stratigraphic column - chronological sequence of sedimentary rock layers.

Structural genes - Genes that code for polypeptides or other structural units of a cell.

Sympatric speciation - Origin of a new lineage totally within the distribution of an ancestral lineage, usually by the formation of absolute reproductive barriers.

Sympatry - the condition of two organisms or groups of organisms inhabiting the same place.

Symplesiomorphy - A plesiomorphy shared by two or more species or clades; almost all plesiomorphies are symplesiomorphies.

Synapomorphy - an apomorphy shared by two or more species or clades.

Syngameon - a group of discrete morphological units (semispecies) that are interfertile.

Systematics - the science of discovering order in the diversity of living organisms, and expressing that order in a classification.

Systematist - one who does systematics. See also Taxonomist.

Tautology - see Circular reasoning.

Taxon - a group of organisms in a taxonomic hierarchy that have affinity to one another.

Taxonomist - one who does taxonomy. See also Systematist.

Taxonomy - used by some in the strict sense to mean the actual process of naming and classifying organisms. See also Systematics.

Teleology - acceptance of underlying directed purpose in biological phenomena.

Terminal translocation - Movement of a chromosome segment to the end (terminus or telomere) of another chromosome.

Test of sympatry - If two organisms have been separated by a vicariance event and cannot or will not interbreed when brought back together (become sympatric), then the act of speciation is considered complete.

Transgressive segregation - in a hybrid F2 generation, the formation of individuals with features surpassing those of the parental species.

Type specimen - specimen to which the name of an organism is attached.

Unaltered fossils - fossils which retain more or less their original chemical and structural composition; most commonly shells of calcite (mollusks) or silica (diatoms).

Vestigial organ - an organ that has no current adaptive value in an organism.

Vicariance speciation - physical separation (by geography, climate, etc.) of two relatively large populations or groups of populations of a single ancestral species and the attainment of lineage independence by these two groups.